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| Modern Genetic Analysis Chapter 7. Gene Mutations The Molecular Basis of Mutation Figure 7-17. 5-Methylcytosine hot spots in E. coli. Nonsense mutations occurring at 15 different sites in lacI were scored. All result from the G·C A·T transition. The asterisks (*) mark the positions of 5-methylcytosines. Open bars depict sites at which the G·C A·T change could be detected but at which no mutations occurred in this particular collection. It can be seen that 5-methylcytosine residues are hot spots for the G·C A·T transition. Of 50 independently occurring mutations, 44 were at the four 5-methylcytosine sites and only 6 were at the 11 unmethylated cytosines. (From C. Coulondre et al., Nature 274, 1978, 775.)

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