CpG site

Not to be confused with CpG oligodeoxynucleotide.

The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG islands. CpGs are the common unit of methylation because their information is double stranded (the GpC complement is also methylated). This allows methylation information to be preserved during cell division.

Cytosines in CpG dinucleotides can be methylated to form 5-methylcytosines. Enzymes that add a methyl group are called DNA methyltransferases. In mammals, 70% to 80% of CpG cytosines are methylated. Methylating the cytosine within a gene can change its expression, a mechanism that is part of a larger field of science studying gene regulation that is called epigenetics. Methylated cytosines often mutate to thymines.

In humans, about 70% of promoters located near the transcription start site of a gene (proximal promoters) contain a CpG island.

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