Question

Why is it necessary that Adenine bonds with Thymine and Guanine bonds with Cytokinine?

Answer 1

Adenine and guanine has a structure that creates a distance between them, they cannot come close to each other to form a stable hydrogen bond.

Thymine and cytosine has a structure that bring them so close which creates a overlap repulsion that does not allow them to form a stable hydrogen bond.

Structures of these four nitrogenous bases are such that adenine can form two hydrogen bonds with thymine and guanine can form three hydrogen bonds with cytosine.