# A student investigating DNA structure notices that the two strands of a DNA molecule are held together by base pairs. Describe the structure of DNA and explain the importance of complementary base pairing during DNA replication.

> WJEC A-Level Biology (Wales) — 1.5 Nucleic acids and DNA replication · Describe and Explain · 5 marks

> DNA is a polymer found in the nucleus of eukaryotic cells. It carries all the genetic information needed for an organism to function.

## Mark scheme (5 marks)

1. DNA is a polymer made of two strands twisted into a double helix
2. The two strands are held together by complementary base pairs
3. During replication, the two strands of the double helix separate / unzip
4. Each original strand acts as a template for a new complementary strand to be built
5. Two identical DNA molecules are produced, each containing one original strand and one new strand

## Key terms

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

## Related

- [Revision notes for WJEC A-Level Biology (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and 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/a-student-investigating-dna-structure-notices-d007cded) · Published by Druglandscape Ltd.