Biology · Cell Cycle and Cell Division · NEET
No. This is the single most tested trap. In S phase the DNA amount doubles (2C to 4C), but the chromosome NUMBER stays exactly the same. A cell with 2n = 8 chromosomes at G1 still has 8 chromosomes after S phase. Each chromosome simply copies itself into two sister chromatids that stay joined at one centromere, so it is still counted as ONE chromosome, not two.
We count chromosomes by counting centromeres. When DNA replicates, each chromosome makes an identical copy, but the two copies (sister chromatids) remain attached at a single centromere. Since there is still only one centromere per chromosome, the number stays the same even though there is now twice as much DNA (4C). The number only changes later, at anaphase, when the centromere splits.
Yes. NCERT states that during the S phase, while DNA replication begins in the nucleus, the centriole duplicates in the cytoplasm. This is a favourite NEET point. Note the timing: the centrosome DUPLICATES in S phase, but it only starts MOVING towards opposite poles later, in prophase of mitosis.
No. There is only a single cycle of DNA replication in the entire meiosis, and it happens in the S phase BEFORE Meiosis I. There is NO S phase and NO DNA replication before Meiosis II. NEET 2022 tested exactly this: 'DNA replication occurs in S phase of Meiosis-II' is an INCORRECT statement.
'C' measures the AMOUNT of DNA, while 'n' measures the NUMBER of chromosome sets. In S phase, the DNA content goes from 2C to 4C (amount doubles), but the ploidy stays 2n (number of sets unchanged). So after S phase a cell is 2n but 4C. Keep 'C' (DNA quantity) and 'n' (chromosome sets) as two separate ideas.
Among eukaryotes, replication of DNA takes place in
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 centriole undergoes duplication during:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In the S (Synthesis) phase, the cell's entire DNA is replicated. The DNA content per cell doubles from 2C to 4C, and the centriole also duplicates in animal cells. The chromosome number stays unchanged.
No. The chromosome number stays the same during S phase. Only the DNA amount doubles (2C to 4C). Each chromosome becomes two sister chromatids joined at a single centromere, so it is still counted as one chromosome.
2C to 4C means the amount of DNA per cell doubles. '2C' is the DNA content of a normal diploid cell before replication, and '4C' is after replication in S phase, before the cell divides.
The centrosome (centriole) duplicates in the S phase, according to NCERT. It only begins to move towards opposite poles later, during prophase of mitosis. This timing is a common NEET question.
No. DNA is replicated only once during meiosis, in the S phase before Meiosis I. There is no S phase and no DNA replication before Meiosis II.
