SAT Subject Biology Practice Question 163: Answer and Explanation

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Question: 163

13. A sample of DNA is sequenced and is found to consist of 32% guanine. Which conclusion can be drawn about the composition of the DNA sample?

A. It consists of 18% cytosine and 18% thymine.
B. It consists of 18% adenine and 18% cytosine.
C. It consists of 32% cytosine and 18% thymine.
D. It consists of 32% adenine and 18% cytosine.
E. It consists of 32% thymine and 18% adenine.

Correct Answer: C


According to Chargoff's rule, equal amounts of adenine and thymine, and of guanine and cytosine, will be present in a DNA sample. Therefore, the amount of cytosine will be equal to the amount of guanine, 32%. The percentages of adenine and thymine will total 36%, or 18% each.

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