Biochemistry, molecular genetics, and biology glossary terms for Companion Sites.

CpG island

CpG islands are discrete clusters of nonmethylated CpG dinucleotide segments composed of large numbers of phosphodiester-linked cytosine "p" guanine nucleobases. CpG islands comprise about 1 to 2% of the mammalian genome (4, 7), and are located near or within approximately 40% of promoters in mammalian genes.

The formal definition of a CpG island is 'a region with at least 200 bp and with a GC percentage that is greater than 50% and with an observed/expected CpG ratio that is greater than 0.6.'

In a CpG island, a cytosine nucleotide occurs phosphodiester-bonded to a guanine nucleotide. The "p" in the CpG notation distinguishes a cytosine adjacent to a phosphodiester-linked guanine from a cytosine base paired to a guanine on a complementary strand.

Unlike CpG sites in the coding region of a gene, bases within CpG islands are unmethylated if the promoted genes are expressed. Most CpG islands are associated with genes or recognition sites for restriction enzmes. This includes all genes that are ubiquitously expressed (housekeeping genes) plus many genes with a tissue-restricted pattern of expression. Many human and mouse major histocompatibility locus (MHC) genes contain CpG-rich regions (64), yet only the β-chain genes for class II MHC have CpG-rich regions [r].

Promoters are normally located at the upstream edge of the CpG island, such that one or more of the 5′ exons of the gene generally fall within the island region. Although most CpG islands are nonmethylated in all tissues, a small proportion of islands become methylated during development [s]. 82% of all Not 1 sites are found in CpG islands.

Toll-like receptors, a type of innate immune system pattern-recognition receptor, recognize and ligate unmethylated CpGs as a subclass of pathogen-associated molecular pattern (PAMP)

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