Question

The fruit fly has 8 chromosomes (2n) in each cell. During interphase of Mitosis if the number of chromosomes at G1 phase is 8, what would be the number of chromosomes after S phase

• A. 32
• B. 8
• C. 16
• D. 4

Answer 1

The Correct Option is B

The question concerns the number of chromosomes present in a cell during the different phases of the cell cycle, specifically in the context of mitosis.

The fruit fly has 8 chromosomes in each diploid cell (2n = 8). Let's understand the process in steps:

1. G1 Phase: During the G1 phase, each cell of the fruit fly contains 8 chromosomes. This is prior to DNA replication.
2. S Phase (Synthesis Phase): During this phase, DNA replication occurs, but it is crucial to note that the number of chromosomes does not increase. Instead, each chromosome is replicated to form two sister chromatids, but these still count as a single chromosome because they are joined at the centromere.
3. Result of the S Phase: After the S phase, the number of chromosomes remains the same at 8. However, each chromosome now consists of two sister chromatids.

This understanding helps us to reason why the number of chromosomes post-S phase is still 8, even though the DNA content has effectively doubled.

Considering the provided options:

• 32: This would be incorrect as it suggests an increase in chromosome number, which does not happen.
• 8: Correct, as the number of chromosomes remains the same post-S phase.
• 16: Incorrect, as this would imply doubling the number of chromosomes, which isn't what occurs.
• 4: Incorrect, as this suggests a decrease, which does not occur.

Thus, the correct answer is 8 chromosomes after the S phase.