A Levels Biology (9700)•9700/12/M/J/25
Explanation
RNA Base Pairing Hydrogen Bonds
Steps:
- Identify complementary bases: guanine pairs with cytosine; uracil pairs with adenine.
- Recall hydrogen bond counts: G-C forms 3 bonds; A-U forms 2 bonds.
- Apply to guanine: binds to cytosine with 3 hydrogen bonds.
- Apply to uracil: binds to adenine with 2 hydrogen bonds.
Why B is correct:
- In RNA, G-C pairing follows Watson-Crick rules with 3 hydrogen bonds, while A-U pairing has 2, matching the 3 2 configuration.
Why the others are wrong:
- A (3 3): Incorrectly assigns 3 bonds to uracil-adenine, which has only 2.
- C (2 3): Reverses the pairs, giving guanine only 2 bonds instead of 3.
- D (2 2): Understates guanine-cytosine bonds, which are 3, not 2.
Final answer: B
Topic: Structure of nucleic acids and replication of DNA
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