Bases in DNA pair by the rules of adenine with thymine (A-T) and gunanine with cytosine (g-c).  Adenine and guanine are both larger, double-ringed molecules called purines and cytosine and thymine are smaller single ringed pyrimidines.  The purines are complementary and form base pairings with  the pyrimidines, thus A-T and G-C form base pairs. The reasons why mismatches don't normally occur is that two pyrimidines cannot pair because they are too far apart for hydrogen bonding to occur. Two purines cannot pair because since the molecules are two close, repulsion occurs instead of bonding. In GT and AC pairings, hydrogen donors and acceptors don't match up.