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From Biolecture.org
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<p style="margin-left: 40px"> It is process which the methyl group is added to the DNA. When the promoter of gene is methylated, then it represses the gene expression. Adenine and cytosine can be methylated, adenine methylation occurs only in prokaryotes. DNA methyltransferase 1 is an enzyme which transfer methyl group to DNA, and they work as tissue specific manner. DNA methylation has important roles in mammalian system. Genomic imprinting, X-chromosome inactivation, Heterochromatin maintenance, Developmental controls, Tissue specific expression controls.</p>
<p style="margin-left: 40px"> Genomic imprinting can be caused by DNA methylation, and histone methylation which is independent of Mendelian inheritance. Expression of gene is inherited from mother or father. X-chromosome inactivation is the process which one of the two X chromosomes in all female cells is inactivated. Females(XX) carry twice as many dosage of gene as males(XY), so in order to solve this problem one X chromosome is inactivated randomly by methylations in the early developmental process inChromatin . Chromatin can be found as two status, heterochromatin and euchromatin. Heterochromatin is tightly packed form of DNA, and controlled by DNA methylation. As I mention above, DNMT1 act as tissue specific manner, so DNA methylation can control gene expression tissue specific manner.</p>
<p><strong> 2) Histone modification.</strong></p>
