<prompt> All are true about DNA methylation except:It usually occurs in the cytosineCan alters the gene expression pattern in cellsRole in genomic imprintingNo role in carcinogenesis  <response>: Ans: D No role in carcinogenesisCytosine in the sequence CG of DNA gets methylated to form 5-methylcytosine. A major poion of CG about 20 in human DNA exists in methylated form.In DNA, methylation usually occurs in the CpG islands, a CG rich region, upstream of the promoter region. In humans, DNA methylation is carried out by a group of enzymes called DNA methyltransferases.DNA methylation stably alters the gene expression pattern in cells such that cells can remember where they have been or decrease gene expressionDNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, suppression of repetitive elements, and carcinogenesis.Adenine or cytosine methylation is pa of the restriction modification system of many bacteria, in which specific DNA sequences are methylated periodically throughout the genome.Within the bacterium, methylation protects the host DNA from cleavage by its own restriction enzyme. Unmethylated foreign DNA is not protected from cleavage. 