A Levels Biology (9700)•9700/13/M/J/18
Explanation
Early Prophase Chromosome DNA Strands
Steps:
- DNA replicates in S phase, so chromosomes enter mitosis with two sister chromatids each.
- Prophase begins with chromosome condensation from chromatin threads.
- Each chromatid contains one DNA double helix composed of two antiparallel strands.
- One chromosome thus has two chromatids, totaling four DNA strands.
Why B is correct:
- B. 4 aligns with the definition of a duplicated chromosome having two chromatids, each with a double-stranded DNA molecule.
Why the others are wrong:
- A. 2: Counts chromatids, not individual DNA strands.
- C. 1: Applies to unreplicated G1 chromosomes with one chromatid.
- D. 2: Duplicates A, undercounting total strands.
Final answer: B
Topic: Chromosome behaviour in mitosis
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