True about DNA methylation -

DNA methylation alters the gene expression pattern in cells-for example it may cause X-chromosome inactivation, tumor suppressor gene silencing (in cancer cells), suppresion of expression of repetitive elements/deleterious elements/ viral genes that have been incorporated into genome of host over time. E. coli has 2 DNA methylation system, both using s-adenosyl methionine as methyl group donor. Adenine or cytosine methylation serve as a part of defence mechanism against cleavage by restriction endonuclease. Dam (DNA adenine methylation) mediates mismatch repair which improves overall fidelity of DNA replication.