DNA methylation is a crucial part of normal organismal development and cellular differentiation in higher organisms. DNA methylation stably alters the gene expression pattern in cells such that cells can “remember where they have been” or decrease gene expression; for example, cells programmed to be pancreatic islets during embryonic development remain pancreatic islets throughout the life of the organism without continuing signals telling them that they need to remain islets.
In addition, DNA methylation suppresses the expression of viral genes and other deleterious elements that have been incorporated into the genome of the host over time.
DNA methylation involves the addition of a methyl group to the 5 position of the cytosine pyrimidine ring or the number 6 nitrogen of the adenine purine ring (cytosine and adenine are two of the four bases of DNA).
This modification can be inherited through cell division. DNA methylation is typically removed during zygote formation and re-established through successive cell divisions during development although the latest research shows that hydroxylation of methyl group occurs rather than complete removal of methyl groups in zygote.
DNA methylation also forms the basis of chromatin structure, which enables cells to form the myriad characteristics necessary for multicellular life from a single immutable sequence of DNA.
DNA methylation also plays a crucial role in the development of nearly all types of cancer.
DNA methylation involves the addition of a methyl group to DNA – for example, to the number 5 carbon of the cytosine pyrimidine ring – in this case with the specific effect of reducing gene expression. DNA methylation at the 5 position of cytosine has been found in every vertebrate examined. In adult somatic tissues, DNA methylation typically occurs in a CpG dinucleotide context; non-CpG methylation is prevalent in embryonic stem cells.
