# Explain how the 5' to 3' directionality of nucleic acid strands arises from the chemistry of nucleotide polymerisation.

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

## Mark scheme (4 marks)

1. Nucleotides are joined by phosphodiester bonds formed between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of the next nucleotide being added.
2. The incoming nucleotide always carries a triphosphate group at its 5' carbon, and it is this 5'-phosphate that attacks / bonds to the 3'-OH of the growing chain.
3. Because new nucleotides are always added to the 3' end, the newly synthesised strand has a free 5'-phosphate at one end and a free 3'-hydroxyl at the other, creating an inherent directionality.
4. The two strands of a DNA double helix run antiparallel because each strand has inherent 5'→3' directionality; one strand runs 5'→3' in one direction whilst the complementary strand runs 5'→3' in the opposite direction.

## Key terms

- [5' to 3' directionality](https://www.gradenine.co.uk/glossary/5-to-3-directionality)
- [nucleotide polymerisation](https://www.gradenine.co.uk/glossary/nucleotide-polymerisation)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-5-to-3-de09db36) · Published by Druglandscape Ltd.