The in vivo methylation of DNA in mouse fibroblasts.

Abstract Studies on the in vivo methylation of DNA occurring in cultured mouse fibroblasts (Strain L929) treated with hydroxyurea have suggested that the only DNA base to be methylated, namely cytosine, is methylated after the initial synthetic stage of DNA replication and is a process which proceeds efficiently only in the presence of a continual supply of nascent DNA. Experiments with synchronised cell cultures demonstrated that cytosine methylation occurred within an hour of the synthesis of DNA in the S phase of the cell cycle. Additional ‘chase’ experiments showed that the 5-methylcytosine of DNA so formed is stable, no evidence being found for its demethylation, deamination or excision from the polymer.

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