Bio H – Cell division
Understanding Chromosomes:
Bio H – Cell division
Chromosome Structure:
DNA is a long chain of nucleotides. Each nucleotide is attached together through dehydration synthesis. In
humans, it has been determined that the genome, or complete set of genetic information, is 3.2 billion
nucleotides long! In the cell however, the DNA doesn’t exist by itself. It is always wrapped around histone
proteins in one of two forms:
- Chromatin: DNA wrapped around histones; so thin and stringy that it is not visible in the nucleus
as anything but a solid color (usually we use a purple/blue stain to color DNA); The information in
the DNA is accessible when it is in the chromatin form; The DNA is copied when it is in Chromatin
form
DNA in Chromatin Form
-
Chromosomes: chromatin that has been tightly folded and wrapped up into tiny, distinct bundles
that are visible in the nucleus; the information in the DNA is NOT accessible when it is in
chromosome form; DNA is ONLY in chromosome form during cell division to make the DNA
easier to move into the two cells;
DNA in Chromosome Form
Ploidy:
All sexually reproducing organisms have two sets of chromosomes: one from the female parent, one from the
male parent. For this reason they are called diploid organisms and their cells referred to as 2N, meaning they
have two sets of chromosomes. The only cells in a diploid organism that do not have two sets of chromosomes
are the gametes, or reproductive cells, that form during meiosis. These cells have one set of chromosomes and
are referred to as haploid, or N, cells.
All asexually reproducing organisms have a single set of chromosomes and are also referred to as haploid or N,
cells.
The ploidy number refers to how many chromosomes the organism normally has. For instance humans are
said to have a ploidy number of 2N = 46. This means we have two copies of each chromosome (2N) and a
total of 46 chromosomes. Simple math tells us that if we have 46 total chromosomes and two of each kind, then
humans must contain 23 different types of chromosomes. A fruit fly is diploid also but has a ploidy number of
2N = 8. This means there are only 4 different types of fly chromosomes and flies inherit two of each for a total
of 8. A haploid organism might have a ploidy number of n = 3 meaning they have three different chromosomes
and only one copy of each.
Bio H – Cell division
Replication:
When the cell prepares to divide by mitosis, it must first copy all of the nucleotides in the DNA (3.2 billion in a
human cell). While the DNA is still in chromatin form, the cell makes an identical copy of each piece of DNA
in the nucleus. THEN each duplicated piece folds into a compact, duplicated chromosome that looks like the
structure below:
-
This structure is why we often draw chromosomes as an X. But remember, this X like shape is only
appropriate for chromosomes AFTER they’ve gone through replication. At this point there are two
identical copies of the chromosome attached at the center.
Chromosomes vs. Chromatids:
When chromosomes are in the duplicated form, each copy of the information is referred to as a sister
chromatid. However, it is still a single chromosome. Once the two identical sister chromatids are separated,
each one will become part of a different cell and is now referred to as a chromosome. Too complicated? Here’s
the trick: to figure out how many chromosomes you are looking at count the centromeres, or the middle of the
chromosomes. However many centromeres you count, that’s how many chromosomes you have.
Example:
So, before replication each piece of DNA is an unfolded piece of chromatin.
After replication, both copies fold up into an X like structure called a
duplicated chromosome made up of two sister chromatids. Each sister
chromatid is identical to the other.
During cell division we will pull apart the sister chromatids and their name
immediately changes to an unduplicated chromosome.
Finally at the end of cell division, the unduplicated chromosome will unfold
back into chromatin.
Homologous Chromosomes:
As we said, all diploid organisms have two sets of the same information. Each of your parents donated one
version of each of the 23 human chromosomes. BUT, unless your parents were identical twins (which would be
odd on several levels) it is unlikely that the version of chromosome #1 that you got from mom was the same as
the version of chromosome #1 that you got from dad. Different versions of the same chromosome are called
homologous chromosomes.
Each chromosome has hundreds if not thousands of genes on it (of the 20,000 – 25,000 genes in the human
genome). But there are many variations of each of those genes in the human population. For example let’s say,
hypothetically, that a person’s hair color gene is on chromosome #1. Is there only one version of this gene? No,
we know there are several different varieties. So, even though you inherit two #1 chromosomes, and they both
have hair color genes, they will likely have different variations of that gene. Variations of a gene are called
alleles.
Bio H – Cell division
Let’s simplify things a bit and use traits that we know about (although for the geneticists out there, we are
REALLY simplifying things) Pretend that there is only one gene for each of the following traits and only two
variations, called alleles, for each gene. Let’s also pretend there are only three genes on each chromosome and
that we know exactly which chromosome contains which genes. We’ll represent the alleles with capital and
lower case letters. (this will be the convention when we get to genetics.)
Chromosome:
Chromosome 1:
Chromosome 2:
Chromosome 3:
Genes:
Hair color
Alleles:
A(brown) or a (blonde)
Eye color
B (brown) or b (blue)
Height
C (tall) or c (short)
Skin color
D (fair) or d (dark)
Eye shape
E (round) or e (oval)
Hair texture
F (curly) or f (straight)
Dimples
G (dimples) or g (no dimples)
Ear lobes
H (attached) or h (unattached)
Freckles
I (no freckles) or i(freckles)
Two homologous chromosomes will have the same genes, but not necessarily the same alleles. Sister
chromatids will have the same genes AND the same alleles because they are identical copies created through
DNA replication. Interestingly, each time a piece of chromatin folds up into a chromosome, the genes end up in
the same place on the chromosome. This location of a particular gene is called a locus and can help geneticists
trace certain traits and diseases in organisms.
Example
chromosome
ABC
Example
Chromosome
abc
Comparison of two examples
These two would be unduplicated (only one copy of each) and
homologous (because carry different variations of the same genes)
(brown hair, brown (blonde hair, blue
eyes, tall)
eyes, short)
DEF
ABC
These are two completely different chromosomes
GHI
GHI
GhI
GhI
These would be duplicated, homologous chromosomes.
If we were to describe GHI and GHI they would be sister
chromatids.
The term homologous chromosomes refers more to the information present than the actual structure.
Homologous chromosomes are never attached to each other, BUT they will need to go through replication and
form sister chromatids before each round of cell division. So you can have homologous chromosomes that are
duplicated or unduplicated, in chromosome or chromatin form depending on where they are in the cycle.
Bio H – Cell division
Directions: Read through the attached article and then answer the questions IN YOUR OWN WORDS. You
will lose credit for “quoting” the reading. You must reword the information to receive credit for the assignment.
1. In your own words explain the difference in structure AND function between chromatin and
chromosomes.
2. Imagine an organism with a ploidy number of 2N = 6
a. First, explain in words what 2N = 6 means.
b. Now draw a circle to represent the nucleus. Inside the circle, clearly draw the chromosomes of
the above organism at a stage where each chromosome is duplicated. Use a different color to
represent each set of chromosomes (mom’s vs. dad’s) Use different sizes to represent different
types of chromosomes. Remember, homologous pairs will be similar in size even if they are part
of different sets. (example: chromosome #1 might be longer than chromosome #2 BUT mom’s
chromosome 1 shown in one color will be same length as dad’s chromosome 1 shown in another
color)
c. Now draw a picture of two different sperm cells that could come from the organism above.
Remember reproductive cells end up with only one of each homologous pair. Also remember
chromosomes will be unduplicated at this point)
d. What would the ploidy number of the reproductive cell above be?
Bio H – Cell division
3. In your words, what is the difference between an allele and a gene? Provide an example NOT already
given in this reading.
4. Use the chart in the reading to draw examples of chromosomes with the following criteria, use the
examples in the reading as a model.
a. Draw a single unduplicated chromosome that would have the information for a blonde, blue eyed, tall person.
b. Draw a single unduplicated chromosome that is homologous to the chromosome in (a).
c. Draw a duplicated chromosome for a fair skinned, oval eyed and straight haired person.
d. Label the sister chromatids in the chromosome for c.
These are thinking questions. You won’t find the answers spelled out in the reading, but you can figure it
out.
Mitosis and meiosis both make more cells. BUT they work differently. Look at the comparison chart below to
help you with the questions.
Mitosis
Meiosis
Types of cells
made
New cells are
Body cells (skin, kidney, liver, etc)
Reproductive cells (sperm or egg)
Identical to each other
All unique
New cells have
Pairs of homologous chromosomes
Only one of each homologous pair.
5. Why is it essential that sperm and egg cells only end up with one of each homologous pair of
chromosomes instead of two?
6. Why would we want the information in all our body cells to be identical, but be different in each
reproductive cell?
Bio H – Cell division
7. Mutations happen all the time in cell division. Sometimes cells end up with extra or missing
chromosomes. Sometimes the chromosomes break and parts of one get attached to another. Thing
about the effects of such a mutation happening in mitosis as compared to meiosis. Considering the types
of cells being made, what do you think the consequences might be? Compare the effects of these
mutations in each mitosis and meiosis.