Origins of spontaneous base substitutions.

Although simple to understand, satisfying to the imagination, and compelling as a teaching aid for beginning students, the evidence is mounting that the tautomeric shift is neither a common nor a likely origin for spontaneous base substitutions. Indeed, other sources, such as ionized base mispairings with nonionized bases, wobble of a bases in the DNA, and transient misalignment of bases causing dislocations at pairing sites, have been shown to induce spontaneous base substitutions. On the other hand, among the 4 common bases in DNA, no experimental evidence exists that tautomeric shifts can induce mutations.