Question

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

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

Answer 1

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

A hydrogen bond is formed between a hydrogen donor and acceptor group; a hydrogen bond acceptor is an electronegative atom having a non-bonding electron pair and thus can form a partial bond to a hydrogen atom. An electronegative atom bonded to a hydrogen atom serve as hydrogen bond donor. The amino and carbonyl groups of purine and pyrimidines serve the purpose. The distance between donor oxygen atom of thymine and the hydrogen atom of adenine molecule exceeds the permissible distance range for hydrogen bonding between the two (<2.5). Both bases need to twist hard to facilitate the hydrogen bond which is energetically expensive event; thus hydrogen bonds forming functional groups are not complementary in adenine and cytosine; correct answer is D. Adenine is purine and cytosine is pyrimidine; their two and one ring structure support the width of double helix which makes options B and C incorrect. Both purine and pyrimidine are non-polar which rules out option A.