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.

Understanding the Cell Cycle Phases:
The cell cycle consists of interphase (which includes G₁, S, and G₂ phases) followed by the M phase (Mitosis).
During the G₁ phase (Gap 1), the cell is metabolically active and grows, but its DNA has not yet replicated. The fruit fly in this phase has 8 chromosomes.

What Happens During the S Phase (Synthesis Phase):
During the S phase, DNA replication occurs. This means the amount of DNA in the cell doubles.
However, even though the DNA content doubles, the DNA replication produces two identical sister chromatids for each chromosome, which remain joined together at the centromere.

Chromosome Count vs. DNA Amount:
A chromosome is counted by the number of centromeres. Because the sister chromatids remain attached at their centromere, they are still considered a single chromosome.
Therefore, the number of chromosomes remains the same before and after the S phase, even though each chromosome now consists of two sister chromatids instead of one.

Thus, if the number of chromosomes at the G₁ phase is 8, the number of chromosomes after the S phase remains 8.