NCERT Page No 164
Onion has 16 chromosomes in each cell.
Q.1. How many chromosomes will the cell have at G1 phase, after S-phase and after M-phase?
Ans: At G1-phase number of chromosome remain same therefore it is 16.
After S-phase number of chromosome remain same but DNA number doubles therefore it is 16.
After M-phase number of chromosome remain same but DNA number halves therefore it is 16.
Q.2. What will be the DNA content of cells at G1 phase, after S-phase and at G2- phase, if the content after M-phase is 2C.
Ans: If after M-phase DNA content is 2C,
Then, at G1- phase it is 2C, because neither chromosome number or DNA content increases or decreases in this phase.
After S-phase DNA content become 4C, because after this phase number of chromosome remain same but DNA number doubles.
At G2- phase DNA content remains as 4C, because after S-phase no change in numbers of chromosome and DNA content.