The Mitotic Cell Cycle: Question 1

Worked solution

Step 1: Place S phase within the cell cycle

The mitotic cell cycle is made up of interphase (G1, S and G2) followed by mitosis and then cytokinesis. Interphase is often mistakenly thought of as a “resting” stage, but the cell is highly active throughout it: G1 and G2 are growth phases in which the cell increases in size and makes the proteins and organelles it will need, while the S phase in between is specifically when the cell’s DNA is copied.

Step 2: Recall what actually happens during S phase

During S phase, every chromosome in the nucleus is replicated by semi-conservative DNA replication. Before S phase, a chromosome consists of a single DNA molecule. After S phase, that same chromosome consists of two identical sister chromatids, joined together at a centromere, because the original DNA molecule and its exact copy remain attached at this point. The chromosome number of the cell does not change, but because each chromosome now contains twice as much DNA, the total amount of DNA in the nucleus doubles.

Step 3: Evaluate each option

• A: incorrect, this describes G1 or G2, where growth and protein synthesis occur without any change in DNA content; S phase specifically changes the amount of DNA.
• B: correct. DNA replication in S phase produces two sister chromatids per chromosome and doubles the DNA content of the nucleus.
• C: incorrect. Chromosome condensation and becoming visible as two chromatids happens during prophase of mitosis, not S phase; the chromatids exist after S phase but are not yet condensed.
• D: incorrect. Nuclear envelope breakdown and spindle formation are also events of prophase, well after S phase has finished.

Final answer

• S phase replicates each chromosome into two identical sister chromatids and doubles the nuclear DNA content, option B.