# Explain the roles of helicase and DNA gyrase (topoisomerase II) in preparing the DNA double helix for replication.

> IB DP Biology Higher Level (2023 syllabus) — D1.1 DNA replication · Explain · 4 marks

## Mark scheme (4 marks)

1. Helicase breaks/disrupts the hydrogen bonds between complementary base pairs, separating the two strands of the double helix.
2. Separation of strands at the replication fork causes positive supercoiling / overwinding ahead of the fork, creating torsional stress in the helix.
3. DNA gyrase (topoisomerase II) cuts both strands of the double helix, allowing the DNA to rotate / unwind, then re-ligates the strands.
4. 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

- [helicase](https://www.gradenine.co.uk/glossary/helicase)
- [DNA gyrase](https://www.gradenine.co.uk/glossary/dna-gyrase)
- [topoisomerase II](https://www.gradenine.co.uk/glossary/topoisomerase-ii)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-roles-of-helicase-and-5fd9f8f6) · Published by Druglandscape Ltd.