Explain how the spindle apparatus ensures that each daughter cell receives a complete set of chromosomes during mitosis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Spindle fibres (microtubules) attach to chromosomes at the centromere via the kinetochore.
Sister chromatids of each chromosome are attached to spindle fibres from opposite poles (bi‑orientation), generating tension.
At anaphase, cohesins holding the sister chromatids together are cleaved, allowing the spindle fibres to pull one chromatid to each pole.
Because one chromatid from every chromosome is delivered to each pole, both daughter cells receive a complete, identical set of chromosomes.
Sister chromatids of each chromosome are attached to spindle fibres from opposite poles (bi‑orientation), generating tension.
At anaphase, cohesins holding the sister chromatids together are cleaved, allowing the spindle fibres to pull one chromatid to each pole.
Because one chromatid from every chromosome is delivered to each pole, both daughter cells receive a complete, identical set of chromosomes.
Examiner tips
- Use the term ‘kinetochore’ and ‘bi‑orientation’ to show understanding of attachment. Show the sequence: attachment → tension → cohesin cleavage → separation. Explain that each pole receives one chromatid per chromosome to justify equal chromosome number.
Common mistakes
- Forgetting to mention the kinetochore. Saying ‘spindle pulls chromosomes’ without specifying sister chromatids. Claiming the spindle ensures identical DNA without explaining the bi‑orientation mechanism.
Mark scheme (4 marks)
- Spindle fibres (microtubules) attach to chromosomes at the centromere via the kinetochore.
- Sister chromatids of each chromosome are attached to spindle fibres from opposite poles (bi-orientation / amphitelic attachment), ensuring tension is generated.
- At anaphase, cohesins holding sister chromatids together are cleaved, allowing the spindle fibres to pull one chromatid (now a chromosome) to each pole.
- Because one chromatid from every chromosome is delivered to each pole, both daughter cells receive a full, identical set of chromosomes (same number as the parent cell).
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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