CpG Islands

DNA methylation is a conspicuous feature of vertebrate genomes. Approximately 4% of total cytosines are methylated, representing about 5 × 107 5-methylcytosine (5-mC) residues per diploid nucleus. All 5-mC is present in the dinucleotide CpG, although only 70 to 80% of the potentially methylatable sites are actually in a methylated form. Most studies on the relative distribution of methylated cytosines in DNA have relied on the ability of some restriction enzymes to distinguish between the methylated and unmethylated version of the same sequence. This approach has provided information about the methylation patterns at specific genes in different tissues during development and has revealed that the vertebrate genome can be divided into two distinct compartments: the CpG island fraction, representing 1 to 2% of the total genome and containing all the sites that are consistently nonmethylated at all stages of development; and the remaining 98% containing all methylated sites.