Explain the roles of helicase and DNA gyrase (topoisomerase II) in preparing the DNA double helix for replication.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Helicase breaks the hydrogen bonds between complementary bases, separating the two strands at the replication fork.
The separation of strands creates positive supercoiling (over‑winding) ahead of the fork, generating torsional stress.
DNA gyrase (topoisomerase II) cuts both strands of the DNA, allowing the helix to rotate and unwind, then re‑ligates the strands.
By relieving the torsional stress, gyrase permits helicase to continue unwinding, enabling replication to proceed smoothly.
The separation of strands creates positive supercoiling (over‑winding) ahead of the fork, generating torsional stress.
DNA gyrase (topoisomerase II) cuts both strands of the DNA, allowing the helix to rotate and unwind, then re‑ligates the strands.
By relieving the torsional stress, gyrase permits helicase to continue unwinding, enabling replication to proceed smoothly.
Examiner tips
- Use the term ‘hydrogen bonds’ for helicase action. Mention ‘positive supercoiling’ and ‘torsional stress’ for the problem. Show that gyrase cuts both strands and re‑ligates. Explain the effect: relief of stress to allow continued unwinding.
Common mistakes
- Confusing helicase with DNA polymerase. Forgetting to mention positive supercoiling. Saying gyrase only removes supercoiling without describing cutting and re‑ligation.
Mark scheme (4 marks)
- Helicase breaks/disrupts the hydrogen bonds between complementary base pairs, separating the two strands of the double helix.
- Separation of strands at the replication fork causes positive supercoiling / overwinding ahead of the fork, creating torsional stress in the helix.
- DNA gyrase (topoisomerase II) cuts both strands of the double helix, allowing the DNA to rotate / unwind, then re-ligates the strands.
- This relieves torsional stress ahead of the fork, allowing helicase to continue unwinding and replication to proceed without the DNA becoming too tightly coiled to unwind.
Key terms in this question
helicase · DNA gyrase · topoisomerase II
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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