DNA methylation

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DNA methylation can occur in either the cytosine or adenine bases, cytosine methylation is generally found in eukaryotic cells whilst both but mainly adenine methylation can be found in bacteria[1]. Methylation is the addition of a methyl group to another chemical substance. DNA methylation results in the addition of a methyl group to carbon number five in the respective base by one of three enzymes called DNMT1, DNMT3A or DNMT3B. DNMT refers to an enzyme DNA methyltransferase. These enzymes usually add a methyl group to CpG (i.e a C that is followed by a G) which are abundantly found in the promoter region of a DNA, called CpG islands, as stated by Professor Adrian Bird in one of his published papers[2]. As an example, when cytosine is methylated it becomes 5-methylcystosine[3]. This change often occurs in the major grooves of the DNA strand, resulting in transcription factors being unable to bind to the area[4].

Methylation of DNA greatly affects gene expression. The effect of DNA methylation is that those methylated areas of the DNA are not sequenced during DNA Transcription and therefore the affected genes are silenced, proceeding to show phenotypic change. In this way, DNA methylation plays a main role in epigenetics.

Methylation of a gene, addition of a methyl group (-CH3), is catalyzed by the family of enzymes DNA-methyltransferases (DNMTs) and occurs at the 5-carbon on a cytosine ring; the methyl group projects into the major groove of the DNA double helix, causing the DNA-histone complex to become more tightly condensed, which prevents/negatively affects the binding of transcriptional factors to promoter regions or induces deacetylation of the histone proteins which inhibits transcription of the DNA sequence to mRNA.


  1. D. T. Meštrović, "DNA Methylation in Bacteria," News Medical Life Sciences and Medicine, 10 09 2014. [Online]. Available: http://www.news-medical.net/health/DNA-Methylation-in-Bacteria.aspx. [Accessed 23 11 2015].
  2. Nessa Carey, The Epigenetics Revolution.
  3. D. L. a. E. W. Jones, "Genetics Analysis of Genes and Genomes 7th Edition," in Epigenetic Mechanisms of Transcriptional Regulation, London, Jones and Bartlett Publishers, 20009, p. 411.
  4. Klug WS, Cummings MR, Spencer CA, Palladino MA. Concepts of genetics. 11th ed. Harlow (England): Pearson; 2016. p. 708-17
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