Explain the role of RNA primers in DNA replication and why they must be replaced with DNA before replication is complete.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA polymerase III can only add nucleotides to an existing 3′‑OH group, so it cannot start a new strand. Primase therefore synthesises a short RNA primer that is complementary to the template strand and provides the required 3′‑OH for polymerase III to begin elongation.
The RNA primers must be removed because the presence of RNA nucleotides in the final DNA would give an incorrect sequence and would compromise the chemical integrity of the double helix. In prokaryotes DNA polymerase I (or RNase H in eukaryotes) removes the RNA primer and fills the gap with DNA nucleotides, after which DNA ligase seals the nick between Okazaki fragments.
Thus, RNA primers initiate synthesis and are later replaced with DNA to ensure a complete, accurate DNA strand.
The RNA primers must be removed because the presence of RNA nucleotides in the final DNA would give an incorrect sequence and would compromise the chemical integrity of the double helix. In prokaryotes DNA polymerase I (or RNase H in eukaryotes) removes the RNA primer and fills the gap with DNA nucleotides, after which DNA ligase seals the nick between Okazaki fragments.
Thus, RNA primers initiate synthesis and are later replaced with DNA to ensure a complete, accurate DNA strand.
Examiner tips
- Mention the 3′‑OH requirement of polymerase III
- Explain primase’s role in providing the primer
- State why RNA cannot remain in the final DNA
- Include the enzymes that remove and replace the primer and the role of ligase
Common mistakes
- Claiming DNA polymerase can start a strand de novo
- Forgetting that RNA primers must be removed
- Confusing the roles of RNase H and DNA polymerase I
Mark scheme (4 marks)
- DNA polymerase III can only add nucleotides to an existing 3′-OH group / cannot initiate a new strand de novo.
- Primase synthesises a short RNA primer complementary to the template strand, providing the 3′-OH group needed for DNA polymerase III to begin elongation.
- 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.
- 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 in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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