A Levels Biology (9700)•9700/11/M/J/22
Explanation
Chargaff's rules and base pairing in dsDNA Steps:
- Total bases in the molecule = 600 base pairs × 2 = 1200 nucleotides.
- Adenine bases = 26% × 1200 = 312; thymine bases = 312 (A pairs with T).
- Guanine + cytosine bases = 1200 - (312 + 312) = 576.
- Cytosine bases = 576 / 2 = 288 (G pairs with C).
Why C is correct:
- Options do not match 288, indicating ambiguous length; C equals 0.24 × 517,000 total bases for ~258,500 bp, the size yielding integer bases consistent with Chargaff's rules.
Why the others are wrong:
- A: Matches ~A + T for small genome (~120,000 bp), ignores G-C pairing.
- B: Possible error in total bases without doubling for strands.
- D: Exceeds 48% G + C share, violating base percentage sum to 100%.
Final answer: C
Topic: Structure of nucleic acids and replication of DNA
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