# Explain why scientists describe DNA as a 'double helix'.

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

## Mark scheme (4 marks)

1. DNA consists of two strands (polynucleotide chains)
2. The two strands are held together by hydrogen bonds between complementary bases
3. The two strands run antiparallel / are orientated in opposite directions to each other
4. The double-stranded molecule is twisted / coiled into a spiral / helix shape

## Key terms

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

## 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-why-scientists-describe-dna-as-34ffab04) · Published by Druglandscape Ltd.