Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologyeasymcq1 pt

A DNA double helix rich in guanine and cytosine separates into single strands at a higher temperature than an equal-length helix rich in adenine and thymine. Why?

A.Guanine and cytosine contain stronger covalent bonds than adenine and thymine do.
B.Each guanine-cytosine pair shares three hydrogen bonds while each adenine-thymine pair shares two.
C.Longer bases span a wider gap and mechanically wedge the strands together.
D.Sugar-phosphate backbones tighten around GC regions and physically clamp them shut.

Explanation

Core Concept

GC-rich DNA generally melts at a higher temperature than AT-rich DNA of the same length and under the same solution conditions. G–C pairs form three canonical hydrogen bonds whereas A–T pairs form two, and base-stacking interactions also contribute importantly to duplex stability. Salt concentration and sequence context can also affect melting temperature, so hydrogen-bond count is an important contributor rather than the sole cause.

Correct Answer

BEach guanine-cytosine pair shares three hydrogen bonds while each adenine-thymine pair shares two.

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