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

A DNA region rich in guanine and cytosine separates into single strands at a higher temperature than an adenine- and thymine-rich region of equal length. What explains this difference?

A.Each guanine-cytosine pair forms three hydrogen bonds, while each adenine-thymine pair forms two
B.Guanine-cytosine pairs contain one extra covalent bond inside the base pair
C.Cytosine carries a phosphate group that anchors it more firmly to the backbone
D.Adenine and thymine carry like charges and push the strands apart at lower heat

Explanation

Core Concept

GC-rich DNA generally has a higher melting temperature than AT-rich DNA of the same length. G–C pairs form three canonical hydrogen bonds while A–T pairs form two, and sequence-dependent base-stacking interactions also contribute importantly to duplex stability. Ionic strength and other solution conditions can shift melting temperature as well, so hydrogen-bond count is not the only stabilizing factor.

Correct Answer

AEach guanine-cytosine pair forms three hydrogen bonds, while each adenine-thymine pair forms two

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