A Levels Biology (9700)•9700/12/O/N/21
Explanation
DNA Base Pairing Rules
Steps:
- Total base pairs = 94, so total bases in double-stranded DNA = 188.
- By Chargaff's rules, adenine (A) pairs with thymine (T), so A = T; cytosine (C) pairs with guanine (G), so C = G.
- Table provides A and C counts in one or both strands; sum A + T + C + G = 188, but A = T and C = G imply total purines (A + G) = total pyrimidines (T + C).
- Guanine total = cytosine total from table (since C = G across both strands).
Why B is correct:
- Matches cytosine count from table, as G equals C in double-stranded DNA per base-pairing rules.
Why the others are wrong:
- A: Underestimates G by ignoring full C pairing.
- C: Overcounts by adding unpaired bases.
- D: Exceeds possible G based on 94 pairs.
Not enough information: Table data not provided.
Final answer: B
Topic: Structure of nucleic acids and replication of DNA
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