4.7 Passive Transport
Created by: CK-12/Adapted by Christine Miller
Letting in the Light
Look at the big windows in this house (Figure 4.7.1). Imagine all the light they must let in on a sunny day. Now imagine living in a house that has walls without any windows or doors. Nothing could enter or leave. Or imagine living in a house with holes in the walls instead of windows and doors. Things could enter or leave, but you couldn’t control what came in or went out. Only when a house has walls with windows and doors that can be opened or closed, can you control what enters or leaves. Windows and doors allow you to let in light and the family dog and keep out rain and bugs, for example.
Transport Across Membranes
If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important function of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport — which requires no energy expenditure by the cell — and active transport — which requires energy from the cell.
Transport Without Energy Expenditure By The Cell
Passive transport occurs when substances cross the plasma membrane without any input of energy from the cell. No energy is required because the substances are moving from an area where they have a higher concentration to an area where they have a lower concentration. Concentration refers to the number of particles of a substance per unit of volume. The more particles of a substance in a given volume, the higher the concentration. A substance always moves from an area where it is more concentrated to an area where it is less concentrated.
There are several different types of passive transport, including simple diffusion, osmosis, and facilitated diffusion. Each type is described below.
Simple Diffusion
Diffusion is the movement of a substance due to a difference in concentration. It happens without any help from other molecules. The substance simply moves from the area where it is more concentrated to the area where it is less concentrated. Picture someone spraying perfume in the corner of a room. Do the perfume molecules stay in the corner? No, they spread out, or diffuse throughout the room until they are evenly spread out. Figure 4.7.2 shows how diffusion works across a cell membrane. Substances that can squeeze between the lipid molecules in the plasma membrane by simple diffusion are generally very small, hydrophobic molecules, such as molecules of oxygen and carbon dioxide.
Osmosis
Osmosis is a special type of diffusion — the diffusion of water molecules across a membrane. Like other molecules, water moves from an area of higher concentration to an area of lower concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane. In Figure 4.7.3, the dotted red line shows a semi-permeable membrane. In the first beaker, there is an uneven concentration of solutes on either side of the membrane, but the solute cannot cross — diffusion of the solute can’t occur. In this case, water will move to even out the concentration as has happened on the beaker on the right side. The water levels are uneven, but the process of osmosis has evened out the concentration gradient.
Facilitated Diffusion
Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules (such as glucose) all need help with diffusion. This help comes from special proteins in the membrane known as transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. There are several types of transport proteins, including channel proteins and carrier proteins. Both are shown in Figure 4.7.4.
- Channel proteins form pores (or tiny holes) in the membrane. This allows water molecules and small ions to pass through the membrane without coming into contact with the hydrophobic tails of the lipid molecules in the interior of the membrane.
- Carrier proteins bind with specific ions or molecules. In doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane.
Transport and Homeostasis
For a cell to function normally, the inside of it must maintain a stable state. The concentrations of salts, nutrients, and other substances must be kept within certain ranges. The state in which stable conditions are maintained inside a cell (or an entire organism) is called homeostasis. Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell. The transport of substances into and out of cells as described in this section plays an important role in homeostasis. By allowing the movement of substances into and out of cells, transport keeps conditions within normal ranges inside the cells and throughout the organism as a whole.
Watch this video “Cell Transport,” by the Amoeba Sisters:
Cell Transport with the Amoeba Sisters, 2016.
4.7 Summary
- Controlling the movement of things in and out of the cell is an important function of the plasma membrane. There are two basic ways that substances can cross the 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.
- Simple diffusion is the movement of a substance due to differences in concentration. It happens 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 is the diffusion of water molecules across a membrane. Water moves in or out of a cell by osmosis until its concentration is the same on both sides of the plasma membrane.
- Facilitated diffusion is the movement of a substance across a membrane due to differences in concentration, but it only occurs with the help of transport proteins (such as channel proteins or carrier proteins) in the membrane. This is how large or hydrophilic molecules and charged ions enter and leave the cell.
- Processes of passive transport play important roles in homeostasis. By allowing the movement of substances into and out of the cell, they keep conditions within normal ranges inside the cell and the organism as a whole.
4.7 Review Questions
- What is the main difference between passive and active transport?
- Summarize three different ways that passive transport can occur. Give an example of a substance that is transported in each way.
- Explain how transport across the plasma membrane is related to homeostasis of the cell.
- In general, why can only very small, hydrophobic molecules cross the cell membrane by simple diffusion?
- Explain how facilitated diffusion assists with osmosis in cells. Define osmosis and facilitated diffusion in your answer.
- Imagine a hypothetical cell with a higher concentration of glucose inside the cell than outside. Answer the following questions about this cell, assuming all transport across the membrane is passive, not active.
- Can the glucose simply diffuse across the cell membrane? Why or why not?
- Assuming that there are glucose transport proteins in the cell membrane, which way would glucose flow — into or out of the cell? Explain your answer.
- If the concentration of glucose was equal inside and outside of the cell, do you think there would be a net flow of glucose across the cell membrane in one direction or the other? Explain your answer.
- What are the similarities and differences between channel proteins and carrier proteins?
-
4.7 Explore More
Osmosis and Water Potential, Amoeba Sisters, 2018.
Structure Of The Cell Membrane – Active and Passive Transport, Professor Dave Explains, 2016.
Attributions
Figure 4.7.1
Windows/ The Oyster Suite in Eureka, CA by Drew Coffman on Unsplash is used under the Unsplash License https://unsplash.com/license).
Figure 4.7.2
Diffusion/ Scheme simple diffusion in cell membrane by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.7.3
Osmosis by OpenStax on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 4.7.4
Scheme facilitated diffusion in cell membrane by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Amoeba Sisters. (2016, June 24). Cell transport. YouTube. https://www.youtube.com/watch?v=Ptmlvtei8hw&feature=youtu.be
Amoeba Sisters. (2018, June 27). Osmosis and water potential. YouTube. https://www.youtube.com/watch?v=L-osEc07vMs&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 3.7 Osmosis [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane
Professor Dave Explains. (2016, September 5). Structure of the cell membrane – Active and passive transport. https://www.youtube.com/watch?v=AcrqIxt8am8&feature=youtu.be
Diagram shows examples of the shapes of different types of fatty acids. Saturated fatty acids form long straight chains. Monounsaturated fatty acids have a slight curve and saturated fatty acids can have multiple curves or bends.
List of types of mutagens. Radiation includes UV radiation and X rays. Chemicals include cigarette smoke a vaping vapor, nitrite and nitrate preservatives, barbecuing, and benzoyl peroxide. Infectious agents include HPV and H. Pylori.
Created by: CK-12/Adapted by Christine Miller
Mutant Cosplay
You probably recognize these costumed comic fans in Figure 5.8.1 as two of the four Teenage Mutant Ninja Turtles. Can a mutation really turn a reptile into an anthropomorphic superhero? Of course not — but mutations can often result in other drastic (but more realistic) changes in living things.
What Are Mutations?
Mutations are random changes in the sequence of bases in DNA or RNA. The word mutation may make you think of the Ninja Turtles, but that's a misrepresentation of how most mutations work. First of all, everyone has mutations. In fact, most people have dozens (or even hundreds!) of mutations in their DNA. Secondly, from an evolutionary perspective, mutations are essential. They are needed for evolution to occur because they are the ultimate source of all new genetic variation in any species.
Causes of Mutations
Mutations have many possible causes. Some mutations seem to happen spontaneously, without any outside influence. They occur when errors are made during DNA replication or during the transcription phase of protein synthesis. Other mutations are caused by environmental factors. Anything in the environment that can cause a mutation is known as a mutagen. Examples of mutagens are shown in the figure below.
Types of Mutations
Mutations come in a variety of types. Two major categories of mutations are germline mutations and somatic mutations.
- Germline mutations occur in gametes (the sex cells), such as eggs and sperm. These mutations are especially significant because they can be transmitted to offspring, causing every cell in the offspring to carry those mutations.
- Somatic mutations occur in other cells of the body. These mutations may have little effect on the organism, because they are confined to just one cell and its daughter cells. Somatic mutations cannot be passed on to offspring.
Mutations also differ in the way that the genetic material is changed. Mutations may change an entire chromosome, or they may alter just one or a few nucleotides.
Chromosomal Alterations
Chromosomal alterations are mutations that change chromosome structure. They occur when a section of a chromosome breaks off and rejoins incorrectly, or otherwise does not rejoin at all. Possible ways in which these mutations can occur are illustrated in the figure below. Chromosomal alterations are very serious. They often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. An example of a human disease caused by a chromosomal duplication is Charcot-Marie-Tooth disease type 1 (CMT1). It is characterized by muscle weakness, as well as loss of muscle tissue and sensation. The most common cause of CMT1 is a duplication of part of chromosome 17.
Point Mutations
A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be silent, missense, or nonsense mutations, as described in Table 5.8.1. The effects of point mutations depend on how they change the genetic code.
| Type | Description | Example | Effect |
|---|---|---|---|
| Silent | mutated codon codes for the same amino acid | CAA (glutamine) → CAG (glutamine) | none |
| Missense | mutated codon codes for a different amino acid | CAA (glutamine) → CCA (proline) | variable |
| Nonsense | mutated codon is a premature stop codon | CAA (glutamine) → UAA (stop) usually | serious |
Frameshift Mutations
A frameshift mutation is a deletion or insertion of one or more nucleotides, changing the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:
AUG-AAU-ACG-GCU = start-asparagine-threonine-alanine
Now, assume that an insertion occurs in this sequence. Let’s say an A nucleotide is inserted after the start codon AUG. The sequence of bases becomes:
AUG-AAA-UAC-GGC-U = start-lysine-tyrosine-glycine
Even though the rest of the sequence is unchanged, this insertion changes the reading frame and, therefore, all of the codons that follow it. As this example shows, a frameshift mutation can dramatically change how the codons in mRNA are read. This can have a drastic effect on the protein product.
Effects of Mutations
The majority of mutations have neither negative nor positive effects on the organism in which they occur. These mutations are called neutral mutations. Examples include silent point mutations, which are neutral because they do not change the amino acids in the proteins they encode.
Many other mutations have no effects on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix mutations in DNA.
Beneficial Mutations
Some mutations — known as beneficial mutations — have a positive effect on the organism in which they occur. They generally code for new versions of proteins that help organisms adapt to their environment. If they increase an organism’s chances of surviving or reproducing, the mutations are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are two such examples:
- Mutations have occurred in bacteria that allow the bacteria to survive in the presence of antibiotic drugs, leading to the evolution of antibiotic-resistant strains of bacteria.
- A unique mutation is found in people in Limone, a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels despite a high-fat diet. The individual in which this mutation first appeared has even been identified and many of his descendants carry this gene.
Harmful Mutations
Imagine making a random change in a complicated machine, such as a car engine. There is a chance that the random change would result in a car that does not run well — or perhaps does not run at all. By the same token, a random change in a gene's DNA may result in the production of a protein that does not function normally... or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer.
- A genetic disorder is a disease, syndrome, or other abnormal condition caused by a mutation in one or more genes, or by a chromosomal alteration. An example of a genetic disorder is cystic fibrosis. A mutation in a single gene causes the body to produce thick, sticky mucus that clogs the lungs and blocks ducts in digestive organs.
- Cancer is a disease in which cells grow out of control and form abnormal masses of cells (called tumors). It is generally caused by mutations in genes that regulate the cell cycle. Because of the mutations, cells with damaged DNA are allowed to divide without restriction.
Feature: My Human Body
Inherited mutations are thought to play a role in roughly five to ten per cent of all cancers. Specific mutations that cause many of the known hereditary cancers have been identified. Most of the mutations occur in genes that control the growth of cells or the repair of damaged DNA.
Genetic testing can be done to determine whether individuals have inherited specific cancer-causing mutations. Some of the most common inherited cancers for which genetic testing is available include hereditary breast and ovarian cancer, caused by mutations in genes called BRCA1 and BRCA2. Besides breast and ovarian cancers, mutations in these genes may also cause pancreatic and prostate cancers. Genetic testing is generally done on a small sample of body fluid or tissue, such as blood, saliva, or skin cells. The sample is analyzed by a lab that specializes in genetic testing, and it usually takes at least a few weeks to get the test results.
Should you get genetic testing to find out whether you have inherited a cancer-causing mutation? Such testing is not done routinely just to screen patients for risk of cancer. Instead, the tests are generally done only when the following three criteria are met:
- The test can determine definitively whether a specific gene mutation is present. This is the case with the BRCA1 and BRCA2 gene mutations, for example.
- The test results would be useful to help guide future medical care. For example, if you found out you had a mutation in the BRCA1 or BRCA2 gene, you might get more frequent breast and ovarian cancer screenings than are generally recommended.
- You have a personal or family history that suggests you are at risk of an inherited cancer.
Criterion number 3 is based, in turn, on such factors as:
- Diagnosis of cancer at an unusually young age.
- Several different cancers occurring independently in the same individual.
- Several close genetic relatives having the same type of cancer (such as a maternal grandmother, mother, and sister all having breast cancer).
- Cancer occurring in both organs in a set of paired organs (such as both kidneys or both breasts).
If you meet the criteria for genetic testing and are advised to undergo it, genetic counseling is highly recommended. A genetic counselor can help you understand what the results mean and how to make use of them to reduce your risk of developing cancer. For example, a positive test result that shows the presence of a mutation may not necessarily mean that you will develop cancer. It may depend on whether the gene is located on an autosome or sex chromosome, and whether the mutation is dominant or recessive. Lifestyle factors may also play a role in cancer risk even for hereditary cancers. Early detection can often be life saving if cancer does develop. Genetic counseling can also help you assess the chances that any children you may have will inherit the mutation.
5.8 Summary
- Mutations are random changes in the sequence of bases in DNA or RNA. Most people have multiple mutations in their DNA without ill effects. Mutations are the ultimate source of all new genetic variation in any species.
- Mutations may happen spontaneously during DNA replication or transcription. Other mutations are caused by environmental factors called mutagens. Mutagens include radiation, certain chemicals, and some infectious agents.
- Germline mutations occur in gametes and may be passed onto offspring. Every cell in the offspring will then have the mutation. Somatic mutations occur in cells other than gametes and are confined to just one cell and its daughter cells. These mutations cannot be passed on to offspring.
- Chromosomal alterations are mutations that change chromosome structure and usually affect the organism in multiple ways. Charcot-Marie-Tooth disease type 1 is an example of a chromosomal alteration in humans.
- Point mutations are changes in a single nucleotide. The effects of point mutations depend on how they change the genetic code and may range from no effects to very serious effects.
- Frameshift mutations change the reading frame of the genetic code and are likely to have a drastic effect on the encoded protein.
- Many mutations are neutral and have no effect on the organism in which they occur. Some mutations are beneficial and improve fitness. An example is a mutation that confers antibiotic resistance in bacteria. Other mutations are harmful and decrease fitness, such as the mutations that cause genetic disorders or cancers.
5.8 Review Question
- Define mutation.
- Identify causes of mutation.
- Compare and contrast germline and somatic mutations.
- Describe chromosomal alterations, point mutations, and frameshift mutations. Identify the potential effects of each type of mutation.
- Why do many mutations have neutral effects?
- Give one example of a beneficial mutation and one example of a harmful mutation.
-
- Why do you think that exposure to mutagens (such as cigarette smoke) can cause cancer?
- Explain why the insertion or deletion of a single nucleotide can cause a frameshift mutation.
- Compare and contrast missense and nonsense mutations.
- Explain why mutations are important to evolution.
5.8 Explore More
https://www.youtube.com/watch?time_continue=51&v=PQjL4ZDuq2o&feature=emb_logo
How Radiation Changes Your DNA, Seeker, 2016.
https://www.youtube.com/watch?v=z9HIYjRRaDE&t=93s
Where do genes come from? - Carl Zimmer, TED-Ed, 2014.
https://www.youtube.com/watch?v=a63t8r70QN0&feature=youtu.be
What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin,
TED, 2019.
Attributions
Figure 5.8.1
Ninja Turtles by Pat Loika on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
Figure 5.8.2
Separate images are all in public domain or CC licensed:
- Beauty treatment face mask by no-longer-here on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).
- HPV by AJC1 on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
- H Pylori by AJC1 on Flickr is used under a CC BY-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) license.
- Vape and Cigarette by Vaping360 on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.
- Hand X-Ray by Hellerhoff on Wikimedia Commons - CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en) license.
- Hot dogs by unknown on PxFuel is used under the Pxfuel Terms (https://www.pxfuel.com/terms-of-use).
- Sunshine face is clipart.
Figure 5.8.3
Scheme of possible chromosome mutations/ Chromosomenmutationen by unknown on Wikimedia Commons is adapted from NIH's Talking Glossary of Genetics. [Changes as described by de:user:Dietzel65]. Further use and adapation (text translated to English) by Christine Miller as image is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Seeker. (2016, April 23). How radiation changes your DNA. YouTube. https://www.youtube.com/watch?v=PQjL4ZDuq2o&feature=youtu.be
TED. (2019, June 5). What you should know about vaping and e-cigarettes | Suchitra Krishnan-Sarin. YouTube. https://www.youtube.com/watch?v=a63t8r70QN0&feature=youtu.be
TED-Ed. (2014, September 22). Where do genes come from? - Carl Zimmer. YouTube. https://www.youtube.com/watch?v=z9HIYjRRaDE&feature=youtu.be
Wikipedia contributors. (2020, July 6). Breast cancer. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Breast_cancer&oldid=966366739
Wikipedia contributors. (2020, July 9). Charcot–Marie–Tooth disease. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Charcot%E2%80%93Marie%E2%80%93Tooth_disease&oldid=966912915
Wikipedia contributors. (2020, July 7). Cystic fibrosis. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Cystic_fibrosis&oldid=966566921
Wikipedia contributors. (2020, June 4). Limone sul Garda. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Limone_sul_Garda&oldid=960771991
Wikipedia contributors. (2020, June 23). Ovarian cancer. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Ovarian_cancer&oldid=964157192
Wikipedia contributors. (2020, May 7). BRCA mutation. In Wikipedia. https://en.wikipedia.org/w/index.php?title=BRCA_mutation&oldid=955463902
Wikipedia contributors. (2020, July 10). Teenage Mutant Ninja Turtles. In Wikipedia. https://en.wikipedia.org/w/index.php?title=Teenage_Mutant_Ninja_Turtles&oldid=967030468
A stem cell can become any type of body cell based on gene regulation. Types of cells a stem cell can become include, but are not limited to: Sex cells, muscle cells, fat cells, immune cells, bone cells, epithelial cells, nervous cells, and blood cells.
The strip of hair growing on the ridge above a person's eye socket.
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
Figure 5.10.1
![Mitotic Prophase [tiny]](https://commons.wikimedia.org/wiki/File:Mitotic_Prophase.svg){kind=link}
