ABSTRACT
The reaction of cytosine methylation consists of the incorporation of a methyl group from the S-adenosyl-methionine in a previously unmethylated cytosine and it is catalysed by DNA methyltransferases. The normal physiological function of DNA methylation is multifaceted. DNA methylation plays a role in the protection of chromosomal integrity. An important function of DNA methylation is to regulate the expression of tissue-specific genes. Acetylation and methylation of conserved lysine residues of histone tail domains are known to be important regulators of gene expression. Epigenetic changes are defined as all meiotically and mitotically heritable changes in gene expression that are not encoded in the DNA sequence itself. Mutations and epigenetic silencing of the same gene or different genes with similar functions seem to be mutually exclusive. Histone methylation is a marker of both active and inactive genes. The chapter reviews histone methylation with regard to DNA methylation activity, grouped by histone methylation mark.
