# Explain how the base pairing rule ensures that the two strands of a DNA molecule are complementary to each other.

> WJEC GCSE Biology (Wales) — 2.3.1 DNA structure · Explain · 4 marks

## Mark scheme (4 marks)

1. Each base on one strand pairs with a specific base on the other strand
2. Adenine pairs with thymine (A–T)
3. Cytosine pairs with guanine (C–G)
4. Because the base sequence on one strand determines/fixes the base sequence on the other strand, the strands are complementary

## Key terms

- [base pairing rule](https://www.gradenine.co.uk/glossary/base-pairing-rule)
- [complementary](https://www.gradenine.co.uk/glossary/complementary)

## Related

- [Revision notes for WJEC GCSE Biology (Wales)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-base-pairing-rule-adf5797a) · Published by Druglandscape Ltd.