The natural process of gene expression is where the inheritable information provided by a gene on a DNA strand is turned into a gene product with some function for the organism. The process involves several stages and the end product will be a protein or an RNA strand. DNA methylation is the process by which an amino acid, found in a protein, has a methyl group, -CH3, added to it (this is catalysed by the DNA methyltransferase enzyme). It is an important process as an epigenetic mechanism in the hypothesised epigenetic code that is thought to act above the cellular genetic code.
To see where DNA methylation fits in to the process of gene expression it is worth giving an overview of the stages of gene expression more generally. Overall the process of gene expression can be described in terms of the processes of transcription and translation for producing a protein, and just the process of transcription for an RNA product. Transcription is the process whereby RNA is made from DNA.
Transcription involves several steps, including the unwinding of the DNA double helix, the use of one of these strands to act as a template for transcription, RNA molecules being synthesised using the RNA polymerase enzyme, and the ending of the synthesis on reaching a terminator signal. The transcribed RNA then leaves the vicinity of the DNA and the two DNA strands reanneal.
Translation is the process whereby the RNA sequence is used to create a new polypeptide chain at the ribosomes. This involves several steps, including the attachment of mRNA to the ribsosomes, the passage of the RNA through the ribosome, the reading of the RNA to bring the correct amino acid and then the joining of the amino acids by peptide bonding, thus creating a polypeptide chain.
So where does DNA methylation fit in to the process of gene expression? DNA methylation can have an effect of modifying the normal gene expression. Adding a methyl group to the cytosine pyramid ring's number 5 carbon can, for example, reduce gene expression. The chemical changes of the DNA brought about by DNA methylation can be added before it is inherited and removed afterwards. Also, DNA methylation is specific to particular cells and tissues, unlike the ordinary genetic endowment, which is common to all cells of the individual.
DNA methylation has been found in a variety of organisms including animals, plants, fungi, and bacteria, and is the epigenetic mechanism with the strongest evidence for it. In mammals, for example, it has been shown that normal development requires DNA methylation for things such as imprinting. Whether claims for it such as it having a significant role in human long term memory storage remain to be proven.