A Levels Biology (9700)•9700/11/M/J/25
Explanation
Chargaff's Rules of Base Pairing
Steps:
- Adenine (A) pairs with thymine (T) in double-stranded DNA, so %A = %T = 29%.
- Guanine (G) pairs with cytosine (C), so %G = %C.
- Total nucleotides sum to 100%: %A + %T + %G + %C = 100%.
- Substitute values: 29% + 29% + %C + %C = 100% → 58% + 2%C = 100% → 2%C = 42% → %C = 21%.
Why A is correct:
- Chargaff's rules state A equals T and G equals C, with all bases totaling 100%, yielding %C = (100% - 2×29%)/2 = 21%.
Why the others are wrong:
- B is incorrect because C pairs with G, not A, so %C ≠ %A.
- C is incorrect because 42% equals combined %G + %C, not %C alone.
- D is incorrect because 58% equals combined %A + %T, not %C.
Final answer: A
Topic: Structure of nucleic acids and replication of DNA
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