5.2 Chromosomes and Genes
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.
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?
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5.2 Explore More
WACE Biology: Coding and Non-Coding DNA, Atomi, 2019.
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
By Christine Miller
DNA Replication: Overview
DNA replication is required for the growth or replication of an organism. You started as one single cell and are now made up of approximately 37 trillion cells! Each and every one of these cells contains the exact same copy of DNA, which originated from the first cell that was you. How did you get from one set of DNA, to 37 million sets, one for each cell? Through DNA replication.
Knowledge of DNA’s structure helped scientists understand DNA replication, the process by which DNA is copied. It occurs during the synthesis (S) phase of the eukaryotic cell cycle. DNA must be copied so that each new daughter cell will have a complete set of chromosomes after cell division occurs.
DNA replication is referred to as "semi-conservative". What this means is when a strand of DNA is replicated, each of the two original strands acts as a template for a new complementary strand. When the replication process is complete, there are two identical sets of DNA, each containing one of the original strands of DNA, and one newly synthesized strand.
DNA replication involves a certain sequence of events. For each event, there is a specific enzyme which facilitates the process. There are four main enzymes that facilitate DNA replication: helicase, primase, DNA polymerase, and ligase.
DNA Replication: The Process
DNA replication begins when an enzyme called helicase unwinds, and unzips the DNA molecule. If you recall the structure of DNA, you may remember that it consists of two long strands of nucleotides held together by hydrogen bonds between complementary nitrogenous bases. This forms a ladder-like structure which is in a coiled shape. In order to start DNA replication, helicase needs to unwind the molecule and break apart the hydrogen bonds holding together complementary nitrogenous bases. This causes the two strands of DNA to separate.
Small molecules called single-stranded binding proteins (SSB) attach to the loose strands of DNA to keep them from re-forming the hydrogen bonds that helicase just broke apart.
Once the nitrogenous bases from the inside of the DNA molecule are exposed, the creation of a new, complementary strand can begin. DNA polymerase creates the new strand, but it needs some help in finding the correct place to begin, so primase lays down a short section of RNA primer (shown in green in Figure 5.4.3). Once this short section of primer is laid, DNA polymerase can bind to the DNA molecule and start connecting nucleotides in the correct order to match the sequence of nitrogenous bases on the template (original) strand.
If we think about the DNA molecule, we may remember that the two strands of DNA run antiparallel to one another. This means that in the sugar-phosphate backbone, one strand of the DNA has the sugar oriented in the "up" position, and the other strand has the phosphate oriented in the "up" position (see Figure 5.4.4). DNA polymerase is an enzyme which can only work in one direction on the DNA molecule. This means that one strand of DNA can be replicated in one long string, as DNA polymerase follows helicase as it unzips the DNA molecule. This strand is termed the "leading strand". The other strand, however, can only be replicated in small chunks since the DNA polymerase replicates in the opposite direction that helicase is unzipping. This strand is termed the "lagging strand". These small chunks of replicated DNA on the lagging strand are called Okazaki fragments.
Take a look at Figure 5.4.5 and find the Okazaki fragments, the leading strand and the lagging strand.
Once DNA polymerase has replicated the DNA, a third enzyme called ligase completes the final stage of DNA replication, which is repairing the sugar-phosphate backbone. This connects the gaps in the backbone between Okazaki fragments. Once this is complete, the DNA coils back into its classic double helix structure.
Semi-Conservative Replication
When DNA replication is complete, there are two identical sets of double stranded DNA, each with one strand from the original, template, DNA molecule, and one strand that was newly synthesized during the DNA replication process. Because each new set of DNA contains one old and one new strand, we describe DNA as being semi-conservative.
Watch this video for a great overview of DNA replication:
https://www.youtube.com/watch?v=Qqe4thU-os8
DNA Replication (Updated), Amoeba Sisters, 2019.
5.4 Summary
- DNA replication requires the action of three main enzymes each with their own specific role:
- Helicase unzips and unwinds the DNA molecule.
- DNA polymerase creates a new complementary strand of DNA on each of the originals halves that were separated by helicase. New nucleotides are added through complementary base pairing: A pairs with T, and C with G.
- Ligase repairs gaps in the sugar-phosphate backbone between Okazaki fragments.
- DNA replication is semi-conservative because each daughter molecule contains one strand from the parent molecule and one new complementary strand.
5.4 Review Questions
2. Why are Okazaki fragments formed?
- Because helicase only unzips DNA in one direction.
- Because DNA is in a double helix.
- Because DNA polymerase only replicates DNA in one direction.
- Because DNA replication is semi-conservative.
3. Drag and drop to label the diagram.
5.4 Explore More
https://www.youtube.com/watch?v=TNKWgcFPHqw
DNA replication - 3D, yourgenome, 2015.
Attributions
Figure 5.4.1
DNA_replication_split.svg by Madprime on Wikimedia Commons is used under a CC0 1.0
Public Domain Dedication license (https://creativecommons.org/publicdomain/zero/1.0/deed.en).
Figure 5.4.2
Helicase and single stranded binding proteins (1) by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 5.4.3
DNA polymerase and primase by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 5.4.4
DNA strands run antiparallel by Christine Miller is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.
Figure 5.4.5
Leading and lagging strand/ DNA Replication/ by yourgenome on Flickr is used under a CC BY-NC-SA 2.0 (https://creativecommons.org/licenses/by-nc-sa/2.0/) license.
References
Amoeba Sisters. (2019, June 28). DNA replication (Updated). YouTube. https://www.youtube.com/watch?v=Qqe4thU-os8&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 3.24 DNA Replication [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-3-the-nucleus-and-dna-replication CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)
yourgenome. (2015, June 26). DNA replication - 3D. YouTube. https://www.youtube.com/watch?v=TNKWgcFPHqw&feature=youtu.be
The smallest particle of an element that still has the properties of that element.
The movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.
