Unity and Diversity A.2 Nucleic Acids (id: 5d4f6ee8c)

本题目来源于试卷: Unity and Diversity A.2 Nucleic Acids，类别为 IB生物学

[单选题]
What bases are P, Q, and R if S is abl7y uiql9nmu5o;21 p pyrimidiner we4 heg 0xe9na)v8/d?

A. $P$ is Guanine ;Q is Cytosine ;R is Thymine
B. $P$ is Guanine ;Q is Adenine;R is Cytosine
C. $P$ is Cytosine ;Q is Guanine ;R is Adenine
D. $P$ is Adenine;Q is Thymine ;R is Cytosine


参考答案:  B


本题详细解析:
Adenine and guanine are purines. Cytosine ah2wn5i 2 )tiucnd thymine are pyrimidines.If S is a pyrimidine (~20%), it is either cyto)22i5uhiw cn tsine or thymine. Purines pair with pyrimidines, so Q, which also makes up about 20% of the molecule, must be guanine or adenine. This means Choices A and D can be eliminated as correct answers.P and R must pair up as they make up approximately the same proportion of the DNA. So P and R must either be adenine and thymine or guanine and cytosine. This means Choice C is incorrect.This leaves us with the correct answer of Choice B. P is guanine, which pairs with R, cytosine, and Q is adenine, which pairs with S, which must be thymine.

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