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

Chemical analysis of a sample of double-stranded DNA shows that adenine makes up 30 percent of all bases. What percentage of the bases are guanine?

A.30 percent, because guanine pairs directly with adenine across the helix.
B.70 percent, since every base that is not adenine must be guanine.
C.20 percent, because adenine equals thymine and the guanine-cytosine pair shares what remains.
D.20 percent only if the sample came from a virus rather than a cellular organism.

Explanation

Core Concept

Chargaff's rules follow directly from complementary base pairing. Thymine must match adenine base for base, so thymine also totals 30 percent, leaving 40 percent of the sample for the guanine-cytosine pair. Guanine pairs exclusively with cytosine, so the two split the remainder evenly at 20 percent each. This compositional arithmetic helped establish the double-stranded pairing model of DNA itself.

Correct Answer

C20 percent, because adenine equals thymine and the guanine-cytosine pair shares what remains.

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