# Explain how the antiparallel orientation of the two strands in a DNA double helix is related to its structural stability.

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

## Mark scheme (4 marks)

1. The two strands run in opposite (5'→3' and 3'→5') directions, making them antiparallel.
2. Complementary base pairing (A–T and C–G) occurs between the two antiparallel strands, forming hydrogen bonds that hold the strands together.
3. The antiparallel arrangement allows the bases of both strands to face inward, maximising hydrophobic stacking interactions between adjacent base pairs along the helix.
4. The sugar–phosphate backbones of both strands run along the exterior of the helix; the antiparallel orientation allows the negatively charged phosphate groups to be positioned on the outside, maintaining a consistent helical geometry.

## Key terms

- [antiparallel](https://www.gradenine.co.uk/glossary/antiparallel)
- [double helix](https://www.gradenine.co.uk/glossary/double-helix)

## Related

- [Revision notes for IB DP Biology Standard 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-41d84d57) · Published by Druglandscape Ltd.