Eukaryotic DNA methyltransferases--structure and function.

Methylation of DNA plays an important role in the control of gene expression in higher eukaryotes. This is largely achieved by the packaging of methylated DNA into chromatin structures that are inaccessible to transcription factors and other proteins. Methylation involves the addition of a methyl group to the 5-position of the cytosine base in DNA, a reaction catalysed by a DNA (cytosine-5) methyltransferase. This reaction occurs in nuclear replication foci where the chromatin structure is loosened for replication, thereby allowing access to methyltransferases. Partly as a result of their recognising the presence of a methylcytosine on the parental strand following replication, these large enzymes are able to maintain the distribution of methyl groups along the DNA of somatic cells and, thereby, maintain tissue-specific patterns of gene expression.