Question

Why does cytosine make pair with guanine and not with Adenine?

• A. C—A pair would be wider than double
• B. Hydrogen bond forming functional groups are not complementary between C and A
• C. Polar nature of C and A
• D. C—A pair would not reach across the double helix

Answer 1

The correct option is B Hydrogen bond forming functional groups are not complementary between C and A

Introduction to base pairing in DNA
The nitrogenous bases on one strand of the DNA double helix, form pairs with the complementary bases of the opposite strand. A purine always pairs a pyrimidine to maintain uniform diameter of the DNA at 20 Å which leaves approximately 11 Å space for the nitrogenous bases. This space can accommodate only one purine and one pyrimidine and not two purines (will occupy too much space) or two pyrimidines (will occupy too less space).

C-A pair
Cytosine is a pyrimidine and adenine is a purine. Pairing between C-A would fit perfectly in the diameter of the double helix without affecting its width. But, in reality a C-A pair does not exist because the hydrogen donor and acceptor sites of adenine and cytosine are not complementary and hence hydrogen bond formation is impossible. Cytosine is complementary to guanine and pairs with it using 3 hydrogen bonds. Adenine, on the other hand, pairs with thymine using 2 hydrogen bonds.


Final answer
(D) Hydrogen bond forming functional groups are not complementary between C and A