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.
Alters gene expression
Role in carcinogenesis
Protective mechanism against cleavage by restriction endonuclease
All of these