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 option is 8.

To understand why the number of chromosomes remains 8 after the S phase, let us break down the events of the cell cycle step-by-step:
1. G₁ Phase (Gap 1 Phase): In this stage of interphase, the cell is metabolically active and grows in size. The chromosome number of the fruit fly is 2n = 8. At this point, each chromosome consists of a single chromatid (a single DNA molecule).
2. S Phase (Synthesis Phase): During the S phase, DNA replication takes place. The amount of DNA in the cell doubles (from 2C to 4C). However, the replicated DNA strands (sister chromatids) remain joined together at a common region called the centromere.
3. Chromosome Count vs. DNA Content: In genetics, a chromosome is counted by the number of centromeres. Since the sister chromatids are still attached at a single centromere, they are still considered a single chromosome. Therefore, while the DNA content has doubled, the actual chromosome count does not change.

Thus, the number of chromosomes after the S phase remains the same as in the G₁ phase, which is 8.