NEET 2021 Zoology Question 152 Solution

During interphase of the mitotic cell cycle, the cell transitions through $G_1$, $S$, and $G_2$ phases: - In $G_1$ phase: The chromosome number is diploid ($2n = 8$) and the DNA content is $2C$. - In $S$ phase (Synthesis phase): Semi-conservative replication of nuclear DNA occurs. The DNA quantity doubles from $2C$ to $4C$. - However, the newly formed sister chromatids remain attached to each other at the same centromere. Because chromosome count is defined strictly by the number of functional centromeres, the chromosome number does not increase.

For Drosophila melanogaster with $2n = 8$: - Chromosomes in $G_1$: 8 - Chromosomes in $S$: 8 (each consisting of two identical sister chromatids) - Chromosomes in $G_2$: 8 - The physical separation of sister chromatids into individual daughter chromosomes only occurs later during mitotic Anaphase.