The fruit fly has $8$ chromosomes $\left(2n\right)$ in each cell. During the interphase of mitosis if the number of chromosomes at ${\mathrm{G}}_1$ phase is $8.$ What would be the number of chromosomes after the S phase?

The number of chromosomes remains the same after S-phase, but the amount of DNA becomes double.

S phase is the phase of the cell cycle in which DNA is replicated, occurring between the G₁ phase and G₂ phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during S-phase are tightly regulated and widely conserved.