Q.Analyse the events during every stage of cell cycle and notice how the following two parameters change

Two things change as a cell passes through the cell cycle: the chromosome number (N) and the DNA amount (C).

• G1 phase: Chromosome number is 2n (diploid) and DNA content is at its starting level, 2C.
• S phase: DNA is replicated, so the amount of DNA doubles from 2C to 4C. The chromosome number does not change — it stays 2n, because only the amount of DNA increases, not the count of chromosomes.
• G2 phase: Chromosome number remains 2n and DNA content stays at 4C. …

The key to this question is to keep two ideas separate: the number of chromosomes and the amount of DNA. These do not change at the same points in the cycle, and confusing them is a common mistake.

G1 phase

• The cell has just come out of mitosis. It is metabolically active and growing, but it has not yet copied its DNA.
• Chromosome number is 2n (diploid); DNA content is at the starting level, which we write as 2C.

S phase

• This is the period of DNA synthesis, when the DNA is replicated.
• The amount of DNA per cell doubles — from 2C up to 4C.
• Crucially, the chromosome number does not change: a cell that was 2n before S phase is still 2n afterwards. Each chromosome now consists of two chromatids, so there is more DNA, but the count of chromosomes is the same.

G2 phase

• The cell continues to grow and makes proteins in readiness for mitosis.
• Chromosome number is still 2n; DNA content remains at 4C.

M phase and the daughter cells

• During mitosis the sister chromatids are separated equally into two daughter nuclei. …

Step 1 — Keep the two quantities strictly separate: N = chromosome number, C = amount of DNA per set.

Step 2 — Track N and C phase by phase:

• G1: 2n chromosomes, 2C DNA (just came out of the previous division, no replication yet).
• S: DNA doubles (2C→4C) as it's replicated, but chromosome number stays 2n (each chromosome now has two chromatids, but is still counted as one chromosome).
• G2: still 2n, still 4C (replication is over, mitosis hasn't started). …