Explain the significance of the G1 and G2 checkpoints in the cell cycle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Checkpoints are control points that monitor conditions before the cell cycle can proceed to the next phase.
1. The G1 checkpoint checks that the cell is large enough and that environmental conditions (nutrients, growth factors) are suitable for DNA replication.
2. The G2 checkpoint verifies that DNA replication has finished accurately and that any DNA damage has been repaired before mitosis.
3. By stopping the cycle if conditions are not met, these checkpoints prevent the transmission of mutations or incomplete/damaged DNA to daughter cells, thereby maintaining genetic integrity.
1. The G1 checkpoint checks that the cell is large enough and that environmental conditions (nutrients, growth factors) are suitable for DNA replication.
2. The G2 checkpoint verifies that DNA replication has finished accurately and that any DNA damage has been repaired before mitosis.
3. By stopping the cycle if conditions are not met, these checkpoints prevent the transmission of mutations or incomplete/damaged DNA to daughter cells, thereby maintaining genetic integrity.
Examiner tips
- Use the word ‘checkpoints’ first, then state what each (G1, G2) checks for, and finish with the consequence for genetic integrity.
- Include the key terms: ‘cell size’, ‘nutrients’, ‘growth factors’, ‘DNA replication’, ‘damage repair’, ‘genetic integrity’.
- Keep each point concise – 1–2 sentences per checkpoint.
Common mistakes
- Mixing up the order of G1 and G2 or forgetting to mention the size/nutrient check at G1.
- Failing to state that the checkpoints prevent mutation transmission or genetic instability.
Mark scheme (4 marks)
- Checkpoints are control points that monitor conditions before the cell cycle can proceed to the next phase.
- The G1 checkpoint assesses whether the cell is large enough and whether environmental conditions (e.g. nutrients, growth factors) are suitable for DNA replication to proceed.
- The G2 checkpoint ensures that DNA replication has been completed accurately and that any DNA damage has been repaired before mitosis begins.
- These checkpoints prevent the transmission of mutations or incomplete/damaged DNA to daughter cells, maintaining genetic integrity.
Key terms in this question
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