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
[{'content': 'True about DNA methylation - \nSelect and explain. \nA. Alters gene expression, B. Role in carcinogenesis, C. Protective mechanism against cleavage by restriction endonuclease, D. All of these', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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.\nE. coli has 2 DNA methylation system, both using s-adenosyl methionine as methyl group donor.\n\n\nAdenine or cytosine methylation serve\xa0as a part of defence mechanism against cleavage by restriction endonuclease.\nDam (DNA adenine methylation) mediates mismatch repair which improves overall fidelity of DNA replication.', 'role': 'assistant'}]