Question

Why does cytosine 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

Hint: A DNA helix is 2nm in diameter and has 3 rings. In DNA, nitrogenous bases are present are purine(A, G) and pyrimidines(T, C).

Step by step answer:A hydrogen bond is a type of intermolecular force that is formed between a hydrogen donor and a hydrogen acceptor (electronegative) group. The amino and carbonyl groups of purine and pyrimidines serve this purpose. The distance between the donor oxygen of thymine and the hydrogen atom of adenine exceeds the permissible distance range for hydrogen bonding between the two atoms(<2.5nm)
In order to form this hydrogen bond, both bases need to twist in order to facilitate the hydrogen bond. This is an energetically expensive event, hence hydrogen bonds forming functional groups are not complementary in the case of adenine and cytosine. This is applicable to thymine and guanine too.
The diameter of a DNA helix is 2nm. This is normally 3 rings wide, purines like adenine and guanine are 2 ringed structures and pyrimidines line thymine and cytosine are one ringed structure. Hence, purines can’t bond within each other (as it will become a 4 ringed structure).
Correct Answer: Option D: Hydrogen bond forming functional groups are not complementary between C and A
Additional Information:
Adenine and guanine are purine bases that are found in both DNA and RNA. Cytosine is a pyrimidine base that is found in both DNA and RNA. Thymine and uracil are pyrimidine bases found respectively in only DNA or RNA.
Pairs of nitrogenous bases are all set in the same plane. These bases interact with each other through hydrogen bonding. These pairs are often referred to as base pairs (bp)
Hydrogen bonds are weak and noncovalent interactions. They are present in large numbers between complementary base pairs in a DNA double helix. They combine to provide great stability to the structure

Note: In DNA, purine and pyrimidine are equal and join the two backbone of DNA by making hydrogen bonds between them and increase the stability of the nucleic acid.