# Explain how the antiparallel orientation of the two strands in a DNA double helix is essential for the process of semi-conservative replication.

> IB DP Biology Higher Level (2023 syllabus) — A1.2 Nucleic acids · Explain · 4 marks

## Mark scheme (4 marks)

1. The two strands of DNA run in opposite (5'→3') directions, meaning one strand runs 5'→3' and the complementary strand runs 3'→5'.
2. DNA polymerase III can only add nucleotides in the 5'→3' direction, so it can only synthesise a new strand by reading the template in the 3'→5' direction.
3. Because the two template strands are antiparallel, one strand (the leading strand) is synthesised continuously towards the replication fork, while the other (the lagging strand) must be synthesised discontinuously away from the fork as Okazaki fragments.
4. Semi-conservative replication is maintained because each resulting double helix retains one original (parental) strand and one newly synthesised strand; the antiparallel arrangement ensures both parental strands can act as templates simultaneously at each replication fork.

## Key terms

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

## 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-orientation-of-d98c977f) · Published by Druglandscape Ltd.