Explain the significance of the G1 and G2 checkpoints in the cell cycle, distinguishing between the conditions each checkpoint monitors.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The G1 checkpoint checks that the cell has enough nutrients, has reached an adequate size and has received the required growth factors before it can commit to DNA replication. The G2 checkpoint verifies that DNA replication has finished correctly and that no DNA damage remains before the cell enters mitosis. If either checkpoint fails, CDK activity is inhibited or cyclin levels do not rise, so the cell cycle is halted. These checkpoints are crucial because they prevent the passage of damaged or incompletely replicated DNA, thereby reducing the risk of mutations that could lead to cancer or uncontrolled cell division.
Examiner tips
- Use the terms ‘G1’ and ‘G2’ checkpoints explicitly; mention nutrients, size, growth factors for G1 and DNA damage for G2.
- Show the consequence: CDK inhibition or cyclin failure, leading to cell cycle arrest.
- Explain the significance: prevention of mutation and cancer risk.
Common mistakes
- Confusing the functions of G1 and G2 checkpoints; e.g., saying G2 checks nutrients.
- Omitting the role of CDK/cyclin regulation in stopping the cycle.
- Failing to mention the link to cancer prevention.
Mark scheme (4 marks)
- G1 checkpoint assesses whether the cell has sufficient nutrients/size/growth factors to proceed with DNA replication
- G2 checkpoint monitors whether DNA replication has been completed accurately and checks for DNA damage before mitosis begins
- If conditions are not met at either checkpoint, cyclin-dependent kinase (CDK) activity is inhibited / cyclin levels do not rise, halting progression through the cycle
- The checkpoints are significant because they prevent propagation of damaged or incompletely replicated DNA, reducing the risk of mutations that could lead to cancer / uncontrolled cell division
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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