Question 1

Why does DNA content double during S phase but chromosome number remain 2n?

Accepted Answer

Each chromosome makes one additional copy, increasing total DNA from 2C to 4C. The number of chromosomes is still counted at the centromere, so it remains 2n even though every chromosome now has two sister chromatids.

Question 2

Why are condensed X-shaped chromosomes not shown during G1 or G2?

Accepted Answer

Chromatin is relatively uncoiled during interphase, so individual chromosomes are not visible as separate X-shaped structures. Condensation into clearly visible chromosomes begins during prophase of M phase.

Question 3

What happens to chromosome number during mitosis?

Accepted Answer

During anaphase, centromeres split and sister chromatids move toward opposite poles. Each daughter nucleus receives 2n chromosomes, and after cytokinesis each daughter cell contains 2C DNA.

Question 4

How does cytokinesis occur in the depicted higher plant cell?

Accepted Answer

A cell plate develops in the centre and extends toward the periphery. It becomes a new partition between the daughter cells; a cleavage furrow is characteristic of animal-cell cytokinesis instead.

Question 5

What distinguishes G0 from the other cell-cycle stages?

Accepted Answer

G0 is a quiescent exit from the active cycle rather than a normal DNA-replicating stage. The cell may remain metabolically active without dividing, while the depicted exit from G1 retains 2C DNA and 2n chromosomes.

The Cell Cycle Clock: G1 to S to G2 to M and the Quiet G0

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