Figure 2.
Schematic representation of unmethylated (a) and methylated (b) genes. The most CpG islands in the promoter region of normal genes are unmethylated (a) [11]. DNA methylation refers to the addition of a methyl (CH3) group to the fifth carbon atom of the cytosine residues resulting in the formation of 5-methylcytosine (b). The process is mediated by DNA methyltransferase enzymes. DNA methylation occurs mainly at cytosine-guanosine dinucleotides (CpGs) which are concentrated in promoter CpG islands. CpG islands are short DNA sequences (<200 bp) with greater than 50% GC content. Methylation of CpGs in promoter regions plays an important role in both chromatin structure control and gene expression [14].