A Levels Biology (9700)•9700/11/O/N/20
Explanation
Nucleotide Structure and Base Properties
Steps:
- Statement 1 is correct: Nucleotides in RNA contain ribose sugar, so the carbohydrate could be ribose.
- Statement 2 is correct: Cytosine is a nitrogenous base, containing nitrogen atoms in its ring structure.
- Statement 3 is correct: In DNA and RNA, cytosine forms three hydrogen bonds with guanine during base pairing.
- Statement 4 is incorrect: Cytosine is a pyrimidine (single-ring base), not a purine (double-ring base).
Why A is correct:
- A selects statements 1, 2, and 3, which accurately describe the nucleotide's possible sugar, base composition, and pairing rules per nucleic acid structure.
Why the others are wrong:
- B includes incorrect statement 4 about cytosine being a purine.
- C includes incorrect statement 4 about cytosine being a purine.
- D includes incorrect statement 4 about cytosine being a purine.
Final answer: A
Topic: Structure of nucleic acids and replication of DNA
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