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,d9 hkmz08in)d+1t x yz and R if S izk65.2 uerb az 0wk5b,u atgg/ ;nl/gks a pyrimidine?

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 and thymine are pyri6nscy+//z3ymjl*b)0 k d* -xvojlh ey midines.If S is a pyrimidine (~20%), it is either cytosine 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 l/m-d6o /zx3j)sl c+hy y0 v*nbk*yej 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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