# A scientist extracts DNA from two different species of organism. They find that in species A, 30% of the bases are adenine. In species B, 40% of the bases are cytosine. Explain what these findings tell us about the base composition of each species' DNA.

> Eduqas GCSE Biology — 2.3.1 DNA structure · Explain · 4 marks

> DNA contains four types of base: adenine (A), thymine (T), cytosine (C) and guanine (G). The bases pair together in a specific way to hold the two strands of the DNA molecule together.

## Mark scheme (4 marks)

1. In species A, 30% of bases are adenine, so 30% must be thymine (because adenine pairs with thymine / complementary base pairing)
2. In species A, the remaining 40% of bases are split equally between cytosine and guanine (20% each), because cytosine pairs with guanine
3. In species B, 40% of bases are cytosine, so 40% must be guanine (because cytosine pairs with guanine / complementary base pairing)
4. In species B, adenine and thymine together make up the remaining 20% of bases (10% each), because adenine pairs with thymine

## Key terms

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

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- [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/a-scientist-extracts-dna-from-two-b09a627f) · Published by Druglandscape Ltd.