# Explain how the antiparallel orientation of the two strands in a DNA double helix arises from the structure of nucleotides and the bonds formed between them.

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

## Mark scheme (4 marks)

1. Each nucleotide has a 5' phosphate group and a 3' hydroxyl group on the deoxyribose sugar, giving each strand a directionality (polarity).
2. Phosphodiester bonds form between the 3'–OH of one nucleotide and the 5' phosphate of the next, so each polynucleotide strand runs in a 5'→3' direction.
3. Hydrogen bonds between complementary bases can only form when the two strands run in opposite (antiparallel) directions, because the geometry of base pairing requires the sugars to face opposite ways.
4. Therefore one strand runs 5'→3' while the complementary strand runs 3'→5', making the two strands antiparallel.

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-antiparallel-orientation-of-6bf51629) · Published by Druglandscape Ltd.