DNA methylation is a mechanism used to regulate genes and protect DNA from some types of cleavage. It is one of the regulatory processes that are referred to as epigenetic, in which an alteration in gene expression occurs without a change in the nucleotide sequence of DNA. Defects in this process cause several types of disease that afflict humans.
In methylation, a methyl group (-CH3) is added to position five of the cyto-sine ring in a DNA molecule (see diagram), attaching itself there by means of a chemical bond. For methylation to occur in DNA, certain conditions must be met. The cytosine must be linked to guanine, with the guanine occurring at the 3′ ("three prime") end of the DNA molecule, in a formation that, in s…
The frequency of occurrence of the CpG dinucleotide in the genome is not random, as would be expected. Instead, the CpG dinucleotide is greatly under represented in eukaryotic genomes, occurring at approximately 5 to 10 percent of its predicted frequency, according to some estimates. Of these occurrences, it is further estimated that 70 to 80 percent are methylated. This under representation of Cp…
The process of methylation was first described in bacteria in 1948. Most bacterial strains contain enzymes called restriction endonucleases. These restriction enzymes recognize certain short sequences of DNA, and cleave the DNA strand at these sites. By modifying its DNA with a pattern of methylation specific to its strain, a bacterium can use this system of modification and restriction to disting…
Changes in DNA methylation patterns have been implicated in the development and progression of many types of cancers. Additionally, defects in DNA methylation have been associated with several genetic diseases, including ICF (Immunodeficiency, Centromere Instability, and Facial Anomalies), Rett, and Fragile X syndromes, all of which result in variable degrees of mental retardation. This common eff…
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