A Levels Biology (9700)•9700/13/M/J/21
Explanation
Thymine Nucleotide Properties
Steps:
- Statement 1: Thymine has a single-ring structure, classifying it as a pyrimidine base.
- Statement 2: Thymine pairs with adenine in DNA, forming two hydrogen bonds.
- Statement 3: Thymine occurs only in DNA, where the sugar is deoxyribose, not ribose (RNA uses uracil).
- Conclusion: Statements 1 and 2 hold true; 3 does not, as the diagram specifies thymine.
Why B is correct:
- Thymine is defined as a pyrimidine that base-pairs with adenine via exactly two hydrogen bonds in DNA.
Why the others are wrong:
- A includes incorrect statement 3.
- C omits accurate statement 2.
- D omits accurate statement 1.
Final answer: B
Topic: Structure of nucleic acids and replication of DNA
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