Tautomeric shifts in DNA bases

Standard keto and rare enol forms of T & G [left and right] differ by a spontaneous proton shift between the adjacent O and N molecules. Note that the molecular formula does not change, only the configuration of bonds among atoms: the keto and enol forms are therefore tautomers of each other.

The standard amino and rare imino forms of A [middle] also differ by a spontaneous tautomeric shift between the the adjacent N molecules. Standard amino and rare imino forms of C [not shown] differ in the same way.

Figure 2012 TA Brown, Introduction to Genetics (1st ed.); additional text 2013 by Steven M. Carr