This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
True about DNA methylation - 
Select and explain. 
A. Alters gene expression, B. Role in carcinogenesis, C. Protective mechanism against cleavage by restriction endonuclease, D. All of these
D: All of these
Answer: D: All of these 
Explanation: 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.