Question

Explain why adenine is chemically Unable to pair with cytosine and why is guanine chemically unable to pair with thymine.

Answer 1

The base-pairing rule, exists as per a consequence of stability in terms of hydrogen bond formation.
Adenine and Guanine are together referred to as purines, as they have an imidazole ring attached to their pyrimidine ring. Cytosine and Thymine are together referred to as pyrimidines, as they do not have any such imidazole ring attached to them.
A double stranded DNA has just barely enough space within its two strands to exactly accommodate only one purine and one pyrimidine. Two of the same type will simply not do, as two purines would be too large while two pyrimidines will fall too short. Thus a purine ends up having to make a hydrogen bond with another pyrimidine.
In terms of the specificity, the molecule structures of these 4 bases are such that since one strand goes parallel and the other anti-parallel, adenine has two possible sites for hydrogen bond formation. Two exactly complement these two sites with minimum distance between polar atoms constituting the H-bond, only thymine pairs up perfectly and not cytosine. The same is true for guanine and cytosine pairing. Since one pair makes 2 hydrogen bonds and the other pair makes 3 hydrogen bonds. Hence specificity is introduced.