Explain why a cell in the metaphase of mitosis has twice the DNA content of a cell that has just completed cytokinesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mitosis is a form of cell division that produces two genetically identical daughter cells. The process is divided into distinct stages, each characterised by specific events involving the chromosomes and the cell.
Model answer (4 marks)
During interphase the cell undergoes S phase, where each chromosome is replicated, producing two identical sister chromatids.
At metaphase the replicated chromosomes are aligned at the metaphase plate; the cell still contains the full set of duplicated DNA, so its DNA content is twice that of a post‑mitotic cell.
In anaphase the sister chromatids separate and are pulled to opposite poles, halving the DNA in each half of the cell.
Cytokinesis then divides the cytoplasm, giving each daughter cell one complete set of chromosomes – the original, unreplicated DNA content.
At metaphase the replicated chromosomes are aligned at the metaphase plate; the cell still contains the full set of duplicated DNA, so its DNA content is twice that of a post‑mitotic cell.
In anaphase the sister chromatids separate and are pulled to opposite poles, halving the DNA in each half of the cell.
Cytokinesis then divides the cytoplasm, giving each daughter cell one complete set of chromosomes – the original, unreplicated DNA content.
Examiner tips
- State that DNA is replicated in S phase before mitosis.
- Explain that metaphase contains duplicated chromosomes.
- Mention separation of chromatids in anaphase/telophase.
- Show that cytokinesis delivers one set of DNA to each daughter cell.
Common mistakes
- Confusing metaphase with anaphase; forgetting that DNA is duplicated before mitosis.
- Claiming the cell has the same DNA content at all stages.
- Not linking cytokinesis to the halving of DNA content.
Mark scheme (4 marks)
- DNA replication occurs during S phase of interphase, before mitosis begins
- At metaphase the replicated chromosomes (consisting of two sister chromatids) are present but have not yet separated
- During anaphase/telophase the sister chromatids are separated and pulled to opposite poles, halving the DNA content at each pole
- Cytokinesis divides the cytoplasm so each daughter cell receives one set of chromatids, giving the original (unreplicated) DNA content
Key terms in this question
metaphase · mitosis · cytokinesis · DNA content
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Cell division, diversity and organisation questions
- Explain why cells in the meristem of a plant root tip have a higher proportion o…
- Explain why cells in the meristem of a plant root tip divide by mitosis rather t…
- Explain why cells in the same organism can have very different structures even t…
- Explain why cells in the meristem of a plant root tip are able to divide repeate…