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| Modern Genetic Analysis Chapter 7. Gene Mutations The Molecular Basis of Mutation Figure 7-25. Model for mismatch repair in E. coli. Because DNA is methylated by enzymatic reactions that recognize the A in a GATC sequence, directly after DNA replication the newly synthesized strand will not be methylated. The "hemimethylated" DNA duplex serves as a recognition point for the mismatch-repair system in discerning the old from the new strand. Here a G·T mismatch is shown. The mismatch- repair system can recognize and bind to this mismatch, determine the correct (old) strand because it is the methylated strand of a hemimethylated duplex, and then excise the mismatched base from the new strand. Repair synthesis restores the normal base pair. (Adapted from E. C. Friedberg, DNA Repair. Copyright © 1985 by W. H. Freeman and Company, New York.)

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