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 ?

The correct answer is 8.

Let us understand the behavior of chromosomes and DNA content during the cell cycle step-by-step:

1. Understanding the Cell Cycle Phases:
The cell cycle consists of Interphase (which includes G₁, S, and G₂ phases) followed by Mitosis (M phase).
During the G₁ phase (Gap 1), the cell is metabolically active and grows, but does not replicate its DNA. The chromosome number of the fruit fly is given as 8 (2n = 8). At this stage, each chromosome is composed of a single chromatid.

2. What Happens During the S Phase (Synthesis Phase)?
During the S phase, DNA replication takes place. The amount of DNA in the nucleus doubles (for example, from 2C to 4C).
However, the replication of DNA does not change the total number of chromosomes. Instead, each chromosome now consists of two sister chromatids joined together at the centromere. Since the centromere count remains the same, the chromosome number does not double.

3. Conclusion:
Because the chromosome number is determined by the count of centromeres and not the number of chromatids, the number of chromosomes remains unchanged after the S phase.
Therefore, since the number of chromosomes at the G₁ phase is 8, the number of chromosomes after the S phase remains 8.