Explain how the spindle apparatus ensures the accurate distribution of chromosomes to daughter cells during mitosis.

IB DP Biology Standard 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)

Spindle fibres (microtubules) attach to chromosomes at the centromere/kinetochore during prometaphase and metaphase. Each sister chromatid is attached to fibres from opposite poles, so chromosomes line up at the metaphase plate. In anaphase the fibres shorten, pulling the sister chromatids apart to opposite poles. This guarantees that each daughter cell receives one copy of every chromosome, maintaining the correct chromosome number.

Examiner tips

Use the terms ‘spindle fibres’, ‘centromere/kinetochore’, ‘metaphase plate’ and ‘shorten’ to show understanding of the process.
Show the sequence: attachment → alignment → separation to demonstrate the logical flow of mitosis.

Common mistakes

Confusing the role of the spindle with the nuclear envelope; the spindle forms after nuclear envelope breakdown.
Omitting the fact that each sister chromatid is attached to opposite poles, which is key to accurate segregation.

Mark scheme (4 marks)

Spindle fibres (microtubules) attach to chromosomes at the centromere / kinetochore during prometaphase/metaphase.
Each sister chromatid of a chromosome is attached to spindle fibres from opposite poles, ensuring chromosomes align at the metaphase plate / equator.
During anaphase, spindle fibres shorten/contract, pulling sister chromatids apart to opposite poles.
This ensures each daughter cell receives one copy of each chromosome (the correct/same chromosome number as the parent cell), maintaining genetic continuity.

Key terms in this question

spindle apparatus