AP Biologymediummcq1 pt
Which nitrogenous bases are purines?
A.Adenine and guanine
B.Cytosine and thymine
C.Thymine and uracil
D.Adenine and thymine
Explanation
Core Concept
Adenine and guanine are two-ring purines, whereas cytosine, thymine, and uracil are one-ring pyrimidines. In canonical Watson–Crick base pairing, a purine pairs with a pyrimidine, helping maintain a consistent duplex width. 'Purines always pair with pyrimidines' is too absolute outside that canonical pairing context.
Correct Answer
AAdenine and guanine
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