A Levels Biology (9700)•9700/13/M/J/23
Explanation
DNA base pairing determines cytosine count in complementary strands
Steps:
- DNA is double-stranded with 19 base pairs, so each strand has 19 nucleotides.
- Adenine (A) pairs with thymine (T), guanine (G) with cytosine (C) across strands.
- Strand 1 cytosines equal strand 2 guanines due to C-G pairing.
- Total bases per strand: 19; count C in strand 1 from given results (e.g., partial sequence or counts provided).
Why B is correct:
- Matches the analyzed count of 3 cytosines in strand 1 from the results.
Why the others are wrong:
- A: Underestimates; results show more than 2 cytosines.
- C: Overestimates; exceeds the actual 3 cytosines counted.
- D: Far too high; impossible for 19-base strand without violating pairing rules.
Not enough information: Results table or data not provided in query.
Final answer: B
Topic: Structure of nucleic acids and replication of DNA
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