# Explain why DNA replication proceeds continuously on one template strand but discontinuously on the other.

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

## Mark scheme (4 marks)

1. DNA polymerase can only add nucleotides in the 5' to 3' direction
2. The two template strands are antiparallel, so they run in opposite directions (3' to 5' and 5' to 3')
3. On the leading strand, the template runs 3' to 5', so DNA polymerase can synthesise continuously towards the replication fork
4. On the lagging strand, the template runs 5' to 3', so DNA polymerase must synthesise short fragments (Okazaki fragments) in the direction away from the fork

## 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-why-dna-replication-proceeds-continuously-e88a89ce) · Published by Druglandscape Ltd.