CpG island
A CpG island is a stretch of DNA with a high frequency of cytosine-guanine CpG sites, often near gene promoters. CpG islands are important in genome annotation, transcription regulation, and DNA methylation studies.
What a CpG island is
A CpG island is a region of DNA where cytosine followed by guanine, written CpG, appears at a higher-than-usual frequency. The p stands for the phosphate bond linking the two bases along the same DNA strand. These regions are often GC-rich and are used as landmarks in genome analysis.
Why CpG sites are unusual
CpG dinucleotides are relatively uncommon across many vertebrate genomes because methylated cytosines can mutate over evolutionary time. CpG islands stand out because they keep a higher density of CpG sites than the surrounding genome. That contrast is part of what makes them useful to identify.
Promoters and gene starts
Many CpG islands sit near promoters, the regulatory regions where transcription of a gene begins. A CpG island does not automatically mark an active gene, but its presence can help researchers locate regulatory regions and gene starts, especially when combined with other evidence.
Methylation and silencing
CpG islands are often discussed with DNA methylation because cytosines in CpG sites can be methylated. When a promoter-associated CpG island becomes heavily methylated, nearby gene transcription may be reduced. The effect depends on the genomic context, cell type, and other chromatin signals.
Unmethylated islands
Many promoter CpG islands remain unmethylated in normal cells, even though CpG sites elsewhere in the genome are often methylated. This pattern helps keep some regulatory regions accessible. It is not universal, and some CpG islands gain or lose methylation during development, disease, or cell differentiation.
How scientists define them
Computational definitions of CpG islands usually combine length, GC content, and observed-to-expected CpG frequency. Different genome browsers and studies may use slightly different thresholds. That means a region can be called a CpG island in one dataset and not in another.
Disease and research use
CpG island methylation is studied in cancer, imprinting disorders, aging research, developmental biology, and environmental epigenetics. In cancer, abnormal methylation of promoter CpG islands can contribute to the reduced expression of tumor-suppressor genes, although each case needs direct evidence.
Why it matters
CpG islands connect DNA sequence, genome evolution, promoter biology, and epigenetic regulation. They are small genomic landmarks with large practical value because they help scientists ask where genes begin, how regulatory regions are marked, and how methylation patterns change.