Explain the role of complementary base pairing in ensuring accurate DNA replication.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The double helix is opened by helicase, breaking the hydrogen bonds between the base pairs and exposing the template strands.
The exposed template strands are read in the 3'→5' direction and DNA polymerase adds free nucleotides in the 5'→3' direction.
Because of complementary base‑pairing rules (A pairs with T, C pairs with G), only the correct nucleotide can bond to each position on the template strand.
Thus each new strand is complementary to its template, giving two identical DNA molecules – one parental and one newly synthesised – i.e. semi‑conservative replication, preserving the genetic information.
The exposed template strands are read in the 3'→5' direction and DNA polymerase adds free nucleotides in the 5'→3' direction.
Because of complementary base‑pairing rules (A pairs with T, C pairs with G), only the correct nucleotide can bond to each position on the template strand.
Thus each new strand is complementary to its template, giving two identical DNA molecules – one parental and one newly synthesised – i.e. semi‑conservative replication, preserving the genetic information.
Examiner tips
- Show the sequence of events – opening, reading, synthesis, base‑pairing rule, result.
- Use the exact terminology: helicase, DNA polymerase, 5'→3', 3'→5', A–T, C–G, semi‑conservative.
- Link each step to the mark scheme points.
- Keep the answer concise – 4 points only.
Common mistakes
- Confusing the direction of synthesis (5'→3' vs 3'→5'), Failing to mention the base‑pairing rules, or Not stating that the replication is semi‑conservative.
Mark scheme (4 marks)
- The two strands of the double helix are separated / hydrogen bonds between base pairs are broken, exposing the template strands.
- Each exposed template strand is read in the 3' to 5' direction and free nucleotides are added in a 5' to 3' direction by DNA polymerase.
- Complementary base pairing rules (A–T and C–G) mean only the correct nucleotide can bond to each position on the template strand.
- The result is two identical DNA molecules each consisting of one original (parental) strand and one newly synthesised strand — semi-conservative replication — preserving the genetic information.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →