Explain how DNA replication during S phase ensures that each daughter cell produced by mitosis receives a complete and identical copy of the genome.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA replication is semi‑conservative, producing two identical double helices from one parental molecule. Each replicated chromosome therefore has two sister chromatids, each carrying a complete copy of the DNA sequence. The entire genome is duplicated before mitosis starts. During anaphase the cohesin that holds the sister chromatids together is cleaved, allowing each chromatid to move to a different spindle pole. Consequently each daughter cell receives one chromatid from every replicated chromosome, so each daughter cell contains a complete and identical copy of the genome.
Examiner tips
- Show the semi‑conservative nature of replication first, then link to sister chromatids and centromere. Mention cohesin cleavage at anaphase and the movement of chromatids to poles. Keep the explanation concise and use the exact terms "sister chromatids", "centromere", "cohesin", "anaphase".
- Use a logical order: replication → sister chromatids → anaphase separation → identical genomes.
- Avoid unnecessary detail about enzymes or base pairing.
Mark scheme (4 marks)
- DNA replication produces two identical DNA double helices from one parental molecule (semi-conservative replication), so each chromatid of a replicated chromosome carries a complete copy of that DNA sequence.
- Each replicated chromosome consists of two sister chromatids joined at the centromere, meaning the entire genome is duplicated before cell division begins.
- During mitosis, sister chromatids are separated at anaphase when cohesin is cleaved, so one chromatid moves to each pole, ensuring each daughter cell receives one copy of every chromosome.
- Because both daughter cells each receive one sister chromatid from every replicated chromosome, the genome in each daughter cell is identical to the original parent cell genome.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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