Question

During the study of mitosis in onion root tip cells, we found 16 chromosomes in each cell. Can you tell how many chromosomes will the cell have at the G1 phase, after the S phase, and after M- phase? Also, what will be the DNA content P of the cells after G1, after S phase, and at G2 phase if the content after M phase is 2 C.?

Answer 1

Hint:During cell cycle in interphase, many metabolic activities are observed. DNA replication takes place during the S phase and the amount of DNA becomes double but the number of chromosomes remains the same after the S phase because chromosomes separate during the anaphase of mitosis. The roots of plant cells divide through mitosis.

Complete answer:
DNA replication takes place at the S phase of interphase and the content of the DNA becomes double. It is given in the question itself that after the M phase DNA content is 2 C. After M Phase the DNA is divided equally into daughter cells. So after the M phase if the DNA content is 2 C it means before the M phase its content should be double than this. After S and G2 phase the DNA content P is 4 C and after G1 it is 2 C because DNA has not yet replicated.
DNA is found in a condensed form that is called chromosomes. The number of chromosomes is given 16. The number of chromosomes remains the same even after the S phase because two sister chromatids (replicated chromosomes) are attached through the centromere. So the number of chromosomes after the G1 phase will be 16 and after the S phase.
Chromosomes are separated only during anaphase. In anaphase, all sister chromatids move towards opposite poles and the number of chromosomes becomes double. In telophase, it will be the same but after cytokinesis, the real number of chromosomes is established again.
During the M phase at prophase and metaphase number of chromosomes is the same as during interphase.

Note: In mitotic division the number of chromosomes always remains the same after division. This type of division is found in somatic cells. In meiosis, the number of chromosomes becomes half after division. This type of division only happens in germ cells.