Question

Onion root tip cells have 16 chromosomes in each cell. How many chromosomes will the cell have at the \[{G_1}\] 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?

Answer 1

Hint: In onion root tip cells, cells undergo mitotic division. Mitosis is also called somatic cell division as it occurs in body cells during normal growth. It is also called equational division as it produces two daughter cells exactly similar to that of the mother cell.

Complete answer:
 We know that the cell cycle consists of two stages. They are interphase and mitotic phases. Interphase which is also called a resting stage consists of three phases. They are ${G_1}$ , S phase and ${G_2}$ phase. During S phase, DNA is synthesised, but there will be no change in chromosome number. Keep in mind that chromosome number remains unchanged throughout the mitotic division. Note that DNA content is doubled in S phase and is again halved after M phase.
In the question, the initial chromosome number is unchanged in all phases.
So, the DNA content will be:
At ${G_1} = 16$, at ${G_2} = 16$ , after S phase $ = 16$ and after M phase $ = 16$
As it is said that the DNA content after M phase is 2C, DNA content in other stages will be:
${G_2} = 4C$ at ${G_1} = 2C,$after S phase$ = 4C,$after ${G_2} = 4C$.
Here, we can see that daughter cells formed have the same chromosome number to that of the mother cell. So, mitosis ensures genetic and metabolic constancy among the cells of an organism as well as the members of a species.

Note: Meiosis is the type of cell division seen in diploid cells. Here, the chromosome number is reduced by half and forms four haploid cells. So, this type of division is also called reduction division.