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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
In species A, 30 % of the bases are adenine, so by base‑pairing 30 % must be thymine. The remaining 40 % are split equally between cytosine and guanine, giving 20 % each.
In species B, 40 % of the bases are cytosine, so 40 % must be guanine. The remaining 20 % are split equally between adenine and thymine, giving 10 % each.
In species B, 40 % of the bases are cytosine, so 40 % must be guanine. The remaining 20 % are split equally between adenine and thymine, giving 10 % each.
Examiner tips
- Use the fact that A pairs with T and C pairs with G; state the percentages explicitly.
- Show the complementary base for each given base and then calculate the remaining percentages.
Common mistakes
- Saying that the remaining bases are all the same type instead of splitting them equally.
- Confusing which base pairs with which (e.g. pairing A with C).
Mark scheme (4 marks)
- In species A, 30% of bases are adenine, so 30% must be thymine (because adenine pairs with thymine / complementary base pairing)
- In species A, the remaining 40% of bases are split equally between cytosine and guanine (20% each), because cytosine pairs with guanine
- In species B, 40% of bases are cytosine, so 40% must be guanine (because cytosine pairs with guanine / complementary base pairing)
- In species B, adenine and thymine together make up the remaining 20% of bases (10% each), because adenine pairs with thymine
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →