Question

You have studied mitosis in onion root tip cells. It has 16 chromosomes in each cell. Can you tell:
(ii) 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

Step 1: $G_1$ phase
This phase is also known as pre-DNA synthesis phase. In this phase, the cell prepares itself for DNA replication by forming proteins involved in the replication machinery. Considering the DNA content present in a diploid onion cell to be 2C, the DNA content per cell at $G_1$ phase is 2C.

Step 2: S phase
In this phase, the amount of DNA per cell doubles. Considering the initial amount of DNA to be 2C per cell, it increases to 4C after DNA replication.

Step 3: $G_2$ phase
Synthesis of proteins occurs in the G2 phase to make the cell ready for M phase. Hence, the amount of DNA per cell at $G_2$ phase is 4C.

Final Answer:
The amount of DNA per cell at $G_1$ phase, after S phase and at $G_2$ phase is 2C, 4C and 4C respectively.