Practice MCAT Question - Q59 - Answer!
Purines are double-ringed nitrogenous bases, while pyrimidines are single-ringed bases. Which of the following must be true?
A. In a double-stranded molecule of DNA, there will normally be an equal number of purines and pyrimidines.
B. In a single-stranded molecule of RNA, there will normally be an equal number of purines and pyrimidines.
C. In a double-stranded molecule of DNA, there will normally be twice as many pyrimidines as purines.
D. In a single-stranded molecule of RNA, there will normally be twice as many pyrimidines as purines.
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The correct answer is (A). The purine adenine, on one strand of DNA, can only bond to the pyrimidine thymine on the complementary strand. Similarly, the purine guanine can only bond to the pyrimidine cytosine. Since there must be one purine for each pyrimidine, only choice (A) can be correct. Because RNA is usually single-stranded (no complementary strand), each nitrogenous base in the chain is independent of every other base, and there need not be a 1:1 ratio. The fact that purines have “double rings” and pyrimidines have “single rings” is totally irrelevant to the question!
