How to count chromosomes and DNA molecules during mitosis?

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:

Metaphase- Anaphase- Telophase
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
  • No. of DNA molecule =8
  • No. of chromosome=8
By telophase and cytokinesis, the chromosomes and DNA molecules are distributed between two cells. 
  • 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
In Mitotic Stages
After cytokinesis of Mitosis

Take a piece of paper. Draw it and feel the difference in understanding.

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