Describe chromosome behaviour during mitosis
During prophase, metaphase, anaphase and telophase, spindle fibres organise and separate sister chromatids into two daughter nuclei.
Section 5.2 · Chromosome behaviour in mitosis
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain cell-cycle concepts and image interpretation. Written practice is self-checked; it does not demonstrate practical microscopy competence or replace teacher-reviewed assessment.
Before you begin
- Recognise DNA as genetic material and use the terms cell, nucleus and chromosome accurately.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At which stage do chromosomes align at the cell equator?
What you will learn
- Describe chromosome and spindle behaviour during prophase, metaphase, anaphase and telophase
- Relate nuclear-envelope changes to the stages of mitosis
Prophase and metaphase organise chromosomes
During prophase, chromosomes condense and become visible. The nuclear envelope breaks down, and a spindle forms. Spindle fibres attach to chromosomes at their centromere regions.
During metaphase, chromosomes line up at the equator of the cell. Spindle attachments from opposite poles position each chromosome so that sister chromatids can be separated.
Anaphase and telophase separate nuclei
During anaphase, centromeres divide and spindle fibres move sister chromatids to opposite poles. Once separated, each chromatid is treated as a chromosome.
During telophase, chromosomes reach the poles, become less condensed, and new nuclear envelopes form around each set. The spindle breaks down. The result is two nuclei with the same chromosome set, before cytokinesis separates the cell.
Pause and explain
What happens to sister chromatids during anaphase?
Worked example
A cell image shows chromosomes aligned across the centre with spindle fibres extending from both poles. Identify the stage and state what follows.
Show the worked solution
- Chromosomes at the equator identify metaphase.
- Anaphase separates sister chromatids to opposite poles.
Answer The cell is in metaphase because chromosomes are aligned at the equator with spindle fibres extending from opposite poles. Anaphase follows, when centromeres divide and sister chromatids move to opposite poles.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the main chromosome events in prophase and metaphase.
Give me a hint
State the chromosome appearance and position in each stage.
Compare my reasoning
- Chromosomes condense and the spindle forms in prophase.
- Chromosomes line up at the equator in metaphase.
In prophase, chromosomes condense and the spindle forms as the nuclear envelope breaks down. In metaphase, chromosomes line up at the cell equator with spindle fibres attached at their centromere regions.
Look for these in your work
- Both stages described
- Chromosome positions accurate
A micrograph shows sister chromatids moving towards opposite poles. Name the stage and explain the movement.
Give me a hint
Identify the stage after metaphase.
Compare my reasoning
- This is anaphase.
- Centromeres divide and spindle fibres move chromatids to opposite poles.
This is anaphase. Centromeres divide, and spindle fibres move sister chromatids to opposite poles; each separated chromatid is now a chromosome.
Look for these in your work
- Stage identified
- Movement mechanism stated
Outline the nuclear events in telophase and state the result.
Give me a hint
Think about chromosomes, spindle and nuclear envelope.
Compare my reasoning
- Chromosomes reach opposite poles and become less condensed.
- New nuclear envelopes form and the spindle breaks down, producing two nuclei.
In telophase, chromosomes reach opposite poles and become less condensed. The spindle breaks down and nuclear envelopes form around each chromosome set, producing two daughter nuclei.
Look for these in your work
- Chromosomes and spindle described
- Two nuclei stated
Common mistakes
- Putting chromosome alignment in prophase
- Saying sister chromatids separate during metaphase
- Calling cytokinesis a stage of nuclear division
What can you explain now?
At which stage do chromosomes align at the cell equator?
Compare with the explanation
Metaphase
Metaphase aligns chromosomes at the equator.
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Make it stick
What distinguishes prophase, metaphase, anaphase and telophase by chromosome position and nuclear-envelope behaviour?
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