Discussion
In DNA, the number of hydrogen bonds that hold each adenine-thymine base pair and each guanine-cytosine base pair, respectively, are ________.
Answer: C
An adenine-thymine pair is held by 2 hydrogen bonds and a guanine-cytosine pair by 3, so the answer is 2 and 3.
In the Watson-Crick A-T pair, the N-H of thymine bonds to a ring nitrogen of adenine and the amino N-H of adenine bonds to a carbonyl oxygen of thymine: two bonds. The G-C pair adds a third, between the amino group of guanine and the carbonyl oxygen of cytosine. That extra bond is one reason DNA rich in G-C pairs melts at a higher temperature.
'3 and 2' reverses the pairs, while '3 and 3' and '2 and 2' ignore the difference between them, so all three are wrong.
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