What is DNA methylation?
In addition to this flexible and dynamic module of the epigenome, which is associated with the genome but is not part of its covalent structure, the genome is covalently modified by addition of a methyl group at the 5th position of the cytosine ring. Since the methyl group is connected to DNA by a strong chemical bond, it is considered a stable and fixed mark. The vast majority of methylated cytosines in vertebrate genomes are found in the dinucleotide sequence CG. Not all CG dinucleotide sequences are methylated, different CGs are methylated in different tissues creating a pattern of methylation that is gene and tissue specific. Thus, there is a good correlation between the state of activity of genes and lack of methylation of CGs in their regulatory regions (Razin and Riggs, 1980). Remarkably, there is also a tight correlation between the chromatin structure and the status of DNA methylation (Razin and Cedar, 1977). We propose that the DNA methylation pattern is steady state equilibrium of DNA methylation and demethylation reactions. DNA methylation is catalyzed by DNA methyltransferase enzyme and demethylation catalyzed by demethylase enzymes. The DNA methylation reaction is stimulated by inactive chromatin and demethylation by active chromatin. Thus the direction of the DNA methylation reaction is determined by chromatin structure.
[General Interests of the Laboratory]