A Levels Biology (9700)•9700/12/M/J/23
Explanation
DNA copies double during interphase and separate by cytokinesis
Steps:
- Cell enters interphase in G1 phase with one unreplicated copy of each DNA molecule per chromosome.
- S phase of interphase replicates DNA, yielding two identical copies (sister chromatids) per molecule.
- Mitosis aligns and separates sister chromatids to opposite poles, forming two nuclei each with one copy.
- Cytokinesis begins with the cell containing both nuclei, thus two copies total before cytoplasmic division completes.
Why B is correct:
- Matches cell cycle definition: unreplicated (1 copy) at interphase start, duplicated and separated (2 copies in one cell) at cytokinesis start.
Why the others are wrong:
- A: Cytokinesis starts with 2 copies in the undivided cell, not 1.
- C: Interphase starts with 1 copy before S-phase replication.
- D: Interphase starts with 1 copy, not 2.
Final answer: B
Topic: Replication and division of nuclei and cells
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