# Explain how complementary base pairing in DNA contributes to the accurate replication of genetic information.

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

## Mark scheme (4 marks)

1. Each base on the template strand pairs with only one specific complementary base (A with T, C with G), ensuring the correct nucleotide is inserted opposite each template base.
2. The two strands of the parent DNA are separated (e.g. by helicase breaking hydrogen bonds), each strand acting as a template for the synthesis of a new complementary strand.
3. Free deoxyribonucleotides align opposite each template base via hydrogen bonds, and DNA polymerase joins them by forming phosphodiester bonds, producing a new complementary strand.
4. The result is two identical DNA molecules each consisting of one original (parental) strand and one newly synthesised strand, preserving the base sequence — this is semi-conservative replication.

## 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-how-complementary-base-pairing-in-d644ac4a) · Published by Druglandscape Ltd.