A Levels Biology (9700)•9700/11/M/J/20

Explanation
Thymine Structure and Classification Steps:
- Recall thymine as a nitrogenous base in DNA, one of four: adenine, guanine, cytosine, thymine.
- Classify bases: purines (adenine, guanine) have two fused rings; pyrimidines (cytosine, thymine) have one ring.
- Confirm thymine's structure: single six-membered heterocyclic ring with keto groups.
- Check base pairing: thymine pairs with adenine via 2 hydrogen bonds, unlike guanine-cytosine's 3.
Why A is correct:
- Thymine possesses a single-ring pyrimidine structure, matching the definition of pyrimidines as monocyclic aromatic heterocycles.
Why the others are wrong:
- B: True but incomplete; omits pyrimidine classification essential for base identity.
- C: True but incomplete; lacks structural detail of the single ring.
- D: False; thymine-adenine pairing involves 2 hydrogen bonds, per Watson-Crick model.
Final answer: A
Topic: Structure of nucleic acids and replication of DNA
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