CpG Site
CpG sites or CG sites are regions of DNA where a cytosine nucleotide occurs next to a guanine nucleotide in the linear sequence of bases along its length. "CpG" is shorthand for "—C—phosphate—G—", that is, cytosine and guanine separated by only one phosphate; phosphate links any two nucleosides together in DNA. The "CpG" notation is used to distinguish this linear sequence from the CG base-pairing of cytosine and guanine.
Cytosines in CpG dinucleotides can be methylated to form 5-methylcytosine. In mammals, methylating the cytosine within a gene can turn the gene off, a mechanism that is part of a larger field of science studying gene regulation that is called epigenetics. Enzymes that add a methyl group are called DNA methyltransferases.
In mammals, 70% to 80% of CpG cytosines are methylated.
Unmethylated CpG sites can be detected by Toll-Like Receptor 9 (TLR 9) on plasmacytoid dendritic cells and B cells in humans. This is used to detect intracellular viral, fungal, and bacterial pathogen DNA.
Read more about CpG Site: Frequency in Vertebrates, CpG Islands, Methylation, Silencing, and Cancer
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