Methylation

In Chemistry, Methylation isaddition ofmethyl group tosubstrate.

In Epigenetics, Methylation can refer toaddition ofmethyl group tocytosine residueDNAconvert it5-methylcytosine oraddition ofmethyl group or groupsarginine or lysine amino acids inprotein.

MethylationDNA occurs at any CpG sites, whichsequencesDNA where cytosine lies nextguanine. The processmethylationmediated by an enzyme known as DNA methyltransferase. CpG sitesquite rare ineukaryotic genome exceptregions nearpromoter ofeukaryotic gene. These regionsknown as CpG islands, andstatemethylationthese CpG sitescriticalgene activity/expression.

In early development (fertilisation8-cell stage),eukaryotic genomedemethylated. From8-cell stage tomorula, de novo methylation ofgenome occurs, modifyingadding epigenetic information togenome. By blastula stage,methylationcomplete. This processreferredas "epigenetic reprogramming". The importancemethylation was shownknockout mutants without DNA methyltransferase. Allresulting embryos died atmorula stage.

The patternmethylation has recently become an important topicresearch. Studies have found thatnormal tissue, methylation ofgenemainly localised tocoding region, whichCpG poor. In contrast,promoter region ofgeneunmethylated, despitehigh densityCpG islands inregion.

Interestingly,cancer cells, methylationvery high even inpromoter region, raising interest inrolemethylation ininductioncancerous properties. Furthermore,patternmethylation has been shownbereliable markercancerous tissue, withheavily methylated gene found90% or more patientsprostate cancer.

Additionally, adenosine methylationpart ofrestriction modification systemmany bacteria. Bacterial DNAsmethylated periodically throughoutgenome,foreign DNAs (whichnot methylatedthis manner) thatintroduced intocelldegraded by restriction enzymes. Bacteria protect themselves from infection by bacteria viruses, called bacteriophage or phage, through this system.

Methylation can occur onargininelysine residuesproteins. Methylationprotein has been most well studied inhistones. The transfermethyl groups from S-adenosyl-methioninehistonescatalyzed by enzymes known as histone methyltransferases. Histones whichmethylated on certain residues can actepigeneticallyrepress or activate "gene" expression.