Explain the role of cohesins and the spindle checkpoint in ensuring accurate chromosome segregation during mitosis.

IB DP Biology Higher Level (2023 syllabus) — D2.1 Cell and nuclear division · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Cohesins are protein complexes that bind sister chromatids together along their length and at the centromere after DNA replication, keeping them linked until anaphase.

At the metaphase‑to‑anaphase transition, separase cleaves the cohesin complexes, releasing the sister chromatids so they can be pulled to opposite poles by the spindle microtubules.

The spindle (assembly) checkpoint monitors kinetochore attachment. If a kinetochore is unattached or improperly attached, it generates a ‘wait‑anaphase’ signal via the mitotic checkpoint complex (MCC).

This signal inhibits the anaphase‑promoting complex/cyclosome (APC/C), preventing premature activation of separase and thus ensuring that chromosomes are correctly bi‑oriented before segregation.

Examiner tips

  • Use the terms ‘cohesin’, ‘separase’, ‘spindle checkpoint’, ‘MCC’, and ‘APC/C’ as they are key words. Show the sequence: cohesin holds chromatids → checkpoint ensures correct attachment → MCC inhibits APC/C → separase cleaves cohesin.
  • common_mistakes
  • :
  • Confusing cohesin with condensin or describing cohesin only at the centromere. Failing to mention the MCC or APC/C as the key inhibitory pathway. Using vague phrases like ‘checks attachment’ without naming the checkpoint mechanism.

Mark scheme (4 marks)

  1. Cohesins hold sister chromatids together along their length / at the centromere following DNA replication.
  2. Cohesins are cleaved/degraded by separase at the metaphase-to-anaphase transition, allowing sister chromatids to be pulled to opposite poles.
  3. The spindle (assembly) checkpoint detects unattached or incorrectly attached kinetochores and halts progression through mitosis until all chromosomes are correctly bi-orientated.
  4. Unattached kinetochores generate a 'wait-anaphase' signal (via the mitotic checkpoint complex / MCC) that inhibits the anaphase-promoting complex (APC/C), preventing premature cohesin cleavage and non-disjunction.

Key terms in this question

cohesin

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