Which is the correct base pairing found in a normal DNA molecule ?
- A.Adenine pairs with Thymine
- B.Adenine pairs with Guanine
- C.Adenine pairs with Cytosine
- D.Thymine pairs with Guanine
Correct answer
Explanation
The correct answer is A, Adenine pairs with Thymine. In the DNA double helix, adenine on one strand always pairs with thymine on the other, and guanine always pairs with cytosine.
The two strands of DNA are held together by hydrogen bonds between their bases. Adenine and thymine are joined by two hydrogen bonds, while guanine and cytosine are joined by three, which makes G-C pairs a little stronger. Each pair links a larger purine, adenine or guanine, with a smaller pyrimidine, thymine or cytosine, so the width of the helix stays the same. This rule, found by Chargaff and used by Watson and Crick in 1953, lets DNA copy itself exactly.
B is wrong because adenine and guanine are both purines; two purines would be too wide to fit. C is wrong because adenine and cytosine cannot form the right hydrogen bonds. D is wrong because thymine pairs only with adenine, and guanine only with cytosine.
Exam tip: A with T by two bonds, G with C by three bonds; in RNA, uracil takes the place of thymine.
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