# Explain how the antiparallel nature of DNA strands presents a challenge during replication, and outline how this challenge is resolved.

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

## Mark scheme (4 marks)

1. DNA polymerase can only synthesise new DNA in the 5′ to 3′ direction
2. Because the two template strands are antiparallel, only one (the leading strand template) runs 3′ to 5′ continuously toward the replication fork, allowing continuous synthesis
3. The other template strand (lagging strand template) runs 5′ to 3′ toward the fork, so synthesis must occur discontinuously / away from the fork
4. Short RNA primers are laid down repeatedly and DNA polymerase extends each primer, producing Okazaki fragments that are later joined by DNA ligase

## Key terms

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

## Related

- [Revision notes for IB DP Biology Higher 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-the-antiparallel-nature-of-7f69ffeb) · Published by Druglandscape Ltd.