ORIGINS OF SPONTANEOUS BASE SUBSTITUTIONS

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