Explain why DNA replication proceeds continuously on one template strand but discontinuously on the other.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
DNA polymerase can only add nucleotides in the 5'→3' direction.
The two template strands are antiparallel, running 3'→5' and 5'→3'.
On the leading strand the template runs 3'→5', so polymerase can synthesise continuously toward the replication fork.
On the lagging strand the template runs 5'→3', so polymerase must synthesise short Okazaki fragments away from the fork.
The two template strands are antiparallel, running 3'→5' and 5'→3'.
On the leading strand the template runs 3'→5', so polymerase can synthesise continuously toward the replication fork.
On the lagging strand the template runs 5'→3', so polymerase must synthesise short Okazaki fragments away from the fork.
Examiner tips
- State the 5'→3' directionality of polymerase first.
- Explain the antiparallel nature of strands.
- Show why the leading strand is continuous and the lagging strand is discontinuous.
- Mention Okazaki fragments as evidence of discontinuity.
Common mistakes
- Confusing the direction of the template strands.
- Forgetting to link polymerase directionality to strand continuity.
- Not naming Okazaki fragments.
Mark scheme (4 marks)
- DNA polymerase can only add nucleotides in the 5' to 3' direction
- The two template strands are antiparallel, so they run in opposite directions (3' to 5' and 5' to 3')
- On the leading strand, the template runs 3' to 5', so DNA polymerase can synthesise continuously towards the replication fork
- 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
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →