Chromosome, chromatin reticulam, chromonemata, DNA molecule, gene. Is it
confusing? All these terms are denoting the same thing that is nucleic acid
protein complex called as chromatin. We call it differently at different stages
of cell cycle. Chromatin reticulum or chromonemata is the chromatin seen as
network under microscope during interphase or prophase. Gene is a segment
of DNA coding for a functional polypeptide, ribozyme or RNAs. Each chromosome
contains many genes. We use the term chromosome to denote the most condensed
form of chromatin that is metaphase chromosome. A functional chromosome
has three essential components; a centromere, a pair of telomeres and origin of
replication (ori).
Now our problem is counting the number
of chromosomes and DNA molecules at different stages of cell cycle. For
more on cell cycle you can refer the post cell cycle the history of cell. I found this as a question in many competitive exams in biology.
Remember, this question will check our level of understanding of the term
‘chromosome or DNA molecule’. Let us first discuss some basics regarding
chromosome.
Sometimes chromosomes possess single
chromatids at times it has two chromatids. This is the case with DNA molecules
also may be one or two. The reason is simple, before replication, chromosome
has one chromatid, after S phase or replication. The DNA content is doubled. So
chromosome has two chromatids after replication.
It is very simple to count number of
DNA molecules or chromosome during different stages of cell cycle.
Rule of thumb:
- The number of chromosome = count the number of
functional centromere
- The number of DNA molecule= count the number
of chromatids.
Now let us have a look into the number
of chromosomes and DNA molecules at different stages of cell cycle. Let
the chromosome number of a cell at G1 phase of interphase is 8. First
think carefully in what all stages changes in number of DNA molecules and
chromosomes takes place.
When the DNA molecule number change?
- The number of DNA molecules increase only when
DNA replicates that is only in S phase or synthetic phase.
- The number of DNA molecules decreases only
when the cell divides
Now when the chromosome number change?
Chromosome number the thing is little
tricky. Remember
- The number of chromosomes increases only when
sister chromatids separated during anaphase of mitosis and anaphase II of
meiosis. Think, in anaphase I of meiosis, only homologous chromosomes are
separated. Only in anaphase II sister chromatids are separated.
- As in the case of DNA molecule, reduction in
chromosome number takes place only by cell division.
Re read these 4 points. Imagine and
make it yours. Now you can answer any question regarding this.
Now let us work out the question with
the above points in mind.
Have a close look at the picture,before going into the detail:
 |
| Interphase-Prophase-Prometaphase |
 |
| Metaphase- Anaphase- Telophase |
We have a cell with chromosome
number 4 at G1.
At G1 phase:
- No. of DNA molecule =4
- No. of chromosome=4 and each chromosome has a single chromatid
At G2 phase:
- In S phase DNA replicates. So no. of DNA molecule in G2 will be =8
- No. of chromosome= 4 and as two DNA molecules are held at a common centromere. Or in other words each chromosome has two chromatids.
During Mitosis, there is no
change in no. of DNA molecule and chromosomes in prophase and metaphase.
At Metaphase
- No. of DNA molecule =8
- No. of chromosome=4
At Anaphase,
Here the sister chromatids separate and become independent chromosome. At this point the chromosome number also increases
Here the sister chromatids separate and become independent chromosome. At this point the chromosome number also increases
- No. of DNA molecule =8
- No. of chromosome=8
- After mitosis the newly formed cells has no. of DNA molecule =4
- No. of chromosome=4 and now in G1
After mitosis the two cells are
formed with chromosome number same as parent cell.
In the figure initial chromosome
number is 4 (2n=4). Then the changes in no. of DNA molecules and chromosomes
during mitosis in the table. Correlate this with
figure
Cell cycle Stages
|
Number of DNA
molecules /cell
|
Number of
Chromosomes/cell
|
G1
|
4
|
4
|
G2
|
8
|
4
|
In Mitotic Stages
|
||
Metaphase
|
8
|
4
|
Anaphase
|
8
|
8
|
After cytokinesis of Mitosis
|
4
|
4
|
Take a piece of paper. Draw it
and feel the difference in understanding.