# Explain the role of complementary base pairing in ensuring accurate DNA replication.

> IB DP Biology Standard Level (2023 syllabus) — D1.1 DNA replication · Explain · 4 marks

## Mark scheme (4 marks)

1. The two strands of the double helix are separated / hydrogen bonds between base pairs are broken, exposing the template strands.
2. Each exposed template strand is read in the 3' to 5' direction and free nucleotides are added in a 5' to 3' direction by DNA polymerase.
3. Complementary base pairing rules (A–T and C–G) mean only the correct nucleotide can bond to each position on the template strand.
4. The result is two identical DNA molecules each consisting of one original (parental) strand and one newly synthesised strand — semi-conservative replication — preserving the genetic information.

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-role-of-complementary-base-3e80ac35) · Published by Druglandscape Ltd.