Question

Why isn't it possible for adenine to pair with cytosine
or guanine?

Answer 1

the hydrogen bonds forming functional groups are not complementary in adenine and cytosine. it also has to do with structure because adenine and guanine are purines and cytosine and thymine are pyrimidines.

Answer 2

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