# Explain how the structure of the DNA double helix allows it to be both stable under normal cellular conditions and yet accessible for transcription and replication.

> IB DP Biology Higher Level (2023 syllabus) — A1.2 Nucleic acids · Explain · 4 marks

## Mark scheme (4 marks)

1. Hydrogen bonds between complementary base pairs (A–T and G–C) contribute to the stability of the double helix
2. The large number of hydrogen bonds along the entire length of the molecule collectively provides strong overall stability, even though each individual hydrogen bond is weak
3. Because hydrogen bonds are individually weak, enzymes such as helicase can break them locally, allowing strand separation for transcription or replication without permanently disrupting the entire molecule
4. The sugar–phosphate backbone (phosphodiester bonds) provides a stable, covalently bonded framework that maintains the integrity of each strand during and after strand separation

## Key terms

- [double helix](https://www.gradenine.co.uk/glossary/double-helix)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-the-b5718e66) · Published by Druglandscape Ltd.