CpGdinukleotideja

CpG dinukleotideja refer to a sequence of two nucleotides in DNA where cytosine (C) is immediately followed by guanine (G) on the same strand. These dinucleotides are relatively rare in the genomes of many vertebrates, including humans, due to a process called CpG methylation. Cytosine residues can be methylated by DNA methyltransferases, converting them to 5-methylcytosine. These methylated CpG dinucleotides are then more likely to be spontaneously deaminated into thymine (T), leading to C-to-T transitions during DNA replication. As a result, CpG dinucleotides are significantly underrepresented in methylated regions of the genome.

However, unmethylated CpG dinucleotides are often found clustered in regions called CpG islands. These islands are typically located in or near the promoter regions of genes, playing a crucial role in gene regulation. The presence of unmethylated CpG islands often indicates that a gene is actively being transcribed. Conversely, methylation of CpG islands in promoter regions is generally associated with gene silencing, a mechanism important for processes like X-chromosome inactivation and the suppression of transposable elements.

The immune system also recognizes unmethylated CpG dinucleotides. Bacterial DNA, which is rich in unmethylated CpG motifs, is recognized by Toll-like receptor 9 (TLR9) in immune cells. This recognition triggers an innate immune response, which is a key defense mechanism against microbial infection. This distinction between methylated (mammalian) and unmethylated (bacterial) CpG dinucleotides is crucial for differentiating self from non-self.