[What is DNA methylation?]
Methylation decreases the expression of particular genes, and during the differentiation of cells in the development of an embryo, genes are methylated and demethylated as the cell adapts to produce the proteins that are involved in the structure and function of a particular tissue.
DNA methylation patterns are normally fairly stable, and can help to account for the transgenerational transmission of acquired adaptations, and for neonatal imprinting that can last a lifetime. But with injury, stress, and aging, the methylation patterns of differentiated tissues can be changed, contributing to the development of tumors, or to the loss of cellular functions.
Most methylations occur on cytosines in a CpG dinucleotide. Cytosines can be methylated to form 5-methylcytosines.
CpG is shorthand for 5'—C—phosphate—G—3' , that is, cytosine and guanine separated by only one phosphate group; phosphate links any two nucleosides together in DNA. The CpG notation is used to distinguish this single-stranded linear sequence from the CG base-pairing of cytosine and guanine for double-stranded sequences.