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Concept
CpG site
Summary
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 (or CG islands).
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.
CpG is shorthand for 5'—C—phosphate—G—3' , that is, cytosine and guanine separated by only one phosphate group; phosphate links any two nucleosides together in DNA. The CpG notation is used to distinguish this single-stranded linear sequence from the CG base-pairing of cytosine and guanine for double-stranded sequences. The CpG notation is therefore to be interpreted as the cytosine being 5 prime to the guanine base. CpG should not be confused with GpC, the latter meaning that a guanine is followed by a cytosine in the 5' → 3' direction of a single-stranded sequence.
CpG dinucleotides have long been observed to occur with a much lower frequency in the sequence of vertebrate genomes than would be expected due to random chance. For example, in the human genome, which has a 42% GC content, a pair of nucleotides consisting of cytosine followed by guanine would be expected to occur of the time. The frequency of CpG dinucleotides in human genomes is less than one-fifth of the expected frequency.
This underrepresentation is a consequence of the high mutation rate of methylated CpG sites: the spontaneously occurring deamination of a methylated cytosine results in a thymine, and the resulting G:T mismatched bases are often improperly resolved to A:T; whereas the deamination of unmethylated cytosine results in a uracil, which as a foreign base is quickly replaced by a cytosine by the base excision repair mechanism.
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