Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologymediummcq1 pt

In double-stranded DNA, adenine pairs only with thymine and guanine only with cytosine. Which structural principle enforces this pairing specificity?

A.Backbone phosphate groups attract adenine toward thymine and push guanine away from cytosine.
B.Covalent bonds join each paired set of bases permanently in place.
C.Sugar rings twist until any base tolerates any partner equally well.
D.A purine paired with a pyrimidine spans the backbone gap uniformly, with matching hydrogen-bond donors and acceptors.

Explanation

Core Concept

Adenine and guanine are double-ringed purines, while thymine and cytosine are single-ringed pyrimidines. Pairing one purine with one pyrimidine spans nearly the same distance between the two backbones, so A-T and G-C pairs preserve a constant helix diameter. Within those pairs the edge chemistry aligns so adenine offers two hydrogen bonds with thymine while guanine offers three with cytosine, and mismatched edges lack complementary donor and acceptor positions. These geometric constraints make Watson-Crick pairing overwhelmingly favored and underlie accurate replication.

Correct Answer

DA purine paired with a pyrimidine spans the backbone gap uniformly, with matching hydrogen-bond donors and acceptors.

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