The idea that DNA modification may play a role in gene regulation is an attractive one that only recently became testable with the tools now available to molecular biologists. The only known DNA modification found in animal cells is 5-methylcytosine, which is localized almost exclusively to CpG residues. Unlike methylated sites in bacteria, the CpG moieties are only 50–80% methylated, depending on the particular organism (Razin and Riggs 1980). Furthermore, these methylations appear to occur at specific sites and to form a methylation pattern that is essentially passed on from cell generation to cell generation with only minor variations. The unique axial symmetry of the CpG dinucleotide is, in fact, the major element in this inheritance scheme. Since all methylated sites are methylated at symmetrical cytosines, each strand serves as a template for a maintenance methylase that is specific for the hemimethylated sites present immediately following DNA replication (Gruenbaum et...
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Cedar et al. (1983) studied this question.