# Explain how the chemical properties of the nitrogenous bases in DNA result in the specificity of base pairing between complementary strands.

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

## Mark scheme (4 marks)

1. Hydrogen bonds form between bases on opposite strands, providing the inter-strand attraction.
2. Adenine pairs only with thymine via two hydrogen bonds, and cytosine pairs only with guanine via three hydrogen bonds.
3. Specificity arises because each base has a precise shape and arrangement of hydrogen bond donors and acceptors that is complementary only to its partner base.
4. Purines (adenine, guanine) always pair with pyrimidines (thymine, cytosine), maintaining a constant width of the double helix because a purine–pyrimidine pair has the correct combined geometry to span the distance between the two sugar–phosphate backbones.

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-chemical-properties-of-a68e0ecd) · Published by Druglandscape Ltd.