Why do you think adenine does not basepair with cytosine and

Why do you think adenine does not base-pair with cytosine; and why thymine does not base-pair with guanine?

Solution

DNA contains four bases adenine (A), cytosine (C), thymine (T) and guanine (G). Adenine and guanine are purines whereas cytosine and thymine are pyrimidines. Adenine (A) always pairs with thymine (T) and cytosine (C) with guanine (G) (purine:pyrimidine). Purines do not pair with purines (A and G) and pyrimidines (C and T) do not pair with pyrimidines because of some following reasons:

·         Two purines base pairs are too big to fit into the double helix.

·         Two pyrimidines base pairs are too far from each other and they need to move close to form hydrogen bond in double helix.