A Levels Biology (9700)•9700/13/O/N/18
Explanation
Purines in nucleic acids are adenine and guanine
Steps:
- Identify purines as the two-ring nitrogenous bases in DNA and RNA.
- List purines: adenine (A) and guanine (G).
- List pyrimidines: cytosine (C), thymine (T in DNA), uracil (U in RNA).
- Match choices to find the pair containing only purines.
Why B is correct:
- Purines are defined as adenine and guanine, the double-ring bases pairing with pyrimidines in nucleic acid structure.
Why the others are wrong:
- A: Cytosine is a pyrimidine, not a purine.
- C: Thymine and cytosine are both pyrimidines.
- D: Uracil is a pyrimidine, not a purine.
Final answer: B
Topic: Structure of nucleic acids and replication of DNA
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