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| Modern Genetic Analysis Chapter 7. Gene Mutations The Molecular Basis of Mutation Figure 7-4. Mutation by tautomeric shifts in the bases of DNA. (a) In the example diagrammed, a guanine undergoes a tautomeric shift to its rare enol form (G*) at the time of replication. (b) In its enol form, it pairs with thymine. (c) and (d) In the next replication, the guanine shifts back to its more stable keto form. The thymine incorporated opposite the enol form of guanine, seen in (b), directs the incorporation of adenine in the subsequent replication, shown in (c) and (d). The net result is a G·C A·T mutation. If a guanine undergoes a tautomeric shift from the common keto form to the rare enol form at the time of incorporation (as a nucleoside triphosphate, rather than in the template strand diagrammed here), it will be incorporated opposite thymine in the template strand and cause an A·T G·C mutation. (From E. J. Gardner and D. P. Snustad, Principles of Genetics, 5th ed. Copyright © 1984 by John Wiley & Sons, New York.)

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