Investigation 1: Homologous chromosomes? Exploring the relationship of human autosomes
and sex chromosomes.
To view and compare information about human chromosomes, visit the National Center for
Biotechnology Information (NCBI) website:
http://www.ncbi.nlm.nih.gov/
From the main page, change the drop-down menu selection from "All Databases" to
“Genome”. Without filling in anything in the search field, hit the “Search” button.
Click “Human Genome” under the heading Custom Resources in the middle of the page.
On the left, you will see "Browse your Genome" and a drop-down menu to choose the type of
chromosome map shown. Each map represents a physical chromosome and provides information
about the chromosome, such as the array of genes on the chromosome.
To learn more about the arrangement of genes on chromosomes, let's first look at the genes on
the largest chromosome, chromosome 1.
View the chromosome by clicking on the appropriate chromosome in the display (it may take
a little while to load the map). Scroll down through the page to view the gene map and the
“Summary of Maps”, which includes how large the chromosome is in base pairs, how many
genes are on the chromosome, and links to other information. Note that you can also zoom in
and zoom out on sections of the chromosomes. To view the entire chromosome and answer the
questions below, you will need to be fully zoomed out to view the entire chromosome at once.
Chromosome 1 is the largest chromosome; that is, chromosome 1 is made up of the most single
nucleotide bases (in the double stranded DNA molecule, adenine base pairs with thymine,
guanine base pairs with cytosine). One megabase is equal to one million nucleotide bases.
Answer the following questions in your laboratory notebook in the “Answers/Conclusions”
section:
Question 1-1 : How many megabases (M bp) long is chromosome 1?
Question 1-2 : How many genes are located on the chromosome (Total genes on chromosome)?
Question 1-3: Why do you think there are so many more transcripts that align on the
chromosome compared to the number of genes on the chromosome?
Question 1-4: What would you expect to find if you were to compare the number of genes
present on the maternally and paternally inherited chromosomes for this autosomal chromosome
pair in a normal individual? Explain.
Now let’s look at the smallest chromosome, chromosome 21.
Return to the Genome Browser page, and select chromosome 21 by clicking on it (it may take
a little while to load the map).
Answer the following questions in your laboratory notebook in the “Answers/Conclusions”
section:
Question 1-5 : How many megabases (M bp) long is chromosome 21?
Question 1-6 : How many genes are located on the chromosome (Total genes on chromosome)?
Question 1-7: Would you expect to observe the same genes in the same order on the maternally
and paternally inherited chromosomes for this autosomal chromosome pair in a normal
individual? Explain.
Now, let’s compare the array of genes on the human X and Y chromosomes.
Return to “Browse your Genome” as before.
View the Y chromosome by clicking on it (it may take a little while to load the map). Scroll
down through the page to view the chromosome gene map and the “Summary of Maps.”
Answer the following questions in your laboratory notebook in the “Answers/Conclusions”
section:
Question 1-8 : How many megabases (M bp) is the Y chromosome?
Question 1-9 : How many genes are located on the Y chromosome (Total genes on
chromosome)?
Open a new tab (or a new window) in your internet browser. Navigate to the Genome Browser
(http://www.ncbi.nlm.nih.gov/ All Databases Human Genome).
View the X chromosome by clicking on the chromosome.
Answer the following questions in your laboratory notebook in the “Answers/Conclusions”
section:
Question 1-10 : How many megabases is the X chromosome?
Question 1-11 : How many genes are located on the X chromosome?
Question 1-12 : Given the gene maps you have just observed, would you consider the X and Y
human chromosomes homologous?
The human X and Y chromosomes are hypothesized to have evolved from homologous
autosomes into the dimorphic sex chromosomes they are today (11-17). Vertebrates employ
different mechanisms of sex determination: not all vertebrates have X and Y chromosomes,
many vertebrates do not have sex chromosomes at all (11).
The X and Y chromosomes do have some shared homologous elements that remain, although
these shared regions are very small compared to the shared regions of homology of autosomal
pairs. Recombination can still occur at these shared regions of the X and Y chromosomes,
particularly at the ends of the chromosomes where telomeres are located. Additionally, there are
several genes located at the ends of the X and Y chromosomes that are homologous and do
participate in recombination during meiosis. These regions are known as pseudoautosomal
regions (PARs) (18-20).
Answer the following question in your laboratory notebook in the “Answers/Conclusions”
section:
Question 1-13: Given what you know about DNA replication, why do all eukaryotic (linear)
chromosomes have telomeres? What purpose do telomeres serve?