Question

Onion root tip cells have 14 chromosomes in each cell. Can you tell 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?

Answer 1

There are 14 chromosomes in each cell of the onion. At the G${}_1$ phase, the cell prepares protein machinery for the S-phase, so the number of chromosomes will be 14. After S-phase, DNA duplication has occurred, but the number of chromosomes will remain the same as 14. After M-phase, the duplicated chromatid will separate equally to the daughter cells. So, each of the daughter cells will have 14 chromosomes.

If the content of DNA is 2C after M-phase, then it is 2C at G${}_1$ because there is no duplication of DNA in this phase. It will be 4C after S-phase and will remain 4C till G${}_2$ phase as the chromosomes now have duplicated DNA.