# Explain the role of RNA primers in DNA replication and why they must be replaced with DNA before replication is complete.

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

## Mark scheme (4 marks)

1. DNA polymerase III can only add nucleotides to an existing 3′-OH group / cannot initiate a new strand de novo.
2. Primase synthesises a short RNA primer complementary to the template strand, providing the 3′-OH group needed for DNA polymerase III to begin elongation.
3. RNA primers must be removed because RNA nucleotides in the final DNA molecule would create an incorrect / non-functional sequence / RNA is chemically different from DNA and would compromise the integrity of the double helix.
4. DNA polymerase I (in prokaryotes) / RNase H and DNA polymerase δ (in eukaryotes) remove the RNA primers and replace them with DNA nucleotides; DNA ligase then seals the remaining nick / joins the Okazaki fragments.

## Key terms

- [RNA primer](https://www.gradenine.co.uk/glossary/rna-primer)

## 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-the-role-of-rna-primers-3d7db10d) · Published by Druglandscape Ltd.