Explain why cells in S phase must complete DNA replication before entering mitosis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During the cell cycle, a cell in S phase replicates its entire genome. Regulatory checkpoints monitor the completion of this process before the cell is permitted to enter mitosis.
Model answer (4 marks)
Each chromosome must consist of two identical sister chromatids joined at the centromere before mitosis begins.
If replication is incomplete, daughter cells would receive insufficient or unequal genetic material.
During mitosis the sister chromatids are separated so that each daughter cell receives one copy of each chromosome; this only produces genetically identical cells if replication was complete.
The G2 checkpoint (DNA‑damage/replication checkpoint) detects unreplicated or damaged DNA and prevents entry into mitosis until replication is verified as complete.
If replication is incomplete, daughter cells would receive insufficient or unequal genetic material.
During mitosis the sister chromatids are separated so that each daughter cell receives one copy of each chromosome; this only produces genetically identical cells if replication was complete.
The G2 checkpoint (DNA‑damage/replication checkpoint) detects unreplicated or damaged DNA and prevents entry into mitosis until replication is verified as complete.
Examiner tips
- Use the exact terminology – ‘sister chromatids’, ‘centromere’, ‘G2 checkpoint’. Show the logical sequence: replication → checkpoint → mitosis. Include the consequence for daughter cells (unequal DNA).
Common mistakes
- Confusing S phase with G1 or G2; forgetting to mention the G2 checkpoint. Using vague phrases like ‘cell needs to finish DNA’ instead of ‘each chromosome must have two identical sister chromatids’.
Mark scheme (4 marks)
- Each chromosome must consist of two identical sister chromatids (joined at the centromere) before mitosis begins.
- If replication is incomplete, daughter cells would receive insufficient or unequal genetic material.
- During mitosis, sister chromatids are separated so that each daughter cell receives one copy of each chromosome; this only produces genetically identical cells if replication was complete.
- The G2 checkpoint (DNA damage / replication checkpoint) detects unreplicated or damaged DNA and prevents entry into mitosis until replication is verified as complete.
Key terms in this question
S phase · DNA replication · mitosis
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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