A Levels Biology (9700)•9700/13/O/N/20
Explanation
Meselson-Stahl experiment demonstrates semi-conservative DNA replication
Steps:
- Bacteria initially grown in ¹⁵N produce fully heavy (¹⁵N/¹⁵N) DNA.
- Transfer to ¹⁴N medium; first replication yields all hybrid (¹⁵N/¹⁴N) DNA.
- Second generation: half hybrid, half light (¹⁴N/¹⁴N) DNA.
- Third generation: one-quarter hybrid, three-quarters light DNA.
Why C is correct:
- C states 25% hybrid and 75% light DNA, matching semi-conservative replication where each strand serves as a template, diluting heavy strands over generations.
Why the others are wrong:
- A (all heavy): Ignores replication in ¹⁴N medium.
- B (all light): Requires more than three generations to fully replace heavy strands.
- D (50% hybrid, 50% light): Describes second generation, not third.
Final answer: C
Topic: Structure of nucleic acids and replication of DNA
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