DNA MoleculeUnder normal circumctances, it is not possible for adenine to pair up with guanine or cytosine, or for any other mismatches to occur....

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.