Biology · Cell Cycle and Cell Division · NEET
No. This is the single most tested trap. In S phase DNA replicates, so DNA content doubles (2C to 4C), but the chromosome NUMBER does not change. A cell with 8 chromosomes still has 8 chromosomes after S phase. Each chromosome now has two sister chromatids joined at one centromere, and one centromere means one chromosome. NEET 2021 asked exactly this with the fruit fly (8 to 8, not 16).
N counts how many chromosomes are present — you count centromeres. C measures the amount of DNA. They are counted differently. Replication in S phase changes only DNA amount (C), because it adds a second chromatid to each chromosome without adding a new centromere. Splitting the centromere later (anaphase) is what raises the chromosome number, because each freed chromatid becomes its own chromosome.
After S phase each of the 2n chromosomes carries two chromatids, so the DNA is doubled to 4C. But there are still only 2n centromeres, so it stays 2n, not 4n. The cell becomes 4n only if the centromeres split and the chromatids separate, which happens in mitotic anaphase (temporarily) or if division is blocked.
Reduction happens in Meiosis I, the reductional division. Homologous chromosomes separate, so number drops from 2n to n. But each n chromosome still has two chromatids, so DNA is 2C, not 1C. The final 1C is reached only after Meiosis II splits the chromatids. So after Meiosis I a cell is n/2C — NEET 2019 Odisha tested this.
DNA content halves when sister chromatids separate at a centromere split. In mitosis this is anaphase (4C back to 2C per daughter). In meiosis, the 1C gamete level is reached only after Meiosis II (anaphase II splits the centromere). Meiosis I reduces chromosome number but not DNA per chromosome.
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?
After meiosis I, the resultant daughter cells have
During cell growth, DNA synthesis takes place in
Try the real previous-year questions from this chapter — each with the answer and a full solution.
2n/2C is a normal diploid cell in G1 (2n chromosomes, 2C DNA). 2n/4C is after S phase and in G2 (number same, DNA doubled). n/2C is a cell after Meiosis I (number halved, each chromosome still 2 chromatids). n/1C is the final gamete after Meiosis II.
No. DNA content stays constant during G1 (2C) and during G2 (4C). It changes only in S phase (replication doubles it) and at centromere splitting during anaphase (halves it per daughter cell).
It is ONE chromosome, because it has one centromere. Two sister chromatids joined at a single centromere count as a single chromosome. Only when the centromere splits do the two chromatids become two separate chromosomes.
NEET repeatedly asks whether chromosome number or DNA content changes in a given phase (fruit fly 2021, meiosis I DNA 2019, S phase 2016). Mixing up N and C is a guaranteed lost mark, so the exact 2n/2C to 2n/4C to n/2C to n/1C track is high-yield.
