Onion root tip cells have 16 chromosomes in each cell. How many chromosomes will the cell have at $G_{1}$ phase, after S phase, and after M phase?
Also, what will be the DNA content of the cells at $G_{1}$ , after S and at $G_{2}$ , if the content after M phase is 2C?

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Solution

During the $G_{1}$ phase the chromosome number will remain the same as the given chromosome number of the onion root tip cell. The DNA content will be the initial DNA content of the cell.
The number of chromosomes present in the cell during the $G_{1}$ phase is 16.
The DNA content during the $G_{1}$ phase is given as 2C.

The doubling of genetic material takes place during the S phase. This is why the DNA content is doubled, but the number of chromosomes present in the cell remains the same.
The number of chromosomes present in the cell during the S phase is 16.
The DNA content during the S phase is given as 4C.

During the $G_{2}$ phase, the DNA content remains double the initial content as the cell enters the M phase. This is because the $G_{2}$ phase occurs just after the S phase and the cell prepares for division during this phase. The chromosome number still remains the same.
The number of chromosomes present in the cell during the $G_{2}$ phase is 16.
The DNA content during the $G_{2}$ phase is given as 4C.

During the anaphase stage of mitosis, the sister chromatids are separated and at the end of the M phase, the two separate nuclei start forming. The process of karyokinesis is completed for the two cells. The number of chromosomes remains the same in the daughter cells, but the DNA content gets back to its initial DNA content due to the separation of sister chromatids.
The number of chromosomes present in the cell during the S phase is 16.
The DNA content during the S phase is given as 2C.

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