A process that takes on a number of meanings depending on the context, but that has ‘addition’ as a common entity. In terms of chemistry, it is a reaction that results in the addition of a methyl (-CH3 radical) group to a chemical substrate (e.g., molecule) or the substitution of an atom or group by a methyl group. Mercury pollution caused by the methyl mercury produced by methylation of organic mercury resulted the epidemic of Miniamata disease in Japan. With regard to a protein, it is also involves an addition, this time the addition of a methyl group (or groups) to the amino acids arginine or lysine in a protein. As for DNA methylation, it amounts to a enzymatically controlled modification of DNA during which methyl groups are added to particular positions on the nitrogen or nucleotide bases such as cytosine or adenine in eukaryotes. Methylation in this respect forms the basis of chromatin structure, which enables cells to form a plethora of structures essential for multicellular organisms to derive from a single sequence of DNA. In vertebrates, it typically occurs at so-called CpG sites or islands (i.e., where cytosine is directly followed by guanine in the DNA sequence). Located in the promotor regions of genes, they are considered to play an important role in gene silencing or suppression. As a consequence, this effect is usually permanent and unidirectional, thereby serving to prevent reverting back to a stem cell or converting to another type of tissue. Given this particular function, DNA methylation is essential for typical ontogenetic development and cell differentiation. By the blastula stage, the methylation of embryonic cells is complete after undergoing of demethylation/remethylation referred to as reprogramming. In rats, it has been demonstrated that stress during early (postnatal) development results in alteration to methylation processes such that functionally adverse alterations to DNA arise (e.g., alterations that prevent the brain responding appropriately to stress). All told, DNA methylation is an important ‘gateway’ for the epigenetic modulation of development. Moreover, it is used in the diagnosing the Prader-Willi and Angelman’s syndromes.
See Adenine, Amino acids, Angelman’s syndrome, Blastula, Chromatin, Cytosine, Differentiation, DNA (deoxyribuncleic acid), DNA double helix, Enzyme, Epigenetics, Eukaryote cell (or organism), Guanine, Histone, MECP2 gene, Methyl mercury, Miniamata disease, Nucleotide, Phosphorylation, Prader-Willi syndrome, Proteins, Psychosocial stress, Social stress