A reversible change in the location of a hydrogen atom in a molecule that alters it from one to another isomer. Thymine and guanine are normally in keto forms, but when in the rare enol forms (see diagram) they can join by three hydrogen bonds with keto forms of guanine or thymine, respectively. Likewise, cytosine and adenine are normally in amino forms, but when in the rare imino forms they can join by two hydrogen bonds with amino forms of adenine or cytosine, respectively. Tautomeric shifts that modify the pairing of nucleotides can result in base substitutions and, as a result, mutations.
Tautomeric forms of DNA bases. Chart of tautomeric shifts by Vikram K. Mulligan.