Question

You pulse label cells in an asynchronous culture for 30 minutes with CH] thymidine, a radioactive label incorporated into newly synthesized DNA. You immediately wash and fix the cells. You observe that 10% of cells in the culture are radioactively labeled. 5% of the cells in the culture have unlabeled mitotic chromosomes. You know that your cells complete the entire cell cycle in 20 hours. How long is M phase?

A) 8 hours
B) 2 hours
C)1 hour
D) 4 hours

Answer 1

The length of the M phase is calculated by multiplying the fraction of cells in M phase (5%) by the total duration of the cell cycle (20 hours), resulting in a duration of 1 hour for the M phase. Therefore, the correct option is C.

Step-by-step explanation:

To determine the length of the M phase given that 5% of the cells in the culture have unlabeled mitotic chromosomes and the cells complete the entire cell cycle in 20 hours, we can apply the following logic:

The M phase is the period during which mitosis occurs, and chromosomes would be visible and not incorporating any new thymidine.

Since 5% of the cells are in mitosis without radioactive labeling, this represents the fraction of the cell cycle dedicated to the M phase.

To calculate the duration of the M phase, we would therefore multiply the fraction of cells in the M phase (5%) by the total duration of the cell cycle (20 hours).

Thus, 0.05 × 20 hours gives us 1 hour. Therefore, the length of the M phase is 1 hour (C).