# Explain how hydrogen bonding between nitrogenous bases contributes to the stability of the DNA double helix.

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

## Mark scheme (4 marks)

1. Hydrogen bonds form between complementary base pairs on opposite strands
2. Adenine pairs with thymine via two hydrogen bonds, and cytosine pairs with guanine via three hydrogen bonds
3. Although individual hydrogen bonds are weak, the large number along the length of the molecule collectively provides considerable stability
4. The relative weakness of individual hydrogen bonds allows the two strands to be separated during replication and transcription

## Key terms

- [hydrogen bond](https://www.gradenine.co.uk/glossary/hydrogen-bond)
- [nitrogenous bases](https://www.gradenine.co.uk/glossary/nitrogenous-bases)
- [double helix](https://www.gradenine.co.uk/glossary/double-helix)
- [stability](https://www.gradenine.co.uk/glossary/stability)

## Related

- [Revision notes for IB DP Biology Standard 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-how-hydrogen-bonding-between-nitrogenous-0c697662) · Published by Druglandscape Ltd.