4.11 Anaerobic Processes
Created by: CK-12/Adapted by Christine Miller
Fast and Furious
These sprinters’ muscles will need a lot of energy to complete this short race, because they will be running at top speed. The action won’t last long, but it will be very intense. The energy each sprinter needs can’t be provided quickly enough by aerobic cellular respiration. Instead, their muscle cells must use a different process to power their activity.
Making ATP Without Oxygen
Living things’ cells power their activities with the energy-carrying molecule ATP (adenosine triphosphate). The cells of most living things make ATP from glucose in the process of cellular respiration. This process occurs in three stages: glycolysis, the Krebs cycle, and electron transport. The latter two stages require oxygen, making cellular respiration an aerobic process. When oxygen is not available in cells, the ETS quickly shuts down. Luckily, there are also ways of making ATP from glucose which are anaerobic, which means that they do not require oxygen. These processes are referred to collectively as anaerobic respiration.
Fermentation
Ome important way of making ATP without oxygen is fermentation. Fermentation starts with glycolysis, which does not require oxygen, but it does not involve the latter two stages of aerobic cellular respiration (the Krebs cycle and electron transport). There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. We make use of both types of fermentation using other organisms, but only lactic acid fermentation actually takes place inside the human body.
Alcoholic Fermentation
Alcoholic fermentation is carried out by single-celled fungi (called yeasts), as well as some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine. The biofuel ethanol (a type of alcohol), for example, is produced by alcoholic fermentation of the glucose in corn or other plants. The process by which this happens is summarized in the diagram below. The two pyruvic acid molecules shown in the diagram come from the splitting of glucose in the first stage of the process (glycolysis). ATP is also made during glycolysis. Two molecules of ATP are produced from each molecule of glucose.
Yeasts in bread dough also use alcoholic fermentation for energy. They produce carbon dioxide gas as a waste product. The carbon dioxide released causes bubbles in the dough and explains why the dough rises. Do you see the small holes in the bread pictured to the right? The holes were formed by bubbles of carbon dioxide gas.
As you have probably guessed, yeast is also used in producing alcoholic beverages. When making beer, brewers will add yeast to a mix of barley and hops. In the absence of oxygen, yeast will carry out alcoholic fermentation in order to convert the glucose in the barley into energy, producing the alcohol content as well as the carbonation present in beer.
Lactic Acid Fermentation
Lactic acid fermentation is carried out by certain bacteria, including the bacteria in yogurt. It is also carried out by your muscle cells when you work them hard and fast. This is how the muscles of the sprinters pictured above get energy for their short-lived — but intense — activity. When this happens, your muscles are using ATP faster than your cardiovascular system can deliver oxygen! The process by which this happens is summarized in the diagram below. Again, the two pyruvic acid molecules shown in the diagram come from the splitting of glucose in the first stage of the process (glycolysis). It is also during this stage that two ATP molecules are produced. The rest of the processes produce lactic acid. Note that, unlike in alcoholic fermentation, there is no carbon dioxide waste product in lactic acid fermentation.
Lactic acid fermentation produces lactic acid and NAD+. The NAD+ cycles back to allow glycolysis to continue so more ATP is made. Each circle represents a carbon atom.
Did you ever run a race, lift heavy weights, or participate in some other intense activity and notice that your muscles start to feel a burning sensation? This may occur when your muscle cells use lactic acid fermentation to provide ATP for energy. The buildup of lactic acid in the muscles causes a burning feeling. This painful sensation is useful if it gets you to stop overworking your muscles and allow them a recovery period, during which cells can eliminate the lactic acid.
Pros and Cons of Anaerobic Respiration
With oxygen, organisms can use aerobic cellular respiration to produce up to 38 molecules of ATP from just one molecule of glucose. Without oxygen, organisms must use anaerobic respiration to produce ATP, and this process produces only two molecules of ATP per molecule of glucose. Although anaerobic respiration produces less ATP, it has the advantage of doing so very quickly. For example, it allows your muscles to get the energy they need for short bursts of intense activity. Aerobic cellular respiration, in contrast, produces ATP more slowly.
Fermentation in Food Production
Anaerobic respiration is also used in the food industry. You read about yeast’s role in making bread and beer, but did you know that there are many microbes that are used to create the food we eat, including cheese, sour cream, yogurt, soy sauce, olives, pepperoni, and many more. Watch the video below to learn more about fermentation in the food industry.
The beneficial bacteria that make delicious food – Erez Garty, TED-Ed, 2016.
4.11 Cultural Connection
Fishing has always been part of the culture and nutrition of Indigenous peoples living on the west coast of Canada. Fish provides delicious important nutrients such as protein, Omega-3 fatty acids, calcium, iron, and Vitamins A, B, C and D. Traditionally, no part of the fish was wasted, including head, eyes, internal organs, and eggs.
Eulachon, also known as candle fish or oolichan, (pictured below) have been prized for their oil for thousands of years. The pathways of these fish dictated “grease trails” and are found from Bristol Bay, Alaska, all the way south to the Klamath River, California. Within BC, the areas of Nass, Knights Inlet, and Bella Coola had large trading centres for this important natural resource.
Photos by Brodie Guy – www.brodieguy.com CC BY-NC-ND 4.0
Euchalon were and are eaten fresh, smoked or dried, and as grease. The grease remains a highly valued food to Indigenous coastal communities. The flavour of the grease varies greatly depending not only on where the fish is from and how it is made, but also how long it is left to ferment. To ferment the eulachon, fish are left in a wood-lined locker dug into the soil for 10 days. Fermentation uses the action of fungi and bacteria to break down the fish making oil extraction much faster and easier.
To learn more, visit the First Nations Health Authority Traditional Foods Fact Sheet and a feature in the Yukon News, “Eulachon, oolicahn, hooligan: A fish by any other name is just as oily.”
4.11 Summary
- The cells of most living things produce ATP from glucose by aerobic cellular respiration, which uses oxygen. Some organisms instead produce ATP from glucose by anaerobic respiration, which does not require oxygen.
- An important way of making ATP without oxygen is fermentation. There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis, the first (anaerobic) stage of cellular respiration, in which two molecules of ATP are produced from one molecule of glucose.
- Alcoholic fermentation is carried out by single-celled organisms, including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
- Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
- Anaerobic respiration produces far less ATP than does aerobic cellular respiration, but it has the advantage of being much faster. For example, it allows muscles to get the energy they need for short bursts of intense activity.
4.11 Review Questions
- Explain the primary difference between aerobic cellular respiration and anaerobic respiration.
- What is fermentation?
- Compare and contrast alcoholic and lactic acid fermentation.
- Identify the major pros and the major cons of anaerobic respiration relative to aerobic cellular respiration.
- What process is shared between aerobic cellular respiration and anaerobic respiration? Describe the process briefly. Why can this process happen in anaerobic respiration, as well as aerobic respiration?
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- What is the reactant (or starting material)common to aerobic respiration and both types of fermentation?
4.11 Explore More
Anaerobic Respiration, Bozeman Science, 2013.
Fermentation, The Amoeba Sisters, 2018.
Science of Beer: Tapping the Power of Brewer’s Yeast, KQED Science, 2014.
Attributions
Figure 4.11.1
Sprinters by Jonathan Chng on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 4.11.2
Alcoholic fermentation by Hana Zavadska/ CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
Figure 4.11.3
Bread [photo] by Orlova Maria on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 4.11.4
Lactic Acid Fermenation by Hana Zavadska/ CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
References
Bozeman Science. (2013, May 2). Anaerobic respiration. YouTube. https://www.youtube.com/watch?v=cDC29iBxb3w&feature=youtu.be
Hana Zavadska/CK-12 Foundation. (2016, August 15). Figure 2: Alcoholic fermentation [digital image]. In Brainard, J., and Henderson, R., CK-12’s College Human Biology FlexBook® (Section 4.11). CK-12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/section/4.11/
Hana Zavadska/CK-12 Foundation. (2016, August 15). Figure 4: Lactic acid fermentation [digital image]. In Brainard, J., and Henderson, R., CK-12’s College Human Biology FlexBook® (Section 4.11). CK-12 Foundation. https://www.ck12.org/book/ck-12-college-human-biology/section/4.11/
First Nations Health Authority. (2019, September 6). First Nations traditional foods facts Sheet [pdf]. https://www.fnha.ca/Documents/Traditional_Food_Fact_Sheets.pdf
Genest, M. (2017, May 24). Eulachon, oolichan, hooligan: A fish by any other name is just as oily [online article]. YukonNews.com. https://www.yukon-news.com/business/eulachon-oolichan-hooligan-a-fish-by-any-other-name-is-just-as-oily/
KQED Science. (2014, February 11). Science of beer: Tapping the power of brewer’s yeast. YouTube. https://www.youtube.com/watch?v=TVtqwWGguFk&feature=youtu.be
TED-Ed. (2016). The beneficial bacteria that make delicious food – Erez Garty. YouTube. https://www.youtube.com/watch?v=eksagPy5tmQ&feature=youtu.be
The Amoeba Sisters. (2018, April 30). Fermentation. YouTube. https://www.youtube.com/watch?v=YbdkbCU20_M&feature=youtu.be
Wikipedia contributors. (2020, June 21). Ethanol fuel. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Ethanol_fuel&oldid=963675942
The rounded head (or tip) of the penis.
6.2 Genetic Variation: Review Questions and Answers
- Compare and contrast the significance of genetic variation at the individual and population levels. At the individual level, most human genetic variation is not very important biologically because it has no apparent adaptive significance. It neither enhances nor detracts from individual fitness. At the population level, genetic variation is crucial for evolution to occur. Genetically-based differences in fitness among individuals are the key to evolution by natural selection.
- Describe genetic variation within and between human populations on different continents. Any two randomly selected individuals differ in only about 0.1 per cent of their DNA base pairs. Of this genetic variation, about 90 per cent occurs between individuals within continental populations, and only about 10 per cent occurs between individuals from different continents.
- Explain why allele frequencies for the Duffy gene may be used as a genetic marker for African ancestry. Allele frequencies for the Duffy gene differ greatly between African (and African-derived) populations and other human populations. The allele for no Duffy antigen is very high in African populations (and relatively high in African Americans) but virtually absent from non-African populations. Therefore, allele frequencies for this gene may be used as a genetic marker for African ancestry.
- Identify factors that increase the level of genetic variation within populations. Factors that tend to increase genetic variation within a population include its age and size. You would expect an older, larger population to have more genetic variation. The older a population is, the longer it has been accumulating mutations. The larger a population is, the more people there are in which mutations can occur.
- Self-marking
- Discuss genetic evidence that supports the out-of-Africa hypothesis of modern human origins. Most studies of human genetic variation find greater genetic diversity in African than in non-African populations. This is consistent with the older age of the African population proposed by the out-of-Africa hypothesis. In addition, most of the genetic variation in non-African populations is a subset of the variation in African populations. This is consistent with the idea that part of the African population left Africa and migrated to other places in the Old World.
- What evidence suggests that modern humans interbred with archaic human populations after modern humans left Africa? Recent comparisons of modern human and archaic human DNA show that interbreeding occurred between their populations to differing degrees. The comparisons reveal greater admixture with archaic humans in modern European, Asian, and Oceanic populations than in modern African populations. Populations with the greatest admixture are those in Melanesia. About eight per cent of their DNA came from archaic Denisovans in East Asia.
- How do population size reductions and gene flow impact the genetic variation of populations?
- Describe the role of genetic variation in human disease. Going through a dramatic reduction in size reduces a population's genetic variation. A high rate of migration between populations may lead to gene flow, which decreases inter-population and increases intra-population variation. Gene flow primarily between nearby populations may contribute to the formation of clines in allele frequencies.
- Explain the reasons why variation in a DNA sequence can have no effect on the fitness of an individual. Variation in a DNA sequence can have no effect on fitness for a number of reasons. First, the variation may not occur in a coding or regulatory region of DNA, and therefore would not affect phenotype. Even if it did occur in a coding region of DNA, it may not affect phenotype because it might not change the amino acid sequence of the encoded protein or it might not affect how the protein functions even if it does change the amino acid sequence. If a genetic variation does not affect the phenotype, it cannot affect fitness. Finally, even if it does affect the phenotype, it does not necessarily mean that it affects fitness — i.e., it could be a neutral phenotypic change.
- Explain why migration between populations decreases inter-population genetic variation. How does this relate to the development of clines in allele frequency? Migration between populations decreases inter-population (between population) genetic variation because when individuals move between populations, their different alleles are included in the gene pool of the population that they move to. Interbreeding will often also occur between individuals who were originally from different populations. For these reasons, there will be fewer genetic differences between these populations if individuals are moving between them. Migration relates to the development of clines (i.e. gradual differences) in allele frequency because it causes gene flow between adjacent populations. If there was no gene flow, you would expect to see discrete areas of more significant differences in allele frequency.
- The amount of mixing of modern human DNA and archaic human DNA is an example of admixture.
6.3 Classifying Human Variation: Review Questions and Answers
- Name the 18th century taxonomist that classified virtually all known living things. Carl Linnaeus
- Describe the typological approach to classifying human variation. The typological approach to classifying human variation involves creating a typology, which is a system of discrete types, or categories, such as races. People are sorted into these categories based on a few readily observable phenotypic traits, such as skin colour, hair texture, facial features, and body build.
- Discuss why typological classifications of Homo sapiens are associated with racism. Typological classifications of Homo sapiens are associated with racism because unrelated and often negative traits are attributed to certain racial categories. This may lead to prejudice and discrimination against people based only on how they look. Race and racism are deeply ingrained in our history and culture.
- Why is the breeding population considered to be the most meaningful biological group? The breeding population is considered to be the most meaningful biological group because it is the unit of evolution. It includes people who have mated and produced offspring together for many generations. As a result members of the same breeding population should share many genetic traits.
- Explain why it is generally unrealistic to apply a populational approach to classifying the human species. It is generally unrealistic to apply a populational approach to classifying the human species because most human populations are not closed breeding populations. There has been and continues to be too much gene flow between populations.
- What does a clinal map show? A clinal map shows the geographic distribution of a trait or allele frequency.
- Explain how gene flow and natural selection can result in a gradual change in the frequency of a trait over geographic space. Gene flow tends to vary with distance between populations. Closer populations are likely to exchange genes more often than populations that are farther apart. Such differences in gene flow could produce a gradual change in the frequency of a trait over geographic space. An environmental stressor may vary gradually over space, creating a geographic continuum of selective pressure. Variation in selective pressure may produce corresponding variation in a trait over space.
- Most human traits vary on a continuum. Explain why this presents a problem for the typological classification approach. Since most human traits vary on a continuum, it is difficult to create a sharp dividing line between categories of people, as is done in the typological classification approach.
- Self-marking
6.4 Human Responses to Environmental Stress: Review Questions and Answers
- List four different types of responses that humans may make to cope with environmental stress. Four different types of responses that humans may make to cope with environmental stress are adaptation, developmental adjustment, acclimatization, and cultural responses.
- Define adaptation. An adaptation is a genetically based trait that has evolved by natural selection because it helps living things survive and reproduce in a given environment.
- Self-marking
- Explain how natural selection may have resulted in most human populations having people who can and people who cannot taste PTC. PTC is an artificial, harmless, bitter-tasting compound similar to toxic bitter compounds found in plants. The ability to taste PTC may have been selected for because it helped people identify bitter-tasting toxic plants so they could avoid eating them. Nontasters are hypothesized to be able to taste a different, yet-to-be-identified bitter compound. The gene for PTC tasting has two alleles, T for tasting PTC and t for nontasting PTC (or for tasting some other bitter compound). People who have both alleles (Tt) should be able to taste the widest range of bitter compounds, so they would be the most fit and favored by natural selection. This would result in both alleles, as well as both taster and nontaster phenotypes, being maintained in populations.
- What is a developmental adjustment? A developmental adjustment is a type of nongenetic response to environmental stress. It consists of a phenotypic change that occurs during development in infancy or childhood and that may persist into adulthood. This type of change may be irreversible.
- Define phenotypic plasticity. Phenotypic plasticity is the ability to change the phenotype in response to changes in the environment, allowing individuals to respond to changes that occur during their lifetime.
- Explain why phenotypic plasticity may be particularly important in a species with a long generation time. Phenotypic plasticity may be particularly important in a species with a long generation time because in such species the evolution of genetic adaptations may occur too slowly to keep up with changing environmental stresses.
- Why may stunting of growth occur in children who have an inadequate diet? Why is stunting preferable to the alternative? Stunting of growth may occur in children who have an inadequate diet because they do not take in enough nutrients and calories to fuel both growth and basic metabolic processes. The nutrients and calories are shunted away from growth and toward maintaining life, allowing children to survive at the expense of increased body size. The alternative would be to use nutrients and calories for growth at the expense of life processes, which could possibly result in death.
- What is acclimatization? Acclimatization is the development of reversible changes to environmental stress that generally occur over a relatively short period of time. When the stress is no longer present, the acclimatized state declines, and the body returns to its normal baseline state.
- How does acclimatization to heat come about, and what are two physiological changes that occur in heat acclimatization? Acclimatization to heat occurs when one gradually works out harder and longer at high temperatures. It may take up to two weeks to attain maximum heat acclimatization. Two physiological changes that occur in heat acclimatization are increased sweat output and earlier onset of sweat production. These changes help the body lose heat through the evaporation of sweat, which is called evaporative cooling.
- Give an example of a cultural response to heat stress. An example of a cultural response to heat stress is the use of air conditioning to maintain a cool environment.
- Which is more likely to be reversible — a change due to acclimatization, or a change due to developmental adjustment? Explain your answer. A change due to acclimatization is more likely to be reversible than a change due to developmental adjustment. This is because in acclimatization, the phenotype reverts back to the baseline state once the stressor is gone. In developmental adjustment, the changes that occur during development may or may not be permanent, depending on the circumstances.
6.5 Variation in Blood Types: Review Questions and Answers
- Define blood type and blood group system. Blood type is a genetic characteristic associated with the presence or absence of antigens on the surface of the red blood cell. Blood group system refers to all of the genes, alleles, and possible genotypes and phenotypes that exist for a particular set of blood type antigens.
- Explain the relationship between antigens and antibodies. Antigens are molecules that the immune system identifies as either self or nonself. If antigens are identified as nonself, the immune system responds by forming antibodies that are specific to the nonself antigens. Antibodies are large, Y-shaped proteins produced by the immune system that recognize and bind to nonself antigens. They fit together like a lock and key. When antibodies bind to antigens, it marks them for destruction by other immune system cells.
- Identify the alleles, genotypes, and phenotypes in the ABO blood group system. The ABO blood group system is controlled by one gene with three common alleles, represented by A, B, and O. There are six possible genotypes with three alleles: AA, AB, BB, BO, AO, and OO. Because A and B are codominant and both are dominant to O, there are four possible phenotypes: type A (AA, AO), type B (BB, BO), type AB (AB), and type O (OO).
- Discuss the medical significance of the ABO blood group system. The ABO blood group system is the most important blood group system in blood transfusions. If red blood cells containing a particular ABO antigen are transfused into a person who lacks that antigen, the person’s immune system will recognize the antigen on the red blood cells as nonself and attack them, causing agglutination.
- Compare the relative worldwide frequencies of the three ABO alleles. The ABO allele for antigen B is the least common worldwide. The allele for antigen A is more common than the allele for antigen B but less common than the allele for antigen O, which is the most common ABO allele.
- Give examples of how different ABO blood types vary in their susceptibility to diseases. Answers may vary. Sample answer: People with type O blood may be more susceptible to cholera, plague, and gastrointestinal ulcers; but they may be less susceptible to malaria. People with type A blood may be more susceptible to smallpox and certain cancers.
- Describe the Rhesus blood group system. The Rhesus blood group system is controlled by two linked genes with many alleles on chromosome 1. There are five main Rhesus antigens: D, C, c, E, and e. The major antigen is D, which is either present (Rh+) or absent (Rh-).
- Relate Rhesus blood groups to blood transfusions. People with Rh+ blood can safely receive a blood transfusion of Rh+ or Rh- blood. People with Rh- blood can safely receive a blood transfusion only of Rh- blood.
- What causes hemolytic disease of the newborn? Hemolytic disease of the newborn is caused by an Rh- mother producing antibodies to the D antigen in the blood of an Rh+ fetus. The maternal antibodies may destroy fetal red blood cells, causing anemia.
- Describe how toxoplasmosis may explain the persistence of the Rh- blood type in human populations. Toxoplasmosis is a common parasitic disease that may have lasting neurological effects such as delayed reaction times, which can lead to an increase in traffic accidents and possibly other accidental injuries. People who are heterozygous for the Rhesus D antigen appear to be less likely to develop these lasting neurological effects, so they might be selected for by natural selection. If so, this could explain why both Rh+ and Rh- phenotypes persist in human populations.
- A woman is blood type O and Rh-, and her husband is blood type AB and Rh+. Answer the following questions about this couple and their offspring.
- What are the possible genotypes of their offspring in terms of ABO blood group? AO or BO
- What are the possible phenotypes of their offspring in terms of ABO blood group? A or B
- Can the woman donate blood to her husband? Explain your answer. Yes, because O is the universal donor since it has no A or B antigens, and in any case, AB is the universal recipient since it has both antigens. Also, since she is Rh-, she can donate to an Rh+ person.
- Can the man donate blood to his wife? Explain your answer. No, because he is AB which contains the antigens for both A and B, and since she is type O she has antibodies against A and B. Also, because he is Rh+ and she is Rh-, her body will create antibodies against his D antigen as well.
- Type O blood is characterized by the presence of O antigens — explain why this statement is false. This statement is false because the O allele actually codes for the absence of an antigen, which means there is no "O" allele, just the absence of an antigen.
- Explain why newborn hemolytic disease may be more likely to occur in a second pregnancy than in a first Hemolytic disease of the newborn may be more likely to occur in a second pregnancy than in a first, because the generation of anti-D antibodies usually requires exposure to Rh+ blood in an Rh- person. This exposure may happen during an Rh- mother’s first birth to an Rh+ baby, and then in a subsequent pregnancy, the fetus is at risk of HDN because the anti-D antibodies are already present.
6.6 Human Responses to High Altitude: Review Questions and Answers
- Define hypoxia. Hypoxia is a lack of oxygen.
- Why does hypoxia occur at high altitudes? Hypoxia occurs at high altitudes because the atmosphere is less dense at high altitudes, so there is less oxygen in each breath and lower air pressure to move air from the lungs across cell membranes into the blood.
- Describe the body’s immediate response to hypoxia at high altitude. The body’s immediate response to hypoxia at high altitude is an increase in the rate of breathing (hyperventilation) and elevation of the heart rate.
- Self-marking
- What is high altitude sickness, and what are its symptoms? High altitude sickness is a collection of symptoms that occur in response to the hypoxia at high altitude in a person who is not acclimated or adapted to this stress. It includes symptoms such as fatigue, shortness of breath, loss of appetite, headache, dizziness, and vomiting.
- What changes occur during acclimatization to high altitude? During acclimatization to high altitude, additional red blood cells are produced, capillaries become more numerous in muscle tissues, the lungs increase slightly in size, and there is a small increase in the size of the right ventricle of the heart, which is the heart chamber that pumps blood to the lungs.
- Where would you expect to find populations with genetic adaptations to high altitude? You would expect to find populations with genetic adaptations to high altitude in high altitude areas above 2,500 metres where people have been living continuously for thousands of years, including the Andes Mountains, Himalaya Mountains, Tibetan Plateau, and Ethiopian Highlands.
- Discuss variation in adaptations to high altitude in different high altitude regions. Different high altitude populations have evolved different adaptations to the same hypoxic stress. Tibetan highlanders, for example, have a faster rate of breathing and large arteries, whereas Peruvian highlanders have larger red blood cells and a greater concentration of the oxygen-carrying protein hemoglobin.
- Why do you think that adaptations to living at high altitude are different in different regions of the world?
- Using human responses to high altitude as an example, explain the difference between acclimatization and adaptation.
- Why are most humans not well-adapted to living at high altitudes?
- If a person that normally lives at sea level wants to climb a very high mountain, do you think it is better for them to move to higher elevations gradually or more rapidly? Explain your answer.
6.7 Human Responses to Extreme Climates: Review Questions and Answers
- Compare and contrast hypothermia and hyperthermia. Both hypothermia and hyperthermia are dangerous responses to temperature extremes. Hypothermia is a decrease in core body temperature that occurs in the cold. Hyperthermia is an in increase in core body temperature that occurs in the heat.
- State Bergmann’s and Allen’s rules. Bergmann’s rule states that populations or species have larger body size in colder climates, and vice versa. Allen’s rule states that populations or species have longer extremities in warmer environments, and vice versa.
- How do the Maasai and Inuit match the predictions based on Bergmann’s and Allen’s rules? The Maasai, who live in the tropics, have long, linear bodies with very long legs, so they have a heat-adapted body build as predicted by Bergmann’s and Allen’s rules. The Inuit, who live in the Arctic, have stocky bodies with relatively short limbs, so they have a cold-adapted body build as predicted by Bergmann’s and Allen’s rules.
- Explain how sweating cools the body. Sweating coats the skin with moisture. When the moisture evaporates, it requires heat. The heat comes from the surface of the body, which cools the body.
- What is the heat index? The heat index is a number that combines air temperature and relative humidity to indicate how hot the air feels due to the humidity.
- Relate the heat index to evaporative cooling of the body. When the heat index is high for a given air temperature, it means that the relative humidity is high. With high humidity, sweat will not evaporate readily from the body, and evaporative cooling will not be very effective.
- Identify three heat-related illnesses, from least to most serious. Three heat-related illnesses from least to most serious are heat cramps, heat exhaustion, and heat stroke.
- How does heat acclimatization occur? Heat acclimatization occurs by gradually increasing the duration and intensity of working out at high temperatures. Maximum acclimatization may take up to 14 days. Changes that occur with acclimatization include greater sweat production, decreased salt concentration in sweat, earlier onset of sweating, and vasodilation near the skin so blood can bring heat to the surface of the body from the body core.
- State two major ways the human body can respond to the cold, and give an example of each. Two major ways the body can respond to cold are by generating more heat (for example, by shivering) and by conserving more body heat (for example, by vasoconstriction).
- Explain how and why the hunting response occurs. The hunting response occurs as a response to cold. It involves alternating vasoconstriction and vasodilation in the extremities. This helps conserve body heat while preventing cold injury to the extremities.
- Define basal metabolic rate. Basal metabolic rate is the amount of energy a person needs to keep the body functioning at rest.
- How does a high-fat diet help prevent hypothermia? A high fat diet helps prevent hypothermia by increasing the basal metabolic rate so the body generates more heat.
- Explain why frostbite most commonly occurs in the extremities, such as the fingers and toes. Frostbite most commonly occurs in the extremities, such as the fingers and toes, because one of the body’s responses to cold is vasoconstriction, which moves blood away from the extremities to protect the internal organs in the body’s core. This leaves the extremities more vulnerable to cold and frostbite.
6.8 Nutritional Adaptation: Review Questions and Answers
- Self-marking
- Distinguish between the terms lactose and lactase. Lactose is a disaccharide found in milk. Lactase is an enzyme that is needed to digest lactose by breaking it down into its two component sugars.
- What is lactose intolerance, and what percentage of all people have it? Lactose intolerance is the inability to synthesize lactase and digest lactose after early childhood, leading to symptoms such as bloating and diarrhea if milk is consumed. Lactose intolerance occurs in about 60 per cent of all people.
- Where and why did lactase persistence evolve? Lactase persistence evolved in populations that herded milking animals for thousands of years. People who were able to synthesize lactase and digest lactose throughout life were strongly favored by natural selection.
- What is the thrifty gene hypothesis? The thrifty gene hypothesis posits that “thrifty genes” were selected for because they allowed people to use calories efficiently and store body fat when food was plentiful so they had a reserve to use when food was scarce. Thrifty genes become detrimental and lead to obesity and diabetes when food is plentiful all of the time.
- How well is the thrifty gene hypothesis supported by evidence? Several assumptions underlying the thrifty gene hypothesis have been called into question, and genetic research has been unable to actually identify thrifty genes.
- Describe an alternative hypothesis to the thrifty gene hypothesis. Several alternative hypotheses to the thrifty gene hypothesis have been proposed, so answers may vary. Sample answer: The drifty gene hypothesis explains variation in the tendency to become obese and develop diabetes by genetic drift on neutral genes.
- Do you think that a lack of exposure to dairy products might affect a person’s lactase level? Why or why not? Answers may vary. Sample answer: I think that a lack of exposure to dairy products might affect a person’s lactase level, because production of lactase may not just depend on genes—it also may depend on exposure to lactose.
- Describe an experiment you would want to do or data you would want to analyze that would help to test the thrifty phenotype hypothesis. Remember, you are studying people, so be sure it is ethical! Discuss possible confounding factors that you should control for, or that might affect the interpretation of your results. Answers will vary. Sample answer: To test the thrifty phenotype hypothesis, I would examine data on the rates of type II diabetes in adulthood for people whose mother was pregnant with them during times and regions of famine. Times of famine might have additional factors, such as types of food available, extreme maternal stress, or other environmental conditions that could also affect the development of diabetes, other than overall lack of food. Also, you may not be able to determine whether an individual’s mother personally experienced famine, or to what extent. It may be completely unknown or you may need to rely on self-reporting of events that happened many years ago.
- Explain the relationship between insulin, blood glucose, and type II diabetes. Diabetes is a disease that occurs when there are problems with the pancreatic hormone insulin, which normally helps cells take up glucose from the blood and controls blood glucose levels. In type II diabetes, body cells become relatively resistant to insulin, leading to high blood glucose.
Chapter 6 Case Study Conclusion: Review Questions and Answers
- Explain why an evolutionarily older population is likely to have more genetic variation than a similarly-sized younger population. The older a population is, the longer it has been accumulating mutations, so therefore an older population is likely to have more genetic variation than a similarly-sized younger population.
- The genetic difference between any two people on Earth is only about 0.1 per cent. Based on our evolutionary history, describe one reason why humans are relatively homogeneous genetically. Answers may vary, but can include: modern humans’ relatively recent evolution (less than a quarter million years ago), which is a relatively short period of time for mutations to accumulate; the relatively small human population size (possibly around 10,000 adults) in the past, which also limited genetic variability.
- What aspect(s) of human skin colour are due to adaptation? Be sure to define adaptation in your answer. What aspect(s) of human skin colour are due to acclimatization? Be sure to define acclimatization in your answer. Adaptation is a genetic change that occurs through natural selection. Adaptations that influence skin colour in humans include the type and amount of melanin produced by the skin. Acclimatization is a temporary physiological change in response to environmental stress. The ability of the skin to become darker, or tan, when exposed to UV radiation is a type of acclimatization that influences skin colour.
- For each of the following human responses to the environment, list whether it can be best described as an example of adaptation, acclimatization, or developmental adjustment:
- Reduction in height due to lack of food in childhood Developmental adjustment.
- Resistance to malaria Adaptation.
- Shivering in the cold Acclimatization.
- Changes in body size and dimensions to better tolerate heat or cold Adaptation.
- Give an example of a human response to environmental stress that involves changes in behavior, instead of changes in physiology. Answers will vary but may include: the creation of shelters, clothing, and technology such as air conditioning.
- What are two types of environmental stresses that caused genetic changes related to hemoglobin in some populations of humans? Malaria and high altitude
- The ability of an organism to change their phenotype in response to the environment is called phenotypic plasticity.
- List three natural selection pressures that differ geographically across the world and contributed to the evolution of human genetic variation in different regions. Answers may vary. Sample answer: Altitude; climate; UV levels; presence of endemic malaria.
- You may have noticed that when a sudden hot day occurs during a cool period, it can feel even more uncomfortable than higher temperatures during a hot period — even with the same humidity levels. Using what you learned in this chapter, explain why you think that happens. Answers may vary. Sample answer: I think that a sudden hot day during a cool period feels particularly uncomfortable because your body has not yet acclimated to the heat. Full heat acclimatization can take weeks. During longer periods of heat, your body acclimatizes physiologically to cool you more effectively.
- Out of all mammals, why are humans the only ones that evolved lactase persistence? Humans are the only mammals that evolved lactase persistence, because humans are the only mammals to consume milk in adulthood, due to our domestication of other species for food. It is energetically costly to produce an enzyme that is not needed, so that is probably why other mammals stop making it after the weaning period.
- If the Inuit people who live in the Arctic were not able to get enough vitamin D from their diet, what do you think might happen to their skin colour over a long period of time? Explain your answer. Answers may vary. Sample answer: I think that if the Inuit people were not able to get enough vitamin D from their diet, over a long period of time their skin colour may become lighter. This is because vitamin D, which is important for health, can be synthesized by the skin from UV light exposure. UV light penetrates lighter skin better than darker skin, so people with lighter skin will produce vitamin D more easily. In the absence of sufficient vitamin D in the diet in the Arctic where UV levels are low, people with lighter skin may have an evolutionary advantage due to better health. Over a long period of time, that may lead to the population as a whole having lighter skin.
- Explain why malaria has been such a strong force of natural selection on human populations. Answers may vary. Sample answer: Malaria has been such a strong force of natural selection on human populations for several reasons. First, it is widespread in areas consistently inhabited by large numbers of humans, particularly after the advent of agriculture, because of the nature of malaria life cycle. Second, malaria has been around for a long period of human history, and natural selection causes evolutionary changes only over many generations. Third, malaria is often deadly, particularly to young children and infants, and can cause miscarriages and stillbirths. Because it affects the reproductive rate in this manner, malaria is a strong force of natural selection, dramatically reducing the fitness of individuals that are susceptible to it.
- Give one example of “heterozygote advantage” (i.e. when the heterozygous genotype has higher relative fitness than the dominant or recessive homozygous genotype) in humans. Answers will vary but may include: hemoglobin adaptations in response to malaria; the Rhesus D antigen; the taster/nontaster alleles.
- What is one way in which humans have evolved genetic adaptations in response to their food sources? Answers will vary but may include: lactose persistence; taster genes; the ability to survive on lower amounts of food.
- Do you think adaptation to high altitude evolved once or several times? Explain your reasoning. Answers may vary. Sample answer: Adaptations to high altitude probably evolved independently several times because the specific adaptations are different in different regions. If it had evolved once, you would expect the adaptation to be the same in different populations.
Created by CK-12/Adapted by Christine Miller
Why Are Humans Such Sweaty Animals?
Combine exercise and a hot day, and you get sweat — and lots of it. Sweating is one of the adaptations humans have evolved to maintain homeostasis, or a constant internal environment. When sweat evaporates from the skin, it uses up some of the excess heat energy on the skin, thus helping to reduce the body's temperature. Humans are among the sweatiest of all species, with a fine-tuned ability to maintain a steady internal temperature, even at very high outside temperatures.
Unifying Principles of Biology
All living things have mechanisms for homeostasis. Homeostasis is one of four basic principles or theories that explain the structure and function of all species (including our own). Whether biologists are interested in ancient life, the life of bacteria, or how humans could live on Mars, they base their understanding of biology on these unifying principles:
- Cell theory
- Gene theory
- Homeostasis
- Evolutionary theory
Cell Theory
According to cell theory, all living things are made of cells, and living cells come only from other living cells. Each living thing begins life as a single cell. Some living things, including bacteria, remain single-celled. Other living things, including plants and animals, grow and develop into many cells. Your own body is made up of an amazing 100 trillion cells. But even you — like all other living things — began life as a single cell.
Watch this TED-Ed video about the origin of cell theory:
https://www.youtube.com/watch?v=4OpBylwH9DU
The Wacky History of Cell Theory - Lauren Royal-Woods, TED-Ed, 2012
Gene Theory
Gene theory is the idea that the characteristics of living things are controlled by genes, which are passed from parents to their offspring. Genes are located on larger structures called chromosomes. Chromosomes are found inside every cell, and they consist of molecules of DNA (deoxyribonucleic acid). Those molecules of DNA are encoded with instructions that "tell" cells how to behave.
Homeostasis
Homeostasis, or the condition in which a system is maintained in a more-or-less steady state, is a characteristic of individual living things, like the human ability to sweat. Homeostasis also applies to the entire biosphere, wherever life is found on Earth. Consider the concentration of oxygen in Earth's atmosphere. Oxygen makes up 21 per cent of the atmosphere, and this concentration is fairly constant. What maintains this homeostasis in the atmosphere? The answer is living things.
Most living things need oxygen to survive, so they remove oxygen from the air. On the other hand, many living things, including plants, give off oxygen when they convert carbon dioxide and water to food in the process of photosynthesis. These two processes balance out so the air maintains a constant level of oxygen.
Evolutionary Theory
Evolution is a change in the characteristics of populations of living things over time. Evolution can occur by a process called natural selection, which results from random genetic mutations in a population. If these mutations lead to changes that allow the living things to better survive, then their chances of surviving and reproducing in a given environment increase. They will then pass more genes to the next generation. Over many generations, this can lead to major changes in the characteristics of those living things. Evolution explains how living things are changing today, as well as how modern living things descended from ancient life forms that no longer exist on Earth.
Traits that help living things survive and reproduce in a given environment are called adaptations. You can see an obvious adaptation in the image below. The chameleon is famous for its ability to change its colour to match its background as camouflage. Using camouflage, the chameleon can hide in plain sight.
Feature: Myth vs. Reality
Misconceptions about evolution are common. They include the following myths:
Myth |
Reality |
| "Evolution is "just" a theory or educated guess." | Scientists accept evolutionary theory as the best explanation for the diversity of life on Earth because of the large body of scientific evidence supporting it. Like any scientific theory, evolution is a broad, evidence-supported explanation for multiple phenomena. |
| "The theory of evolution explains how life on Earth began." | The theory of evolution explains how life changed on Earth after it began. |
| "The theory of evolution means that humans evolved from apes like those in zoos." | Humans and modern apes both evolved from a common ape-like ancestor millions of years ago. |
2.3 Summary
- Four basic principles or theories unify all fields of biology: cell theory, gene theory, homeostasis, and evolutionary theory.
- According to cell theory, all living things are made of cells and come from other living cells.
- Gene theory states that the characteristics of living things are controlled by genes that pass from parents to offspring.
- All living things strive to maintain internal balance, or homeostasis.
- The characteristics of populations of living things change over time through the process of micro-evolution as organisms acquire adaptations, or traits that better suit them to a given environment.
Use the flashcards below to review the four principles:
2.3 Review Questions
-
- How does sweating help the human body maintain homeostasis?
- Explain cell theory and gene theory.
- Describe an example of homeostasis in the atmosphere.
- Describe how you can apply the concepts of evolution,natural selection, adaptation, and homeostasis to the human ability to sweat.
- Which of the four unifying principles of biology is primarily concerned with:
- how DNA is passed down to offspring?
- how internal balance is maintained?
- _____________ are located on ______________.
- chromosomes; genes
- genes;chromosomes
- genes; traits
- none of the above
- Define an adaptation and give one example.
- Explain how gene theory and evolutionary theory relate to each other.
- Does evolution by natural selection occur within one generation? Why or why not?
- Explain why you think chameleons evolved the ability to change their colour to match their background, as well as how natural selection may have acted on the ancestors of chameleons to produce this adaptation.
2.3 Explore More
https://www.youtube.com/watch?v=Wg5DBH6uMCw&feature=emb_logo
Myths and misconceptions about evolution - Alex Gendler, TEDEd, 2013
Attributions
Figure 2.3.1
Photo(perspiration), by Hans Reniers on Unsplash. is used under the Unsplash license (https://unsplash.com/license).
Figure 2.3.2
Mediterranean Chameleon Reptile Lizard, by user:1588877 on Pixabay, is used under the Pixabay license (https://pixabay.com/de/service/license/).
References
TED-Ed. (2012, June 4). The wacky history of cell theory - Lauren Royal-Woods. YouTube. https://www.youtube.com/watch?v=4OpBylwH9DU&feature=youtu.be
TED-Ed. (2013, July 8). Myths and misconceptions about evolution - Alex Gendler. YouTube. https://www.youtube.com/watch?v=mZt1Gn0R22Q&t=10s
Created by: CK-12/Adapted by Christine Miller
After reading this chapter, you should be able to see numerous connections between chemistry, human life, and health. In Joseph’s situation, chemistry is involved in the reasons why his father has diabetes, why his personal risk of getting diabetes is high, and why the dietary changes he is considering could be effective.
Type 2 diabetes affects populations worldwide and is caused primarily by a lack of response in the body to the hormone insulin, which causes problems in the regulation of blood sugar, or glucose. Insulin is a peptide hormone, and as you have learned, peptides are chains of amino acids. Therefore, insulin is in the class of biochemical compounds called proteins. Joseph is at increased risk of diabetes partly because there is a genetic component to the disease. DNA, which is a type of chemical compound called a nucleic acid, is passed down from parents to their offspring, and carries the instructions for the production of proteins in units called genes. If there is a problem in a gene (or genes) that contributes to the development of a disease, such as type 2 diabetes, this can get passed down to the offspring and may raise that child’s risk of getting the disease.
But genetics is only part of the reason why Joseph is at an increased risk of diabetes. Obesity itself is a risk factor, and one that can be shared in families due to shared lifestyle factors (such as poor diet and lack of exercise), as well as genetics. Consumption of too many refined carbohydrates (like white bread and soda) may also contribute to obesity and the development of diabetes. As you probably now know, these simple carbohydrates are more easily and quickly broken down in the digestive system into glucose than larger complex carbohydrate molecules, such as those found in vegetables and whole grains. This can lead to dramatic spikes in blood sugar levels, which is particularly problematic for people with diabetes because they have trouble maintaining their blood sugar at a safe level. You can understand why Joseph’s father limits his consumption of refined carbohydrates, and in fact, some scientific studies have shown that avoiding refined carbohydrates may actually help reduce the risk of getting diabetes in the first place.
Joseph’s friend recommended eating a low fat, high carbohydrate diet to lose weight, but you can see that the type of carbohydrate — simple or complex — is an important consideration. Eating a large amount of white bread and rice may not help Joseph reduce his risk of diabetes, but a healthy diet that helps him lose weight may lower his risk of diabetes, since obesity itself is a factor. Which specific diet will work best to help him lose weight probably depends on a variety of factors, including his biology, lifestyle, and food preferences. Joseph should consult with his doctor about his diet and exercise plan, so that his specific situation can be taken into account and monitored by a medical professional.
Drinking enough water is usually good advice for everyone, especially if it replaces sugary drinks like soda. You now know that water is important for many of the chemical reactions that take place in the body. But you can have too much of a good thing — as in the case of marathon runners who can make themselves sick from drinking too much water! As you can see, proper balance, or homeostasis, is very important to the health of living organisms.
Finally, you probably now realize that “chemicals” do not have to be scary, toxic substances. All matter consists of chemicals, including water, your body, and healthy fresh fruits and vegetables, like the ones pictured in Figure 3.12.2. When people advocate “clean eating” and avoiding “chemicals” in food, they are usually referring to avoiding synthetic — or man-made — chemical additives, such as preservatives. This can be a healthy way to eat because it involves eating a variety of whole, fresh, unprocessed foods. But there is no reason to be scared of chemicals in general — they are simply molecules and how they react depends on what they are, what other molecules are present, and the environmental conditions surrounding them.
Chapter 3 Summary
By now, you should have a good understanding of the basics of the chemistry of life. Specifically, you have learned:
- All matter consists of chemical substances. A chemical substance has a definite and consistent composition and may be either an element or a compound.
- An element is a pure substance that cannot be broken down into other types of substances.
- An atom is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative electrons, positive protons, and neutral neutrons. The number of protons in an atom determines the element it represents.
- Atoms have equal numbers of electrons and protons, so they have no charge. Ions are atoms that have lost or gained electrons, so they have either a positive or negative charge. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
- There are almost 120 known elements. The majority of elements are metals. A smaller number are nonmetals, including carbon, hydrogen, and oxygen.
- A compound is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a molecule. Chemical bonds hold together the atoms of molecules. Compounds can form only in chemical reactions, and they can break down only in other chemical reactions.
- Biochemical compounds are carbon-based compounds found in living things. They make up cells and other structures of organisms and carry out life processes. Most biochemical compounds are large molecules called polymers that consist of many repeating units of smaller molecules called monomers.
- There are millions of different biochemical compounds, but all of them fall into four major classes: carbohydrates, lipids, proteins, and nucleic acids.
- Carbohydrates are the most common class of biochemical compounds. They provide cells with energy, store energy, and make up organic structures, such as the cell walls of plants. The basic building block of carbohydrates is the monosaccharide.
- Sugars are short-chain carbohydrates that supply us with energy. Simple sugars, such as glucose, consist of just one monosaccharide. Some sugars, such as sucrose (or table sugar) consist of two monosaccharides and are called disaccharides.
- Complex carbohydrates, or polysaccharides, consist of hundreds or even thousands of monosaccharides. They include starch, glycogen, cellulose, and chitin.
- Starch is made by plants to store energy and is readily broken down into its component sugars during digestion.
- Glycogen is made by animals and fungi to store energy and plays a critical part in the homeostasis of blood glucose levels in humans.
- Cellulose is the most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Humans cannot digest cellulose, but it makes up most of the crucial dietary fibre in the human diet.
- Chitin makes up organic structures, such as the cell walls of fungi and the exoskeletons of insects and other arthropods.
- Lipids include fats and oils. They store energy, form cell membranes, and carry messages.
- Lipid molecules consist mainly of repeating units called fatty acids. Fatty acids may be saturated or unsaturated, depending on the proportion of hydrogen atoms they contain. Animals store fat as saturated fatty acids, while plants store fat as unsaturated fatty acids.
- Types of lipids include triglycerides, phospholipids, and steroids.
- Triglycerides contain glycerol (an alcohol) in addition to fatty acids. Humans and other animals store fat as triglycerides in fat cells.
- Phospholipids contain phosphate and glycerol in addition to fatty acids. They are the main component of cell membranes in all living things.
- Steroids are lipids with a four-ring structure. Some steroids, such as cholesterol, are important components of cell membranes. Many other steroids are hormones.
- In living things, proteins include enzymes, antibodies, and numerous other important compounds. They help cells keep their shape, make up muscles, speed up chemical reactions, and carry messages and materials (among other functions).
- Proteins are made up of small monomer molecules called amino acids.
- Long chains of amino acids form polypeptides. The sequence of amino acids in polypeptides makes up the primary structure of proteins. Secondary structure refers to configurations such as helices and sheets within polypeptide chains. Tertiary structure is a protein's overall three-dimensional shape, which controls the molecule's basic function. A quaternary structure forms if multiple protein molecules join together and function as a complex.
- The chief characteristic that allows proteins' diverse functions is their ability to bind specifically and tightly with other molecules.
- Nucleic acids include DNA and RNA. They encode instructions for making proteins, helping make proteins, and passing the encoded instructions from parents to offspring.
- Nucleic acids are built of monomers called nucleotides, which bind together in long chains to form polynucleotides. DNA consists of two polynucleotides, and RNA consists of one polynucleotide.
- Each nucleotide consists of a sugar molecule, phosphate group, and nitrogen base. Sugars and phosphate groups of adjacent nucleotides bind together to form the "backbone" of the polynucleotide. Bonds between complementary bases hold together the two polynucleotide chains of DNA and cause it to take on its characteristic double helix shape.
- DNA makes up genes, and the sequence of nitrogen bases in DNA makes up the genetic code for the synthesis of proteins. RNA helps synthesize proteins in cells. The genetic code in DNA is also passed from parents to offspring during reproduction, explaining how inherited characteristics are passed from one generation to the next.
- A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms in a chemical reaction is called a product. During the chemical reaction, bonds break in reactants and new bonds form in products.
- Chemical reactions can be represented by chemical equations. According to the law of conservation of mass, mass is always conserved in a chemical reaction, so a chemical equation must be balanced, with the same number of atoms of each type of element in the products as in the reactants.
- Many chemical reactions occur all around us each day, such as iron rusting and organic matter rotting, but not all changes are chemical processes. Some changes, such as ice melting or paper being torn into smaller pieces, are physical processes that do not involve chemical reactions and the formation of new substances.
- All chemical reactions involve energy, and they require activation energy to begin. Exothermic reactions release energy. Endothermic reactions absorb energy.
- Biochemical reactions are chemical reactions that take place inside living things. The sum of all the biochemical reactions in an organism is called metabolism. Metabolism includes catabolic reactions (exothermic reactions) and anabolic reactions (endothermic reactions).
- Most biochemical reactions require a biological catalyst called an enzyme to speed up the reaction by reducing the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, called the enzyme's substrate.
- Virtually all living things on Earth require liquid water. Only a tiny per cent of Earth's water is fresh liquid water. Water exists as a liquid over a wide range of temperatures, and it dissolves many substances. These properties depend on water's polarity, which causes water molecules to "stick" together through weak bonds called hydrogen bonds.
- The human body is about 70 per cent water (outside of fat). Organisms need water to dissolve many substances and for most biochemical processes, including photosynthesis and cellular respiration.
- A solution is a mixture of two or more substances that has the same composition throughout. Many solutions consist of water and one or more dissolved substances.
- Acidity is a measure of the hydronium ion concentration in a solution. Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the pH scale. Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have a lower hydronium ion concentration than pure water and a pH higher than 7.
- Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly.
Now you understand the chemistry of the molecules that make up living things. In the next chapter, you will learn how these molecules make up the basic unit of structure and function in living organisms — cells — and you will be able to understand some of the crucial chemical reactions that occur within cells.
Chapter 3 Review
-
- The chemical formula for the complex carbohydrate glycogen is C24H42O21.
- What are the elements in glycogen?
- How many atoms are in one molecule of glycogen?
- Is glycogen an ion? Why or why not?
- Is glycogen a monosaccharide or a polysaccharide? Besides memorizing this fact, how would you know this based on the information in the question?
- What is the function of glycogen in the human body?
- What is the difference between an ion and a polar molecule? Give an example of each in your explanation.
- Define monomer and polymer.
-
- What is the difference between a protein and a polypeptide?
-
- People with diabetes have trouble controlling the level of glucose in their bloodstream. Knowing this, why do you think it is often recommended that people with diabetes limit their consumption of carbohydrates?
- Identify each of the following reactions as endothermic or exothermic.
- cellular respiration
- photosynthesis
- catabolic reactions
- anabolic reactions
- Pepsin is an enzyme in the stomach that helps us digest protein. Answer the following questions about pepsin:
- What is the substrate for pepsin?
- How does pepsin work to speed up protein digestion?
- Given what you know about the structure of proteins, what do you think are some of the products of the reaction that pepsin catalyzes?
- The stomach is normally acidic. What do you think would happen to the activity of pepsin and protein digestion if the pH is raised significantly?
Attributions
Figure 3.13.1
Prevalence_of_Diabetes_by_Percent_of_Country_Population_(2014)_Gradient_Map by Walter Scott Wilkens [Wwilken2], University of Illinois - Urbana Champaign Department of Geography and GIScience, on Wikimedia Commons, is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.
Figure 3.13.2
Healthy plate by Melinda Young Stuart on Flickr is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/) license.
5.2 Chromosomes and Genes: Review Questions and Answers
- What are chromosomes and genes? How are the two related? Chromosomes are coiled structures made of DNA and proteins that form during cell division and are encoded with genetic instructions for making RNA and proteins. These instructions are organized into units called genes, which are segments of DNA that code for particular pieces of RNA. The RNA molecules can then act as a blueprint for proteins, or directly help regulate various cellular processes. There may be hundreds or even thousands of genes on a single chromosome.
- Describe human chromosomes and genes. Most human cells contain 23 pairs of chromosomes, for a total of 46 chromosomes. One set of chromosomes is inherited from each parent. Of the 23 pairs of chromosomes, 22 pairs are autosomes, which control traits unrelated to sex, and the remaining pair consists of sex chromosomes (XX or XY). Human chromosomes contain a total of 20,000 to 22,000 genes, the majority of which have two more possible versions, called alleles.
- Explain the difference between autosomes and sex chromosomes. Autosomes are chromosomes that contain genes unrelated to sex. They are the same in males and females. Sex chromosomes differ in males and females. Normal males have the chromosomes XY and females the chromosomes XX. Genes on the X chromosome are not related to sex. Only genes on the Y chromosome play a role in determining an individual's sex.
- What are linked genes, and what does a linkage map show? Linked genes are genes that are located on the same chromosome. A linkage map shows the location of specific genes on a chromosome.
- Explain why females are considered the default sex in humans. Females are considered the default sex in humans because only genes on the Y chromosome determine sex and trigger the development of the embryo into a male. Without a Y chromosome, an embryo will develop as female.
- Explain the relationship between genes and alleles. Alleles are different versions of the same gene.
- Most males and females have two sex chromosomes. Why do only females have Barr bodies? Only females usually have Barr bodies because Barr bodies refer to an inactivated X chromosome. This X chromosome is inactivated because cells should only have one functioning X chromosome. Since females have two X chromosomes, they need a Barr body, but since males are XY and only have one X chromosome, they do not have a Barr body.
- Self-marking
- Self-marking
5.3 DNA: Review Questions and Answers
- Outline the discoveries that led to the determination that DNA (not protein) is the biochemical molecule that contains genetic information. The first discovery that led to the determination that DNA is the biochemical molecule that contains genetic information was made in the 1920s, when Frederick Griffith showed that something in virulent bacteria could be transferred to nonvirulent bacteria and make them virulent as well. In the early 1940s, Oswald Avery and colleagues showed that the "something" Griffith found in his research was DNA and not protein. This result was confirmed by Alfred Hershey and Martha Chase who demonstrated that viruses insert DNA into bacterial cells so the cells will make copies of the viruses.
- State Chargaff's rules. Explain how the rules are related to the structure of the DNA molecule. Chargaff's rules state that, within the DNA of any given species, the concentration of adenine is always the same as the concentration of thymine, and the concentration of guanine is always the same as the concentration of cytosine. Bonds between nitrogen bases hold together the two polynucleotide chains of DNA. Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If two-ring adenine, for example, were to bond with two-ring guanine as well as with one-ring thymine, the distance between the two chains would be variable. However, when two-ring adenine bonds only with one-ring thymine, and two-ring guanine bonds only with one-ring cytosine, the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix and explains how Chargaff's rules are related to DNA's structure.
- Explain how the structure of a DNA molecule is like a spiral staircase. Which parts of the staircase represent the various parts of the molecule? The DNA molecule has a double-helix structure, which is similar to the structure of a spiral staircase. The sugar-phosphate backbones of the two polynucleotide chains of DNA are like the two outside edges, or sides, of the spiral staircase. The bonded nitrogen bases are like the steps.
- Self-marking
- Why do you think dead S-strain bacteria injected into mice did not harm the mice, but killed them when mixed with living (and normally harmless) R-strain bacteria? Answers may vary. Sample answer. The DNA from the S strain bacteria was what was making the R strain bacteria harmful. It appears that the S strain DNA requires living bacteria (such as the R bacteria) to be harmful to a host organism. Therefore, it could not hurt the mice when injected alone.
- In Griffith’s experiment, do you think the heat treatment that killed the bacteria also inactivated the bacterial DNA? Why or why not? No, because after heat treatment, the DNA from the S strain bacteria was able to make the R strain bacteria, which is normally harmless, deadly. So the DNA was still causing the same effects after the heat treatment, and therefore seemed to be functioning normally.
- Give one example of the specific evidence that helped rule out proteins as genetic material. Answers may vary, but may include evidence from Avery’s or Hershey and Chase’s experiments. Sample answer: When proteins were inactivated, the dead S strain bacteria were still able to cause the normally harmless R strain bacteria to become deadly. Therefore, proteins were not the genetic material being passed to the R strain bacteria.
5.4 DNA Replication: Review Questions and Answers
- Self-marking
- Why are Okazaki fragments formed? Because DNA polymerase only replicates DNA in one direction.
- Self-marking
5.5 RNA: Review Questions and Answers
- State the central dogma of molecular biology. The central dogma of molecular biology states that the genetic instructions encoded in DNA are first transcribed to RNA and then translated from RNA into a protein.
- Self-marking
- Self-marking
- Self-marking
5.6 Genetic Code: Review Questions and Answers
- Describe the genetic code and explain how it is "read". The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA (A, G, C, and T or U). The four bases make up the "letters" of the code. The letters are combined in groups of three to form "words" called codons. There are 64 possible codons, and each codon codes for one amino acid or for a start or stop signal. The codon AUG is the start codon that establishes the reading frame of the code. After the AUG start codon, the next three bases are read as the second codon. The next three bases after that are read as the third codon, and so on. The sequence of bases is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons.
- Identify three important characteristics of the genetic code. The genetic code is universal, which means that the same code is found in all living things, providing evidence of common evolutionary origins of all organisms. The genetic code is unambiguous. This means that each codon codes for just one amino acid (or for start or stop). As a result, there is no mistaking which amino acid is encoded by a given codon. The genetic code is also redundant. This means that each amino acid is encoded by more than one codon. This helps prevent errors in protein synthesis because an accidental change in a single base often has no effect on which amino acid the codon encodes.
- Summarize how the genetic code was deciphered. The genetic code was deciphered by a series of ingenious experiments carried out mainly by Marshall Nirenberg, along with his colleague Heinrich Matthaei. These researchers added contents of bacterial cells to 20 test tubes. This was done to provide the "machinery" needed to synthesize proteins. They also added all 20 amino acids to the test tubes, with a different amino acid "tagged" by a radioactive element in each test tube. Then they added synthetic RNA containing just one type of base to each test tube, starting with the base uracil. They discovered that an RNA molecule consisting only of uracil bases produces a polypeptide chain of the amino acid phenylalanine. The researchers used similar experiments to determine that each codon consists of three bases and eventually to discover the codons for all 20 amino acids.
- Use the decoder above to answer the following questions:
- Is the code depicted in the table from DNA or RNA? How do you know? RNA, because RNA has U (uracil) as a base instead of T (thymine) which is found in DNA. This code shows only U, no T, so therefore it represents the code of RNA not DNA.
- Which amino acid does the codon CAA code for? Glutamine.
- What does UGA code for? UGA is a stop codon, so it causes the code to stop being read.
- Look at the codons that code for the amino acid glycine. How many of them are there and how are they similar and different? There are 4 codons for the amino acid glycine. They all start with GG, but what is different in each of them is their final base, which may be U, C, A, or G. (Note: this is a common pattern in the redundancy of the genetic code).
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5.7 Protein Synthesis: Review Questions and Answers
- Relate protein synthesis and its two major phases to the central dogma of molecular biology. The way proteins are synthesized in cells is summed up by the central dogma of molecular biology: DNA → RNA → Protein. The first phase of protein synthesis, called transcription, is the DNA → RNA part of the central dogma. The second phase of protein synthesis, called translation, is the RNA → Protein part of the central dogma.
- Explain how mRNA is processed before it leaves the nucleus. Before it leaves the nucleus, mRNA may be processed in several ways, including splicing, editing, and polyadenylation. Splicing removes introns (noncoding regions) from mRNA. Editing changes some of the nucleotides in mRNA, which allows different versions of proteins to be synthesized. Polyadenylation adds adenine bases to the mRNA, which serves several functions, such as helping mRNA leave the nucleus and protecting mRNA from enzymes that might break it down.
- What additional processes might a polypeptide chain undergo after it is synthesized? After a polypeptide chain is synthesized, it may assume a folded shape due to interactions among its amino acids. It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates.
- Where does transcription take place in eukaryotes? Transcription takes place in the nucleus of eukaryotic cells.
- Where does translation take place? Translation takes place at ribosomes, which are in the cytoplasm of a cell.
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5.8 Mutations: Review Questions and Answers
- Define mutation. Mutation is a random change in the sequence of bases in DNA or RNA.
- Identify causes of mutation. Mutations may occur spontaneously when errors occur during DNA replication or during the transcription of DNA during protein synthesis. Other mutations are caused by mutagens. Mutagens are environmental factors that cause mutations. They include radiation, certain chemicals, and some infectious agents.
- Compare and contrast germline and somatic mutations. Germline mutations occur in gametes and may be passed on to offspring. Every cell in the body of the offspring will then carry the mutation. Somatic mutations occur in other cells of the body than gametes. They are confined to a single cell and its daughter cells, and they cannot be passed on to offspring. They are likely to have little or no effect on the organism in which they occur.
- Describe chromosomal alterations, point mutations, and frameshift mutations. Identify the potential effects of each type of mutation. Chromosomal alterations are mutations that cause major changes in the structure of chromosomes. They are very serious and often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. Point mutations are changes in a single nucleotide. Their effects depend on how they change the genetic code and may range from no effects to serious effects. Frameshift mutations change the reading frame of the genetic code. They are likely to have drastic effects on the encoded protein.
- Why do many mutations have neutral effects? Many mutations are neutral in their effects because they do not change the amino acids in the proteins they encode or because they are repaired before protein synthesis occurs.
- Give one example of a beneficial mutation and one example of a harmful mutation. Answers will vary. Sample answer: An example of a beneficial mutation is a mutation that is found in people in a small Italian town that protects from atherosclerosis. An example of a harmful mutation is the mutation that causes the genetic disorder cystic fibrosis.
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- Why do you think that exposure to mutagens (such as cigarette smoke) can cause cancer? Mutagens are things in the environment that can cause mutations. Mutations in genes that control the cell cycle can cause cancer. Therefore, mutagens can cause cancer by causing mutations in these genes.
- Explain why the insertion or deletion of a single nucleotide can cause a frameshift mutation. Because the genetic code is read in sets of three nucleotides (a set of three is a codon), adding or removing a single nucleotide throws the whole reading frame off by changing which three nucleotides make up each codon. All of the codons after the insertion or deletion will be changed because of this, resulting in what is known as a frameshift mutation.
- Compare and contrast missense and nonsense mutations. Missense and nonsense mutations are both point mutations, where a single nucleotide is changed. The difference is that missense mutations cause an amino acid to be changed, while nonsense mutations cause a premature stop codon to be produced.
- Explain why mutations are important to evolution. Mutations are important for evolution because they are the source of new genetic variation. This variation can lead to organisms being more or less well adapted to their environments, which, over time, leads to evolutionary changes through natural selection.
5.9 Regulation of Gene Expression: Review Questions and Answers
- Define gene expression. Gene expression means using a gene to make a protein.
- Why must gene expression be regulated? Gene expression must be regulated so that the correct proteins are made where and when they are needed. This is necessary, for example, so that different types of cells have different shapes and other traits that suit them for their particular functions.
- Explain how regulatory proteins may activate or repress transcription. Regulatory proteins regulate the transcription phase of protein synthesis by binding to regions of DNA called regulatory elements, which are located near promoters. Regulatory proteins typically either activate or repress transcription. Activators promote transcription by enhancing the interaction of RNA polymerase with the promoter, thus initiating transcription of DNA to mRNA. Repressors prevent transcription by impeding the progress of RNA polymerase along the DNA strand so the DNA cannot be transcribed to mRNA.
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- What is the TATA box, and how does it work? The TATA box is a regulatory element that is part of the promoter of almost every eukaryotic gene. A number of regulatory proteins bind to the TATA box, forming a multi-protein complex. It is only when all of the appropriate proteins are bound to the TATA box that RNA polymerase recognizes the complex and binds to the promoter so transcription can begin.
- Describe homeobox genes and their role in an organism's development. Homeobox genes are a large group of similar genes that direct the formation of many body structures during the embryonic stage. In humans, homeobox genes code for chains of 60 amino acids called homeodomains. Proteins containing homeodomains are transcription factors that bind to and control the activities of other genes. They turn on certain genes in particular cells at just the right time so the individual develops normal organs and organ systems.
- Discuss the role of regulatory gene mutations in cancer. Mutations in regulatory genes that normally control the cell cycle can lead to certain types of cancer. Cancer-causing mutations most often occur in two types of regulatory genes, called proto-oncogenes genes and tumor-suppressor genes. Proto-oncogenes normally help cells divide. When a proto-oncogene mutates to become an oncogene, it is expressed continuously, so the cell keeps dividing out of control, which can lead to cancer. Tumor-suppressor genes normally slow down or stop cell division. When a tumor-suppressor gene mutates, cell division can't be slowed or stopped. The cell keeps dividing out of control, which can lead to cancer.
- Explain the relationship between proto-oncogenes and oncogenes. Proto-oncogenes are genes that normally help cells divide. An oncogene is a mutated form of a proto-oncogene that causes the gene to be expressed continuously. This can cause cancer.
- If a newly fertilized egg contained a mutation in a homeobox gene, how do you think this would affect the developing embryo? Explain your answer. Answers may vary. Sample answer: Because homeobox genes are important for the development of body structures in the embryo, including the development of organs and organ systems, I think that a mutation in one of these genes might cause the embryo to be significantly malformed. This might even result in death.
- Compare and contrast enhancers and activators. Enhancers and activators both promote gene expression. However, enhancers are distant regions of DNA and activators are proteins that bind to regulatory elements on the DNA, near the promoter region of the gene.
5.10 Mendel's Experiments and Laws of Inheritance: Review Questions and Answers
- Why were pea plants a good choice for Mendel's experiments? Pea plants were a good choice for Mendel's experiments because they are fast growing and easy to raise. They also have several visible characteristics that vary, such as seed form, flower colour, and stem length.
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- How did the outcome of Mendel's second set of experiments lead to his second law? In Mendel's second set of experiments, he investigated two characteristics at a time. For example, he crossed plants with yellow round seeds and plants with green wrinkled seeds. The plants in the F1 generation were all alike and had the same combination of characteristics (yellow round seeds) like one of the parents, whereas the plants in the F2 generation showed all possible combinations of the two characteristics, such as greenround seeds and yellow wrinkled seeds. This outcome showed that the factors controlling different characteristics are inherited independently of each other.
- Discuss the development of Mendel's legacy. During Mendel's lifetime, his work was largely ignored. It was only after Mendel's results were obtained by other researchers in 1900 that his work was rediscovered and he was given the credit he was due. Now, Mendel is considered the father of genetics for his experiments and the laws he derived from them.
- If Mendel’s law of independent assortment was not correct, and characteristics were always inherited together, what types of offspring do you think would have been produced by crossing plants with yellow round seeds and green wrinkled seeds? Explain your answer. There would be only offspring with yellow round seeds and green wrinkled seeds because those characteristics would always be inherited together. The other combinations would not have been observed.
5.11 Genetics of Inheritance: Review Questions and Answers
- Define genetics. Genetics is the science of heredity.
- Why is Gregor Mendel called the father of genetics if genes were not discovered until after his death? Gregor Mendel is called the father of genetics because his laws of inheritance form the basis of the science of heredity. Mendel thought some type of "factors" control traits and are passed on to offspring, and his laws describe how the factors and the traits they control are inherited. We now call Mendel's factors genes, and his laws of inheritance are now expressed in genetic terms.
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- Imagine that there are two alleles, R and r, for a given gene. R is dominant to r. Answer the following questions about this gene:
- What are the possible homozygous and heterozygous genotypes? The homozygous genotypes would be RR and rr, and the heterozygous genotype would be Rr.
- Which genotype or genotypes express the dominant R phenotype? Explain your answer. RR and Rr would express the dominant R phenotype because only one dominant allele (in this case, R) is needed to express the dominant phenotype.
- Are R and r on different loci? Why or why not? R and r cannot be on different loci because they are alleles of the same gene.
- Can R and r be on the same exact chromosome? Why or why not? If not, where are they located? No, because there is only one version of a gene on a single chromosome. They are on homologous chromosomes.
5.12 Sexual Reproduction, Meiosis, and Gametogenesis: Review Questions and Answers
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- Explain how sexual reproduction happens at the cellular level. At the cellular level, sexual reproduction occurs when two parents produce reproductive cells called gametes that unite to form offspring. Gametes are haploid cells and when they unite in the process of fertilization, they produce a diploid cell called a zygote.
- Summarize what happens during Meiosis. During meiosis, homologous chromosomes separate and the cell undergoes two cell divisions to form four haploid daughter cells. Each daughter cell has just one chromosome from each homologous pair. The two cell divisions that occur during meiosis are called meiosis I and meiosis II, and each of them occurs in four phases: prophase, metaphase, anaphase, and telophase.
- Compare and contrast gametogenesis in males and females. Gametogenesis is the process in which haploid daughter cells from meiosis change to become mature gametes. Gametes produced by males are called sperm, and they mature during a process known as spermatogenesis. Gametes produced by females are called eggs, and they mature during a process known as oogenesis.
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction. Mechanisms that increase genetic variation in offspring produced by sexual reproduction include crossing-over, independent assortment, and the random union of gametes. Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes that may occur during meiosis. It results in new combinations of genes on each chromosome. Independent assortment refers to the way in which different chromosomes are distributed randomly to daughter cells during meiosis. It results in gametes that have unique combinations of chromosomes. Which two of the millions of possible gametes that are produced by two parents actually unite to produce an offspring is likely to be a matter of chance and is another major source of genetic variation in offspring.
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid? Gametes need to be haploid (i.e. half the number of chromosomes) because during fertilization, two of them join together to make a diploid (i.e. the usual number of chromosomes) zygote. If gametes were diploid, the resulting zygote would have twice the normal amount of chromosomes, which would be problematic.
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis. Answers may vary. Sample answer: In prophase I of meiosis, homologous chromosomes pair up, which does not occur in prophase of mitosis.
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not? This would be highly unlikely, because homologous chromosomes segregate independently from each other into daughter cells during meiosis. So the chances of all 23 of the chromosomes that you got from your mother going into one of your gametes is very low.
5.13 Mendelian Inheritance: Review Questions and Answers
- Define genetic traits and Mendelian inheritance. Genetic traits are characteristics that are encoded in DNA. Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other.
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- Explain why autosomal and X-linked Mendelian traits have different patterns of inheritance. Autosomal Mendelian traits do not differ between males and females. They are inherited in the same way regardless of the sex of the parent or offspring. For example, a dominant autosomal trait will show up in anyone who inherits even one copy of the dominant allele, whereas the recessive trait will show up only in people who inherit two copies of the recessive allele. X-linked Mendelian traits, in contrast, have a different pattern of inheritance than autosomal Mendelian traits because males have just one X chromosome, which they always inherit from their mother and pass on to all of their daughters but none of their sons. A recessive X-linked trait, for example, will show up in males who inherit just one copy of the recessive allele, whereas females must inherit two copies of the recessive allele (one on each of their two X chromosomes) to express the recessive trait.
- Identify examples of human autosomal and X-linked Mendelian traits. Answers may vary. Sample answer: Examples of human autosomal Mendelian traits include albinism and Huntington's disease. Examples of human X-linked Mendelian traits include red-green colour blindness and hemophilia.
- Imagine a hypothetical human gene that has two alleles,Q and q. Q is dominant to q and the inheritance of this gene is Mendelian. Answer the following questions about this gene.
- If a woman has the genotype Q q and her husband has the genotype QQ, list each of their possible gametes. What proportion of their gametes will have each allele? The woman’s gametes will be 50%Q and 50% q. Her husband’s gametes will be 100% Q.
- What are the likely proportions of their offspring being QQ, Qq, or qq? Their offspring have a 50% chance of being QQ, a 50% chance of being Qq, and a zero per cent chance of being qq. (Hint: if you are having trouble figuring this out, make a Punnett square).
- Is this an autosomal trait or an X-linked trait? How do you know? This must be an autosomal trait because the man has two alleles for the gene (i.e. he isQQ). X-linked traits have only one copy of the gene in males because males have only one X chromosome.
- What are the chances of their offspring exhibiting the dominant Q trait? Explain your answer. Their offspring will all have the dominant Q trait, because their genotypes will either be QQ or Qq, and only one copy of the dominant Q allele is needed to express the dominant trait.
- Explain why fathers always pass their X chromosome down to their daughters. Fathers pass their single X chromosome down to their daughters because women have two X chromosomes and therefore receive one X chromosome from each parent.
5.14 Non-Mendelian Inheritance: Review Questions and Answers
- What is non-Mendelian inheritance? Non-Mendelian inheritance is the inheritance of traits that have a more complex genetic basis than one gene with two alleles and complete dominance.
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- Explain why the human ABO blood group is an example of a multiple allele trait with codominance. The human ABO blood group is an example of a multiple allele trait because the gene for ABO blood type has more than two commonly occurring alleles: IA, IB, and i. ABO blood group is an example of codominance because the IAand IB alleles are codominant to one another. As a result, heterozygotes who inherit one copy of each allele produce both A and B antigens, giving them type AB blood.
- What is incomplete dominance? Give an example of this type of non-Mendelian inheritance in humans. Incomplete dominance is the case in which the dominant allele for a gene is not completely dominant to a recessive allele for the gene, so an intermediate phenotype occurs in heterozygotes who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional enzyme as normal homozygotes.
- Explain the genetic basis of human skin colour. Human skin colour is a polygenic trait. It is controlled by several different genes, each with more than one allele. The alleles of each gene have a minor additive effect on the phenotype, producing a whole continuum of possible phenotypes and gradations of skin colour.
- How can the human trait of adult height be influenced by the environment? The human trait of adult height is a polygenic trait, and the environment may affect the phenotypic expression of the trait. For example, if a child's growth is negatively affected by poor nutrition or illness, the child may grow up to be shorter in stature than would otherwise be the case given the child's genes for height.
- Define pleiotropy, and give a human example. Pleiotropy is the situation in which one gene has multiple phenotypic effects. Examples may vary. Sample answer: A human example of pleiotropy involves the gene that codes for the main protein in collagen, a substance that helps form bones and is also important in the ears and eyes. Mutations in the gene result in problems not only in bones but also in these sensory organs, which is how the gene's pleiotropic effects were discovered.
- Compare and contrast epistasis and dominance. Epistasis is the case in which a gene affects the expression of other genes. For example, a mutation in one gene may not allow other genes to be expressed in the phenotype. This occurs with albinism, for example. Dominance is the case in which one allele masks the expression of another allele for the same gene. Epistasis is similar to dominance, except that it occurs between different genes rather than between different alleles for the same gene.
- What is the difference between pleiotropy and epistasis? Pleiotropy is when one gene affects more than one phenotypic trait. Epistasis is when one gene affects the expression of other genes.
5.15 Genetic Disorders: Review Questions and Answers
- Define genetic disorder. A genetic disorder is a disease, syndrome, or other abnormal condition that is caused by gene mutation(s) or by chromosomal alterations.
- Identify three genetic disorders caused by mutations in a single gene. Answers may vary. Sample answer: Three genetic disorders caused by mutations in a single gene are Marfan syndrome (autosomal dominant), sickle cell anemia (autosomal recessive), and hemophilia A (X-linked recessive).
- Why are single-gene genetic disorders more commonly controlled by recessive than dominant mutant alleles? Single-gene genetic disorders are more commonly controlled by recessive than dominant mutant alleles because a dominant allele is always expressed. If it causes a serious genetic disorder, individuals who inherit even one copy of the allele may not live long enough to reproduce and pass on the allele to offspring. As a result, the allele is likely to die out of the population. A recessive mutant allele, in contrast, is not expressed in people who inherit just one copy of it. They carry the mutant allele and their offspring can inherit it. Thus, a recessive mutant allele is more likely than a dominant mutant allele to pass on to the next generation rather than die out.
- What is nondisjunction? Why can it cause genetic disorders? Nondisjunction is the failure of replicated chromosomes to separate properly during meiosis. Some of the resulting gametes will be missing all or part of a chromosome, while others will have an extra copy of all or part of the chromosome. If such a gamete is fertilized and forms a new individual, the individual is likely to have a serious genetic disorder.
- Explain why genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome. Genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome because the X and Y chromosomes are very different in size, making nondisjunction more frequent for the sex chromosomes.
- How is Down syndrome detected in utero? One way of detecting Down syndrome in utero is to extract a few fetal cells from the fluid surrounding the fetus and examine the fetal chromosomes. If an extra copy of chromosome 21 is present, the fetus has Down syndrome.
- Use the example of PKU to illustrate how the symptoms of a genetic disorder can sometimes be prevented. PKU is a genetic disorder in which the individual lacks a normal enzyme needed to break down the amino acid phenylalanine, which builds up in the body and causes the symptoms of PKU. If a low-phenylalanine diet is followed throughout life, the symptoms of PKU can be prevented.
- Explain how gene therapy works. Gene therapy works by inserting a normal gene in cells with a mutant gene, so the protein encoded by the gene can be synthesized in cells. A vector, such as a virus, is genetically engineered to deliver the normal gene by infecting cells. If the treatment is successful, the new gene delivered by the vector will allow the synthesis of a functioning protein.
- Compare and contrast genetic disorders and congenital disorders. Genetic disorders and congenital disorders both can be present at birth, but genetic disorders are specifically caused by problems in genes or chromosomes, while congenital disorders may be due to any cause.
- Explain why parents that do not have Down syndrome can have a child with Down syndrome. Answers may vary. Sample answer: Down syndrome is caused by a mistake during meiosis that produces gametes with an extra copy (complete or partial) of chromosome 21. It is only the parent’s gamete or gametes that are affected. If a gamete with this chromosomal abnormality goes on to create a zygote, the child that results will have Down syndrome.
- Hemophilia A and Turner’s syndrome both involve problems with the X chromosome. In terms of how the X chromosome is affected, what is the major difference between these two types of disorders? Answers may vary. Sample answer: Hemophilia A is a single gene mutation on the X chromosome, while Turner’s syndrome involves the loss of an entire X chromosome (XO).
- Can you be a carrier of Marfan syndrome and not have the disorder? Explain your answer. No, because Marfan syndrome is dominant. Even one copy of the gene will cause the disorder. Carriers refer to people with one copy of a recessive gene.
5.16 Genetic Engineering: Review Questions and Answers
Review Questions
- Define genetic engineering Genetic engineering is the use of technology to change the genetic makeup of living things for human purposes.
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- What is recombinant DNA? Recombinant DNA is DNA that is formed by combining DNA from two different species of organisms.
- Identify the steps of gene cloning. The steps of gene cloning are isolation, ligation, transformation, and selection. During isolation, a gene is isolated by using an enzyme to break DNA. During ligation, another enzyme is used to combine the isolated gene with plasmid DNA from bacteria, producing recombinant DNA. In transformation, the recombinant DNA is inserted into another cell, usually a bacterial cell. During selection, transformed bacteria are grown to make sure they have the recombinant DNA and only those that do are selected for further use.
- What is the purpose of the polymerase chain reaction? The purpose of the polymerase chain reaction is to make many copies of a gene or other DNA segment. This might be done in order to have large quantities of the gene for genetic testing.
- Make a flow chart outlining the steps involved in creating a transgenic crop. Flow charts may vary but should include the following steps in creating a transgenic crop: a. Plasmid DNA is obtained from bacteria that infect plants. b. Recombinant DNA is created by combining a desired gene with the plasmid DNA from the bacteria. c. The recombinant DNA is re-inserted into a bacterium. d. The transformed bacterium is used to insert the recombinant DNA into the chromosome of a plant cell. e. The plant cell is grown in culture. f. A plant cell clone from the culture is used to generate a plant with the desired gene.
- Explain how bacteria can be genetically engineered to produce a human protein. To genetically engineer bacteria to produce a human protein, gene cloning is used to form recombinant DNA that contains the normal human gene for the protein and plasmid DNA from bacteria. The recombinant DNA is re-inserted into the bacteria. The bacteria can multiply rapidly and produce large amounts of the human protein.
- Identify an ethical, legal, or social issue raised bygenetic engineering. State your view on the issue, and develop a logical argument to support your view. Answers may vary but should identify an ethical, legal, or social issue raised by genetic engineering; a clearly stated view on the issue; and a logical argument to support the view. Sample topics might include health, safety, privacy, and environmental issues.
- Explain what primers are and what they do in PCR. Primers are short pieces of DNA that have a complementary base sequence to a DNA strand that is being used to make copies of a gene. Primers bind to the DNA strand during the annealing stage of PCR. Then an enzyme adds nucleotides to the primer to make new DNA molecules, which contain copies of the gene.
- The enzyme Taq polymerase was originally identified from bacteria that live in very hot environments, such as hotsprings. Why does this fact make Taq polymerase particularly useful in PCR reactions? Taq polymerase is particularly useful for PCR reactions because it can function in the hot temperatures necessary for PCR, due to the fact that it comes from bacteria that live in extremely hot environments.
5.17 The Human Genome: Review Questions and Answers
Review Questions
- Describe the human genome. The human genome refers to all the DNA of the human species. It consists of 3.3 billion base pairs divided into 20,500 genes on 23 pairs of chromosomes.
- What is the Human Genome Project? The Human Genome Project is a multi-billion dollar, international biological research project that began in 1990, continued to 2003, and involved researchers at 20 universities in several different countries.
- Identify two main goals of the Human Genome Project. Two main goals of the Human Genome Project were to sequence all of the DNA base pairs in the human genome, and to map the location and determine the function of all the genes in the human genome.
- What is the reference genome of the Human Genome Project? What is it based on? The reference genome of the Human Genome Project is the sequence of DNA base pairs in a complete set of human chromosomes. It is based on a combined mosaic of a small number of anonymous donors, all of European origin.
- Explain how knowing the sequence of DNA bases in the human genome is beneficial for molecular medicine. Knowing the sequence of DNA bases in the human genome is beneficial for molecular medicine because it is helping researchers identify mutations linked to different forms of cancer, yielding insights into the genetic basis of many diseases, such as cystic fibrosis, and helping researchers tailor medications to individual genotypes.
- What was one surprising finding of the Human Genome Project? Answers may vary. Sample answer: One surprising finding of the HGP was the relatively small number of genes in humans.
- Why do you think scientists didn’t just sequence the DNA from a single person for the Human Genome Project? Along those lines, why do you think it is important to include samples from different ethnic groups and genders in genome sequencing efforts? Answers may vary. Sample answer: Although all humans share the same basic genes, there is some variation in the specific sequences between individuals. If only one person was sequenced, that sequence would not necessarily be a good representative of the human species as a whole. That is why scientists sequenced several individuals and came up with a composite reference sequence. This is also why different ethnic groups and genders should be included in genome sequencing efforts, because the range of human variation should be represented to better reflect the genome of the human species as a whole.
- What is pharmacogenomics? Pharmacogenomics is the study of how an individual’s genes affect the way they respond to drugs.
- If a patient were to have pharmacogenomics done to optimize their medication, what do you think the first step would be? Answers may vary.Sample answer: I think the first step would be for the patient to have a test to find out the sequence of a gene or genes in their body that could affect how the medication is activated or deactivated.
- List one advantage and one disadvantage of pharmacogenomics. Answers may vary.Sample answer: One advantage of pharmacogenomics is that doctors might be able to find the most effective medication for a specific patient more quickly. One disadvantage is that this technique is currently often not covered by insurance and can be expensive.
- Explain how the sequencing of the human genome relates to ethical concerns about genetic discrimination. Answers may vary.Sample answer: By sequencing the human genome, genes associated with certain diseases can be discovered. This can lead to ethical concerns about potential discrimination against individuals with these genetic sequences, for instance by insurance companies or employers, who have a vested interest in having healthy clients or employees.
Chapter 5 Case Study Conclusion: Review Questions and Answers
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- What are the differences between a sequence of DNA and the sequence of mature mRNA that it produces? Answers may vary. Sample answer: Directly after transcription, an RNA sequence is complementary to the DNA sequence that it is transcribed from, but RNA contains uracil (U) instead of the thymine (T) base that is used in DNA. Then the pre-mRNA is spliced to remove introns, possibly edited, and a “tail” of adenines is added through polyadenylation. Therefore, the mature mRNA sequence is significantly different than simply being the complementary sequence to the DNA sequence.
- Scientists sometimes sequence DNA that they “reverse transcribe” from the mRNA in an organism’s cells, which is called complementary DNA (cDNA). Why do you think this technique might be particularly useful for understanding an organism’s proteins versus sequencing the whole genome (i.e. nuclear DNA) of the organism? Answers may vary. Sample answer: I think this technique might be useful because the mRNA only contains the exons that code for amino acids and, ultimately, proteins. Nuclear DNA contains a lot of regions that do not code for proteins. Therefore, you might be able to gain insight into the proteins that an organism produces more quickly if you sequence the cDNA made from mRNA, rather than starting with the nuclear DNA of the entire genome.
- A person has a hypothetical Aa genotype. Answer the following questions about this genotype:
- What do A and a represent? A and a are different alleles of the same gene.
- If the person expresses only the phenotype associated with A, is this an example of complete dominance, codominance, or incomplete dominance? Explain your answer. Also, describe what the observed phenotypes would be if it were either of the two incorrect answers. This is an example of complete dominance because A completely dominates the phenotype over a. If it were codominance, the phenotypes for A and a would both be expressed. If it were incomplete dominance, you might see an intermediate phenotype that is between the phenotypes for A and a.
- Explain how a mutation that occurs in a parent can result in a genetic disorder in their child. Be sure to include which type of cell or cells in the parent must be affected in order for this to happen. Answers may vary. Sample answer: A gene mutation in a parent’s gametes, otherwise known as a germline mutation, can be passed down to their offspring. If this mutation results in a protein that does not function normally, it can cause a genetic disorder in the child.
- What is the term for an allele that is not expressed in a heterozygote? A recessive allele.
- What might happen if codons encoded for more than one amino acid? Answers may vary. Sample answer. If codons encoded for more than one amino acid, tRNA would bring various amino acids to the ribosome for each codon, resulting in varied proteins. These may have different functions and be detrimental to the organism.
- Explain why a human gene can be inserted into bacteria and can still produce the correct human protein, despite being in a very different organism. A human gene inserted into bacteria still produces the same human protein because the genetic code is universal, meaning that it is the same among all living organisms.
- What is gene therapy? Why is gene therapy considered a type of biotechnology? Gene therapy is an experimental technique to treat genetic disorders. In gene therapy, a normal gene is inserted into human cells to compensate for an abnormally functioning gene. This is often done using viruses as vectors to carry and insert the new DNA. Gene therapy is a type of genetic engineering because it involves changing the genetic makeup of an organism.
Created by: CK-12/Adapted by Christine Miller
So Many Cells!
This baby girl (Figure 4.12.1) has a lot of growing to do before she's as big as her mom. Most of her growth will be the result of cell division. By the time she is an adult, her body will consist of trillions of cells. Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells, causing them to grow out of control. In fact, this is how cancer cells cause illness. In this concept, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.
The Cell Cycle
Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division. The cell cycle in prokaryotes is quite simple: the cell grows, its DNA replicates, and the cell divides. In eukaryotes, the cell cycle is more complicated.
Eukaryotic Cell Cycle
The diagram in Figure 4.12.2 represents the cell cycle of a eukaryotic cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.
Interphase
The interphase of the eukaryotic cell cycle can be subdivided into the three phases described below, which are represented in Figure 4.12.2.
- Growth Phase 1 (G1): During this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes proteins needed for DNA replication and copies some of its organelles in preparation for cell division. A cell typically spends most of its life in this phase. This phase is also known as gap phase 1.
- Synthesis Phase (S): During this phase, the cell’s DNA is copied in the process of DNA replication, in order to prepare for the upcoming mitotic phase.
- Growth Phase 2 (G2): During this phase, the cell makes final preparations to divide. For example, it makes additional proteins and organelles. This phase is also known as gap phase 2.
Control of the Cell Cycle
If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure 4.12.3. There are a number of main checkpoints.
Checkpoints in the eukaryotic cell cycle ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.
- The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.
- The S checkpoint determines if the DNA has been replicated properly.
- The mitosis checkpoint ensures that all the chromosomes are properly aligned before the cell is allowed to divide.
Cancer and the Cell Cycle
Cancer is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell’s DNA becomes damaged. Damage can occur due to exposure to hazards, such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. which may end up forming a mass of abnormal cells called a tumor (see Figure 4.12.4). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death.
Cell Division
Cell division is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic or eukaryotic. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no nucleus, and few other organelles. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus and many other organelles. All of these cell parts must be duplicated and separated when the cell divides.
Before a eukaryotic cell divides, all of the DNA in the cell’s multiple chromosomes is replicated. Its organelles are also duplicated. Cell division occurs in two major steps, called mitosis and cytokinesis, both of which are described in greater detail in Chapter 5.
- The first step in the division of a eukaryotic cell is mitosis, a multi-phase process in which the nucleus of the cell divides. During mitosis, the nuclear envelope (membrane) breaks down and later reforms. The chromosomes are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.
- The second major step is cytokinesis. This step, which also occurs in prokaryotic cells, is when the cytoplasm divides, forming two daughter cells.
Feature: Human Biology in the News
Henrietta Lacks sought treatment for her cancer at Johns Hopkins University Hospital at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks's doctor, Howard Jones, took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, George Gey, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing... and dividing and dividing and dividing. Copies of Henrietta's cells — called HeLa cells, for her name (Henrietta Lacks) — are still alive today. In fact, there are currently billions of HeLa cells in laboratories around the world!
Why Henrietta's cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. Jonas Salk, for example, used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.
You would think that Henrietta's name would be well known in medical history for her unparalleled contributions to biomedical research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot published a nonfiction book, The Immortal Life of Henrietta Lacks. Based on a decade of research, this riveting account became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.
Ironically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. There is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.
If you want to learn more about Henrietta Lacks and her immortal cells, read Rebecca Skloot's The Immortal Life of Henrietta Lacks (or watch the movie, if it is available). You can also watch the short video below about Henrietta Lacks and her immortal cells by Robin Bulleri:
https://www.youtube.com/watch?v=22lGbAVWhro
The immortal cells of Henrietta Lacks - Robin Bulleri, TED-Ed, 2016.
4.12 Summary
- The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division. The cycle is more complicated in eukaryotic than prokaryotic cells.
- In a eukaryotic cell, the cell cycle has two major phases: mitotic phase and interphase. During mitotic phase, first the nucleus and then the cytoplasm divide. During interphase, the cell grows, performs routine life processes, and prepares to divide.
- The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. There are a number of main checkpoints in the regulation of the cell cycle.
- Cancer is a disease that occurs when the cell cycle is no longer regulated, often because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.
- The cell division phase of the cell cycle in a eukaryotic cell occurs in two major steps: mitosis — when the nucleus divides — and cytokinesis, when the cytoplasm divides and two daughter cells form.
4.12 Review Questions
-
- Explain why cell division is more complex in eukaryotic than prokaryotic cells.
- Using a technique called flow cytometry, scientists can distinguish between cells with the normal amount of DNA and those that contain twice the normal amount of DNA as they go through the cell cycle. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.
- What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal?
4.12 Explore More
https://www.youtube.com/watch?v=QVCjdNxJreE
The Cell Cycle (and cancer) [Updated], The Amoeba Sisters, 2018.
Attributions
Figure 4.12.1
Mom and baby by Taiying Lu on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 4.12.2
Cell Cycle by LadyofHats; CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under 
• Terms of Use • Attribution
Figure 4.12.3
Cell Cycle Checkpoints by LadyofHats; CK-12 Foundation is used and adapted by Christine Miller under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 4.12.4
Cancer cells forming a tumour by Ed Uthman, MD on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.12.5
Henrietta Lacks by Oregon State University on Flickr is used under a CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/) license.
References
Amoeba Sisters. (2018, March 20). The cell cycle (and cancer) [Updated]. YouTube. https://www.youtube.com/watch?v=QVCjdNxJreE&feature=youtu.be
TED-Ed. (2016, February 8). The immortal cells of Henrietta Lacks - Robin Bulleri. YouTube. https://www.youtube.com/watch?v=22lGbAVWhro&feature=youtu.be
Wikipedia contributors. (2020, June 23). Henrietta Lacks. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Henrietta_Lacks&oldid=964020268
Wikipedia contributors. (2020, May 11). Howard W. Jones. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Howard_W._Jones&oldid=956033806
Wikipedia contributors. (2020, July 1). George Otto Gey. In Wikipedia. https://en.wikipedia.org/w/index.php?title=George_Otto_Gey&oldid=965394045
Wikipedia contributors. (2020, July 6). Johns Hopkins Hospital. In ,Wikipedia. https://en.wikipedia.org/w/index.php?title=Johns_Hopkins_Hospital&oldid=966348552
Wikipedia contributors. (2020, June 28). Jonas Salk. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Jonas_Salk&oldid=964883129
Wikipedia contributors. (2020, April 14). Rebecca Skloot. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Rebecca_Skloot&oldid=950837115
Wikipedia contributors. (2020, February 21). The immortal life of Henrietta Lacks. In Wikipedia. https://en.wikipedia.org/w/index.php?title=The_Immortal_Life_of_Henrietta_Lacks&oldid=941942679
Created by: CK-12/Adapted by Christine Miller
Divide and Split
Can you guess what the colourful image in Figure 4.13.1 represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the cell in a part of cell division called anaphase, where the DNA is being pulled to opposite ends of the cell. Normally, DNA is located in the nucleus of most human cells. The nucleus divides before the cell itself splits in two, and before the nucleus divides, the cell’s DNA is replicated (or copied). There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To answer that question, you first need to know more about DNA and the forms it takes.
The Forms of DNA
Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only once a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown below.
Most cells in the human body have two pairs of 23 different chromosomes, for a total of 46 chromosomes. Cells that have two pairs of chromosomes are called diploid. Because DNA has already replicated when it coils into a chromosome, each chromosome actually consists of two identical structures called sister chromatids. Sister chromatids are joined together at a region called a centromere.
Mitosis
The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. This is shown in the figure below.
Mitosis actually occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase.
Prophase
The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells. They help ensure that the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The spindle consists of fibres made of microtubules.
Metaphase
During metaphase, spindle fibres attach to the centromere of each pair of sister chromatids. As you can see in Figure 4.13.7, the sister chromatids line up at the equator (or center) of the cell. The spindle fibres ensure that sister chromatids will separate and go to different daughter cells when the cell divides.
Anaphase
During anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibres. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole. At the end of anaphase, each pole of the cell has a complete set of chromosomes.
Telophase
During telophase, the chromosomes begin to uncoil and form chromatin. This prepares the genetic material for directing the metabolic activities of the new cells. The spindle also breaks down, and new nuclear envelopes form.
Cytokinesis
Cytokinesis is the final stage of cell division. During cytokinesis, the cytoplasm splits in two and the cell divides, as shown below. In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. Thus, the goal of mitosis and cytokinesis is now complete, because one parent cell has given rise to two daughter cells. The daughter cells have the same chromosomes as the parent cell.
4.13 Summary
- Until a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each chromosome actually consists of two sister chromatids, which are joined together at a centromere.
- Mitosis is the process during which the nucleus of a eukaryotic cell divides. During this process, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases: prophase, metaphase, anaphase, and telophase.
- Cytokinesis is the final stage of cell division, during which the cytoplasm splits in two and two daughter cells form.
4.13 Review Questions
- Describe the different forms that DNA takes before and during cell division in a eukaryotic cell.
-
- Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase.
- Order the diagrams of the stages of mitosis:
- Explain what happens during cytokinesis in an animal cell.
- What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis?
- True or False:
4.13 Explore More
https://www.youtube.com/watch?time_continue=3&v=C6hn3sA0ip0&feature=emb_logo
Mitosis, NDSU Virtual Cell Animations project (ndsuvirtualcell), 2012.
https://www.youtube.com/watch?time_continue=19&v=EA0qxhR2oOk&feature=emb_logo
Nondisjunction (Trisomy 21) - An Animated Tutorial, Kristen Koprowski, 2012.
Attributions
Figure 4.13.1
Anaphase_IF by Roy van Heesbeen on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.2
Chromosomes by OpenClipArt-Vectors on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
Figure 4.13.3
Chromosome/ Chromatid/ Sister Chromatid by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.4
Simple Mitosis by Mariana Ruiz Villarreal [LadyofHats] via CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 4.13.5
Mitotic Prophase [tiny] by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.6
Prophase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.7
Mitotic_Metaphase by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.8
Metaphase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.9
Anaphase [adapted] by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.10
Anaphase_eukaryotic_mitosis.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.11
Mitotic Telophase by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.12
Telophase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.13
Mitotic Cytokinesis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.13.14
Cytokinesis Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Koprowski, K., Cabey, R. [Kristen Koprowski]. (2012). Nondisjunction (Trisomy 21) - An Animated Tutorial. YouTube. https://www.youtube.com/watch?v=EA0qxhR2oOk&feature=youtu.be
NDSU Virtual Cell Animations project [ndsuvirtualcell]. (2012). Mitosis. YouTube. https://www.youtube.com/watch?v=C6hn3sA0ip0&t=21s
Created by CK12/Adapted by Christine Miller
Jasmin discovered that her extreme fatigue, muscle pain, vision problems, and vomiting were due to problems in her mitochondria, like the damaged mitochondria shown in red in Figure 4.14.1. Mitochondria are small, membrane-bound organelles found in eukaryotic cells that provide energy for the cells of the body. They do this by carrying out the final two steps of aerobic cellular respiration: the Krebs cycle and electron transport. This is the major way that the human body breaks down the sugar glucose from food into a form of energy cells can use, namely the molecule ATP.
Because mitochondria provide energy for cells, you can understand why Jasmin was experiencing extreme fatigue, particularly after running. Her damaged mitochondria could not keep up with her need for energy, particularly after intense exercise, which requires a lot of additional energy. What is perhaps not so obvious are the reasons for her other symptoms, such as blurry vision, muscle spasms, and vomiting. All of the cells in the body require energy in order to function properly. Mitochondrial diseases can cause problems in mitochondria in any cell of the body, including muscle cells and cells of the nervous system, which includes the brain and nerves. The nervous system and muscles work together to control vision and digestive system functions, such as vomiting, so when they are not functioning properly, a variety of symptoms can emerge. This also explains why Jasmin’s niece, who has a similar mitochondrial disease, has symptoms related to brain function, such as seizures and learning disabilities. Our cells are microscopic, and mitochondria are even tinier — but they are essential for the proper functioning of our bodies. When they are damaged, serious health effects can occur.
One seemingly confusing aspect of mitochondrial diseases is that the type of symptoms, severity of symptoms, and age of onset can vary wildly between people — even within the same family! In Jasmin’s case, she did not notice symptoms until adulthood, while her niece had more severe symptoms starting at a much younger age. This makes sense when you know more about how mitochondrial diseases work.
Inherited mitochondrial diseases can be due to damage in either the DNA in the nucleus of cells or in the DNA in the mitochondria themselves. Recall that mitochondria are thought to have evolved from prokaryotic organisms that were once free-living, but were then infected or engulfed by larger cells. One of the pieces of evidence that supports this endosymbiotic theory is that mitochondria have their own, separate DNA. When the mitochondrial DNA is damaged (or mutated) it can result in some types of mitochondrial diseases. However, these mutations do not typically affect all of the mitochondria in a cell. During cell division, organelles such as mitochondria are replicated and passed down to the new daughter cells. If some of the mitochondria are damaged, and others are not, the daughter cells can have different amounts of damaged mitochondria. This helps explain the wide range of symptoms in people with mitochondrial diseases — even ones in the same family — because different cells in their bodies are affected in varying degrees. Jasmin’s niece was affected strongly and her symptoms were noticed early, while Jasmin’s symptoms were more mild and did not become apparent until adulthood.
There is still much more that needs to be discovered about the different types of mitochondrial diseases. But by learning about cells, their organelles, how they obtain energy, and how they divide, you should now have a better understanding of the biology behind these diseases.
Apply your understanding of cells to your own life. Can you think of other diseases that affect cellular structures or functions. Do they affect people you know? Since your entire body is made of cells, when cells are damaged or not functioning properly, it can cause a wide variety of health problems.
Chapter 4 Summary
Type your learning objectives here.
In this chapter you learned many facts about cells. Specifically, you learned that:
- Cells are the basic units of structure and function of living things.
- The first cells were observed from cork by Hooke in the 1600s. Soon after, van Leeuwenhoek observed other living cells.
- In the early 1800s, Schwann and Schleiden theorized that cells are the basic building blocks of all living things. Around 1850, Virchow saw cells dividing, and added his own theory that living cells arise only from other living cells. These ideas led to cell theory, which states that all organisms are made of cells, all life functions occur in cells, and all cells come from other cells.
- The invention of the electron microscope in the 1950s allowed scientists to see organelles and other structures inside cells for the first time.
- There is variation in cells, but all cells have a plasma membrane, cytoplasm, ribosomes, and DNA.
-
- The plasma membrane is composed mainly of a bilayer of phospholipid molecules and forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell. Some cells have extensions of their plasma membrane with other functions, such as flagella or cilia.
- Cytoplasm is a thick solution that fills a cell and is enclosed by the plasma membrane. It helps give the cell shape, holds organelles, and provides a site for many of the biochemical reactions inside the cell. The liquid part of the cytoplasm is called cytosol.
- Ribosomes are small structures where proteins are made.
- Cells are usually very small, so they have a large enough surface area-to-volume ratio to maintain normal cell processes. Cells with different functions often have different shapes.
- Prokaryotic cells do not have a nucleus. Eukaryotic cells have a nucleus, as well as other organelles. An organelle is a structure within the cytoplasm of a cell that is enclosed within a membrane and performs a specific job.
- The cytoskeleton is a highly organized framework of protein filaments and tubules that criss-cross the cytoplasm of a cell. It gives the cell shape and helps to hold cell structures (such as organelles) in place.
- The nucleus is the largest organelle in a eukaryotic cell. It is considered to be the cell's control center, and it contains DNA and controls gene expression, including which proteins the cell makes.
- The mitochondrion is an organelle that makes energy available to cells. According to the widely accepted endosymbiotic theory, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
- The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum (RER) is studded with ribosomes. Smooth endoplasmic reticulum (SER) has no ribosomes.
- The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
- Vesicles and vacuoles are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are vesicles that break down foreign matter, dead cells, or poisons.
- Centrioles are organelles located near the nucleus that help organize the chromosomes before cell division so each daughter cell receives the correct number of chromosomes.
- There are two basic ways that substances can cross the cell’s plasma membrane: passive transport (which requires no energy expenditure by the cell) and active transport (which requires energy).
- No energy is needed from the cell for passive transport because it occurs when substances move naturally from an area of higher concentration to an area of lower concentration. Types of passive transport in cells include:
-
- Simple diffusion, which is the movement of a substance due to differences in concentration without any help from other molecules. This is how very small, hydrophobic molecules, such as oxygen and carbon dioxide, enter and leave the cell.
- Osmosis, which is the diffusion of water molecules across the membrane.
- Facilitated diffusion, which is the movement of a substance across a membrane due to differences in concentration, but only with the help of transport proteins in the membrane (such as channel proteins or carrier proteins). This is how large or hydrophilic molecules and charged ions enter and leave the cell.
- Active transport requires energy to move substances across the plasma membrane, often because the substances are moving from an area of lower concentration to an area of higher concentration or because of their large size. Two examples of active transport are the sodium-potassium pump and vesicle transport.
-
- The sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, both against a concentration gradient, in order to maintain the proper concentrations of both ions inside and outside the cell and to thereby control membrane potential.
- Vesicle transport uses vesicles to move large molecules into or out of cells.
- Energy is the ability to do work. It is needed by every living cell to carry out life processes.
- The form of energy that living things need is chemical energy, and it comes from food. Food consists of organic molecules that store energy in their chemical bonds.
- Autotrophs (producers) make their own food. Think of plants that make food by photosynthesis. Heterotrophs (consumers) obtain food by eating other organisms.
- Organisms mainly use the molecules glucose and ATP for energy. Glucose is the compact, stable form of energy that is carried in the blood and taken up by cells. ATP contains less energy and is used to power cell processes.
- The flow of energy through living things begins with photosynthesis, which creates glucose. The cells of organisms break down glucose and make ATP.
- Cellular respiration is the aerobic process by which living cells break down glucose molecules, release energy, and form molecules of ATP. Overall, this three-stage process involves glucose and oxygen reacting to form carbon dioxide and water.
-
- Glycolysis, the first stage of cellular respiration, takes place in the cytoplasm. In this step, enzymes split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.
- Transition Reaction takes place between glycolysis and Krebs Cycle. It is a very short reaction in which the pyruvate molecules from glycolysis are converted into Acetyl CoA in order to enter the Krebs Cycle.
- Krebs Cycle, the second stage of cellular respiration, takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their chemical bonds being stored in a total of 16 energy-carrying molecules (including four from glycolysis).
- The Electron Transport System, he third stage of cellular respiration, takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions across the membrane, creating an electrochemical gradient that drives the synthesis of many more molecules of ATP.
- In all three stages of aerobic cellular respiration combined, as many as 38 molecules of ATP are produced from just one molecule of glucose.
- Some organisms can produce ATP from glucose by anaerobic respiration, which does not require oxygen. Fermentation is an important type of anaerobic process. There are two types: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis.
-
- Alcoholic fermentation is carried out by single-celled organisms, including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
- Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
- Anaerobic respiration produces far less ATP (typically produces 2 ATP) than does aerobic cellular respiration, but it has the advantage of being much faster.
- The cell cycle is a repeating series of events that includes growth, DNA synthesis, and cell division.
- In a eukaryotic cell, the cell cycle has two major phases: interphase and mitotic phase. During interphase, the cell grows, performs routine life processes, and prepares to divide. During mitotic phase, first the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis), which produces two daughter cells.
-
- Until a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each chromosome consists of two sister chromatids, which are joined together at a centromere.
- During mitosis, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases: prophase, metaphase, anaphase, and telophase.
- The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle at key checkpoints.
- Cancer is a disease that occurs when the cell cycle is no longer regulated, often because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.
In this chapter, you learned about cells and some of their functions, as well as how they pass genetic material in the form of DNA to their daughter cells. In the next chapter, you will learn how DNA is passed down to offspring, which causes traits to be inherited. These traits may be innocuous (such as eye colour) or detrimental (such as mutations that cause disease). The study of how genes are passed down to offspring is called genetics, and as you will learn in the next chapter, this is an interesting topic that is highly relevant to human health.
Chapter 4 Review
- Sequence:
- Drag and Drop:
- True or False:
- Multiple Choice:
- Briefly explain how the energy in the food you eat gets there, and how it provides energy for your neurons in the form necessary to power this process.
- Explain why the inside of the plasma membrane — the side that faces the cytoplasm of the cell — must be hydrophilic.
- Explain the relationships between interphase, mitosis, and cytokinesis.
Attributions
Figure 4.14.1
Mitochondrial Disease muscle sample by Nephron is used under a CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) license.
Figure 4.14.2
Aunt and Niece by Tatiana Rodriguez on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Reference
Wikipedia contributors. (2020, June 6). Mitochondrial disease. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Mitochondrial_disease&oldid=961126371
Created by: CK-12/Adapted by Christine Miller
Identical Twins, Identical Genes
You probably can tell by their close resemblance that these two young ladies are identical twins (Figure 5.2.1). Identical twins develop from the same fertilized egg, so they inherit copies of the same chromosomes and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same genes as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are? Let's find out!
Introducing Chromosomes and Genes
Chromosomes are coiled structures made of DNA and proteins. They are encoded with genetic instructions for making RNA and proteins. These instructions are organized into units called genes. There may be hundreds (or even thousands!) of genes on a single chromosome. Genes are segments of DNA that code for particular pieces of RNA. Once formed, some RNA molecules go on to act as blueprints for building proteins, while other RNA molecules help regulate various processes inside the cell. Some regions of DNA do not code for RNA and serve a regulatory function, or have no known function.
Human Chromosomes
Each species is characterized by a set number of chromosomes. Humans cells normally have two sets of chromosomes in each of their cells, one set inherited from each parent. Because chromosomes occur in pairs, these cells are called diploid or 2N. There are 23 chromosomes in each set, for a total of 46 chromosomes per diploid cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape, and they also contain the same genes. The chromosomes in a pair are known as homologous chromosomes.
All human cells (except gametes, which are sperm and egg cells) have the 23 pairs of chromosomes as shown in Figure 5.2.2.
https://www.youtube.com/watch?v=veB31XmUQm8&feature=youtu.be
Secrets of the X chromosome - Robin Ball, TED-Ed, 2019.
Autosomes
Of the 23 pairs of human chromosomes, 22 pairs are called autosomes (pairs 1-22 in the Figure 5.2.2), or autosomal chromosomes. Autosomes are chromosomes that contain genes for characteristics that are unrelated to biological sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes.
Sex Chromosomes
The remaining pair of human chromosomes consists of the sex chromosomes, X and Y (Pair 23 in Figure 5.2.2 and in Figure 5.2.3). Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.
As you can see from Figure 5.2.3, the X chromosome is much larger than the Y chromosome. The X chromosome has about two thousand genes, whereas the Y chromosome has fewer than 100, none of which is essential to survival. Virtually all of the X chromosome genes are unrelated to sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of female as the default sex of the human species.
Human Genes
Humans have an estimated 20 thousand to 22 thousand genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Only about 25 per cent of the nitrogen base pairs of DNA in human chromosomes make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear, but with more time and research their roles may become understood.
The majority of human genes have two or more possible versions, called alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA base pairs within alleles.
Linkage
Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair colour and eye colour are linked, so certain hair and eye colours tend to be inherited together, such as dark hair with dark eyes and blonde hair with blue eyes. Can you think of other human traits that seem to occur together? Do you think they might be controlled by linked genes?
Genes located on the sex chromosomes are called sex-linked genes. Most sex-linked genes are on the X chromosome, because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are X-linked genes, but the term sex-linked is often used to refer to them. The diagram below is called a linkage map: a linkage map shows the locations of specific genes on a chromosome. The linkage map below (Figure 5.2.4) shows the locations of a few of the genes on the human X chromosome.
Figure 5.2.4 Linkage Map for the Human X Chromosome. This linkage map shows the locations of several genes on the X chromosome. Some of the genes code for normal proteins. Others code for abnormal proteins that lead to genetic disorders.
5.2 Summary
- Chromosomes are coiled structures made of DNA and proteins that are encoded with genetic instructions for making RNA and proteins. The instructions are organized into units called genes, which are segments of DNA that code for particular pieces of RNA. The RNA molecules can then act as a blueprint for proteins, or directly help regulate various cellular processes.
- Each species is characterized by a set number of chromosomes. The normal chromosome complement of a human cell is 23 pairs of chromosomes. Of these, 22 pairs are autosomes, which contain genes for characteristics unrelated to sex. The other pair consists of sex chromosomes (XX in females, XY in males). Only the Y chromosome contains genes that determine sex.
- Humans have an estimated 20 thousand to 22 thousand genes. The majority of human genes have two or more possible versions, which are called alleles.
- Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together. A linkage map shows the locations of specific genes on a chromosome.
5.2 Review Questions
- What are chromosomes and genes? How are the two related?
- Describe human chromosomes and genes.
- Explain the difference between autosomes and sex chromosomes.
- What are linked genes, and what does a linkage map show?
- Explain why females are considered the default sex in humans.
- Explain the relationship between genes and alleles.
- Most males and females have two sex chromosomes. Why do only females have Barr bodies?
-
-
5.2 Explore More
https://www.youtube.com/watch?v=M4ut72kfUJM
WACE Biology: Coding and Non-Coding DNA, Atomi, 2019.
https://www.youtube.com/watch?time_continue=3&v=jhHGCvMlrb0&feature=emb_logo
How Sex Genes Are More Complicated Than You Thought, Seeker, 2015.
Attributions
Figure 5.2.1
Twins5 [photo] by Bùi Thanh Tâm on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 5.2.2
Human_male_karyotype by National Human Genome Research Institute/ NIH on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain). (Original from the Talking Glossary of Genetics.)
Figure 5.2.3
Comparison between X and Y chromosomes byJonathan Bailey, National Human Genome Research Institute, National Institutes of Health [NIH] Image Gallery, on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
Figure 5.2.4
Linkage Map of Human X Chromosome by Christine Miller is used under a
CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
References
Atomi. (2019, October 27). WACE Biology: Coding and Non-Coding DNA. YouTube. https://www.youtube.com/watch?v=M4ut72kfUJM&feature=youtu.be
Seeker. (2015, July 26). How Sex Genes Are More Complicated Than You Thought. YouTube. https://www.youtube.com/watch?v=jhHGCvMlrb0&feature=youtu.be
TED-Ed. (2017, April 18). Secrets of the X chromosome - Robin Ball. YouTube. https://www.youtube.com/watch?v=veB31XmUQm8&feature=youtu.be
Created by: CK-12/Adapted by Christine Miller
What Makes You...You?
This young woman has naturally red hair (Figure 5.3.1). Why is her hair red instead of some other colour? In general, what gives her the specific traits she has? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.
Introducing DNA
Today, it is commonly known that DNA is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed — that is, they were aware that genetic information is contained within biochemical molecules. What they didn’t know was which specific molecules play this role. In fact, for many decades, scientists thought that proteins were the molecules that contain genetic information.
Discovery that DNA is the Genetic Material
Determining that DNA is the genetic material was an important milestone in biology. It took many scientists undertaking creative experiments over several decades to show with certainty that DNA is the molecule that determines the traits of organisms. This research began in the early part of the 20th century.
Griffith's Experiments with Mice
One of the first important discoveries was made in the 1920s by an American scientist named Frederick Griffith. Griffith was studying mice and two different strains of a bacterium, called R (rough)-strain and S (smooth)-strain. He injected the two bacterial strains into mice. The S-strain was virulent and killed the mice, whereas the R-strain was not virulent and did not kill the mice. You can see these details in Figure 5.3.2. Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the dead bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died.
Based on his observations, Griffith deduced that something in the dead S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. What was this "something?" What type of substance could change the characteristics of the organism that received it?
Avery and His Colleagues Make a Major Contribution
In the early 1940s, a team of scientists led by Canadian-American Oswald Avery tried to answer the question raised by Griffith’s research results. First, they inactivated various substances in the S-strain bacteria. Then they killed the S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind that the R-strain bacteria normally did not harm the mice.) When they inactivated proteins, the R-strain was deadly to the injected mice. This ruled out proteins as the genetic material. Why? Even without the S-strain proteins, the R-strain was changed (or transformed) into the deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA — and not protein — is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.
Hershey and Chase Confirm the Results
The conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. Instead, they are basically DNA (or RNA) inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then, it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.
Chargaff Focuses on DNA Bases
Other important discoveries about DNA were made in the mid-1900s by Erwin Chargaff. He studied DNA from many different species and was especially interested in the four different nitrogen bases of DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). Chargaff found that concentrations of the four bases differed between species. Within any given species, however, the concentration of adenine was always the same as the concentration of thymine, and the concentration of guanine was always the same as the concentration of cytosine. These observations came to be known as Chargaff’s rules. The significance of the rules would not be revealed until the double-helix structure of DNA was discovered.
Discovery of the Double Helix
After DNA was shown to be the genetic material, scientists wanted to learn more about its structure and function. James Watson and Francis Crick are usually given credit for discovering that DNA has a double helix shape, as shown in Figure 5.3.3. In fact, Watson and Crick's discovery of the double helix depended heavily on the prior work of Rosalind Franklin and other scientists, who had used X-rays to learn more about DNA’s structure. Unfortunately, Franklin and these others have not always been given credit for their important contributions to the discovery of the double helix.
The DNA molecule has a double helix shape — the same basic shape as a spiral staircase. Do you see the resemblance? Which parts of the DNA molecule are like the steps of the spiral staircase?
The double helix shape of DNA, along with Chargaff’s rules, led to a better understanding of DNA. As a nucleic acid, DNA is made from nucleotide monomers. Long chains of nucleotides form polynucleotides, and the DNA double helix consists of two polynucleotide chains. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of the four bases (adenine, cytosine, guanine, or thymine). The sugar and phosphate molecules in adjacent nucleotides bond together and form the "backbone" of each polynucleotide chain.
Scientists concluded that bonds between the bases hold together the two polynucleotide chains of DNA. Moreover, adenine always bonds with thymine, and cytosine always bonds with guanine. That's why these pairs of bases are called complementary base pairs. Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If adenine were to bond with guanine, as well as thymine, for example, the distance between the two DNA chains would vary. When a one-ring molecule (like thymine) always bonds with a two-ring molecule (like adenine), however, the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix. The bonded base pairs (A-T and G-C) stick into the middle of the double helix, forming the "steps" of the spiral staircase.
5.3 Summary
- Determining that DNA is the genetic material was an important milestone in biology. One of the first important discoveries was made in the 1920s, when Griffith showed that something in virulent bacteria could be transferred to nonvirulent bacteria, making them virulent, as well.
- In the early 1940s, Avery and colleagues showed that the "something" Griffith found in his research was DNA and not protein. This result was confirmed by Hershey and Chase, who demonstrated that viruses insert DNA into bacterial cells so the cells will make copies of the viruses.
- In the mid-1950s, Chargaff showed that, within the DNA of any given species, the concentration of adenine is always the same as the concentration of thymine, and that the concentration of guanine is always the same as the concentration of cytosine. These observations came to be known as Chargaff's rules.
- Around the same time, James Watson and Francis Crick, building on the prior X-ray research of Rosalind Franklin and others, discovered the double-helix structure of the DNA molecule. Along with Chargaff's rules, this led to a better understanding of DNA's structure and function.
- Knowledge of DNA's structure helped scientists understand how DNA replicates, which must occur before cell division occurs so each daughter cell will have a complete set of chromosomes.
5.3 Review Questions
- Outline the discoveries that led to the determination that DNA (not protein) is the biochemical molecule that contains genetic information.
- State Chargaff's rules. Explain how the rules are related to the structure of the DNA molecule.
- Explain how the structure of a DNA molecule is like a spiral staircase. Which parts of the staircase represent the various parts of the molecule?
-
- Why do you think dead S-strain bacteria injected into mice did not harm the mice, but killed them when mixed with living (and normally harmless) R-strain bacteria?
- In Griffith’s experiment, do you think the heat treatment that killed the bacteria also inactivated the bacterial DNA? Why or why not?
- Give one example of the specific evidence that helped rule out proteins as genetic material.
5.3 Explore More
https://www.youtube.com/watch?v=V6bKn34nSbk
The Discovery of the Structure of DNA, OpenMind, 2017.
https://www.youtube.com/watch?time_continue=5&v=JiME-W58KpU&feature=emb_logo
Rosalind Franklin: Great Minds, SciShow, 2013.
Attributions
Figure 5.3.1
Redhead [photo] by Hichem Dahmani on Unsplash is used under the Unsplash License (https://unsplash.com/license).
Figure 5.3.2
Griffith’s mice by Mariana Ruiz Villarreal [LadyofHats] for CK-12 Foundation is used under a
CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.
©CK-12 Foundation Licensed under • Terms of Use • Attribution
Figure 5.3.3
DNA_Overview by Michael Ströck [mstroeck] on Wikimedia Commons is used under a CC BY SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/) license.
References
Brainard, J/ CK-12. (2012). Concentration. In Physical Science [website]. CK12.org. https://www.ck12.org/c/physical-science/concentration/?referrer=crossref
OpenMind. (2017, September 11). The discovery of the structure of DNA. YouTube. https://www.youtube.com/watch?v=V6bKn34nSbk&feature=youtu.be
SciShow. (2013, July 9). Rosalind Franklin: Great minds. YouTube. https://www.youtube.com/watch?v=JiME-W58KpU&feature=youtu.be
Wikipedia contributors. (2020, June 27). Alfred Hershey. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Alfred_Hershey&oldid=964789559
Wikipedia contributors. (2020, June 5). Erwin Chargaff. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Erwin_Chargaff&oldid=960942873
Wikipedia contributors. (2020, June 29). Francis Crick. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Francis_Crick&oldid=965135362
Wikipedia contributors. (2020, July 6). Frederick Griffith. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Frederick_Griffith&oldid=966352134
Wikipedia contributors. (2020, July 5). James Watson. In Wikipedia. https://en.wikipedia.org/w/index.php?title=James_Watson&oldid=966111944
Wikipedia contributors. (2020, March 31). Martha Chase. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Martha_Chase&oldid=948408219
Wikipedia contributors. (2020, July 2). Oswald Avery. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Oswald_Avery&oldid=965632585
Wikipedia contributors. (2020, June 30). Rosalind Franklin. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Rosalind_Franklin&oldid=965334881
Created by: CK-12/Adapted by Christine Miller
A Deceptively Simple Model
This simple model sums up one of the most important ideas in biology, which is called the central dogma of molecular biology (you'll read more about it below). You probably recognize the spiral-shaped structure in the nucleus. It represents a molecule of DNA, the biochemical molecule that stores genetic information in most living cells. The yellow chain represents a newly formed polypeptide — the beginning stage of creating a protein. Proteins are the class of biochemical molecules that carry out virtually all life processes. What is the structure in the center of the model? It appears to resemble DNA, but it is smaller and simpler. This molecule is the key to the central dogma, and it may have been the first type of biochemical molecule to evolve.
Central Dogma of Molecular Biology
DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain in the nucleus, but proteins are made at ribosomes in the cytoplasm. How do the instructions in DNA get to the site of protein synthesis outside the nucleus?
Another type of nucleic acid is responsible. This nucleic acid is RNA, or ribonucleic acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in the nucleus to a ribosome in the cytoplasm and then helps assemble the protein. In short:
DNA → RNA → Protein
This expresses in words what the diagram in Figure 5.5.1 shows. The genetic instructions encoded in DNA in the nucleus are transcribed to RNA. Then, RNA carries the instructions to a ribosome in the cytoplasm, where they are translated into a protein. Discovering this sequence of events was a major milestone in molecular biology. It's called the central dogma of molecular biology.
Introducing RNA
DNA alone cannot “tell” your cells how to make proteins. It needs the help of RNA, the other main player in the central dogma of molecular biology. Like DNA, RNA is a nucleic acid, so it consists of repeating nucleotides bonded together to form a polynucleotide chain. RNA differs from DNA in several ways: it exists as a single stranded molecule, contains the sugar ribose (as opposed to deoxyribose) and uses the base uracil instead of thymine.
Functions of RNA
The main function of RNA is to help make proteins. There are three main types of RNA involved in protein synthesis:
-
Figure 5.5.3 The three types of RNA take very different forms. Messenger RNA (mRNA) copies (or transcribes) the genetic instructions from DNA in the nucleus and carries them to the cytoplasm.
- Ribosomal RNA (rRNA) helps form ribosomes, where proteins are assembled. Ribosomes also contain proteins.
- Transfer RNA (tRNA) brings amino acids to ribosomes, where rRNA catalyzes the formation of chemical bonds between them to form a protein.
In section 5.7 Protein Synthesis, you can read in detail about how these three types of RNA build primary structure of proteins.
RNA is a very versatile molecule which plays multiple roles in living things. In addition to helping to make proteins, for example, there are RNA molecules that regulate the expression of genes, and RNA molecules that catalyze other biochemical reactions needed to sustain life. Because of the diversity of roles that RNA molecules play, they have been called the Swiss Army knives of the cellular world.
It's an RNA World
The function of DNA is to store genetic information inside cells. It does this job well, but that's about all it can do. DNA can't act as an enzyme, for example, to catalyze biochemical reactions that are needed to keep us alive. Proteins are needed for this and many other life functions. Proteins work exceptionally well to keep us alive, but they are unable to store genetic information. Proteins need DNA for that. Without DNA, proteins could not exist. On the other hand, without proteins, DNA could not survive. This poses a chicken-and-egg sort of problem: Which evolved first? DNA or proteins?
Some scientists think that the answer is neither. They speculate instead that RNA was the first biochemical to evolve. The reason? RNA can do more than one job. It can store information as DNA does, but it can also perform various jobs (such as catalysis) to keep cells alive, as proteins do. The idea that RNA was the first biochemical to evolve, predating both DNA and proteins, is called the RNA world hypothesis. According to this hypothesis, billions of years ago, RNA molecules evolved that could both survive and make copies of themselves. According to the hypothesis, early RNA molecules eventually evolved the ability to make proteins, and at some point RNA mutated to form DNA.
Feature: Reliable Sources
The RNA world hypothesis has not gained enough support in the scientific community to be accepted as a scientific theory. In fact, there are probably as many detractors as supporters of the hypothesis. Do a web search to learn more about the RNA world hypothesis and the evidence and arguments for and against it. When weighing the information you gather, consider the likely reliability of the different websites you visit. Based on what you determine are the most reliable sources and the most convincing arguments, form your own opinion about the hypothesis. You may decide to accept or reject the hypothesis. Alternatively, you may decide to reserve judgement until — or if — more evidence or arguments are forthcoming.
5.5 Summary
- The central dogma of molecular biology can be summed up as: DNA → RNA → Protein. This means that the genetic instructions encoded in DNA are first transcribed to RNA, and then from RNA they are translated into a protein.
- Like DNA, RNA is a nucleic acid. Unlike DNA, RNA consists of just one polynucleotide chain instead of two, contains the base uracil instead of thymine, and contains the sugar ribose instead of deoxyribose.
- The main function of RNA is helping to make proteins. There are three main types of RNA involved in protein synthesis: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). RNA has additional functions, including regulating gene expression and catalyzing other biochemical reactions.
- According to the RNA world hypothesis, RNA was the first type of biochemical molecule to evolve, predating both DNA and proteins. The hypothesis is based mainly on the multiple functions of RNA, which can store genetic information like DNA and carry out life processes (like proteins).
5.5 Review Questions
- State the central dogma of molecular biology.
- Drag and drop to compare the structure and function of DNA and RNA:
3.
4.
5.5 Explore More
https://www.youtube.com/watch?time_continue=4&v=VYQQD0KNOis&feature=emb_logo
The RNA Origin of Life, NOVA PBS Official, 2014.
https://www.youtube.com/watch?v=JQByjprj_mA
DNA vs RNA (Updated), Amoeba Sisters, 2019.
Attributions
Figure 5.5.1
From DNA to Protein: Transcription through Translation by OpenStax College on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 5.5.2
Molbio-Header by Squidonius on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 5.5.2
ARNm-Rasmol by Corentin Le Reun on Wikimedia Commons is is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).ublic domain.
References
Amoeba Sisters. (2019, August 29). DNA vs RNA (Updated). YouTube. https://www.youtube.com/watch?v=JQByjprj_mA&feature=youtu.be
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.29 From DNA to Protein: Transcription through Translation [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-4-protein-synthesis#fig-ch03_04_05
NOVA PBS Official. (2014, April 23). The RNA origin of life. YouTube. https://www.youtube.com/watch?v=VYQQD0KNOis&feature=youtu.be
Wikipedia contributors. (2020, June 28). RNA world. In Wikipedia. https://en.wikipedia.org/w/index.php?title=RNA_world&oldid=964998696
A YouTube element has been excluded from this version of the text. You can view it online here: https://pressbooks.bccampus.ca/humanbiology053/?p=1589
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