True about DNA methylation -
Alters gene expression
Role in carcinogenesis
Protective mechanism against cleavage by restriction endonuclease
All of these
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.