5metylocytozyna

5-metylocytozyna, commonly referred to as 5-methylcytosine (5mC), is a methylated form of cytosine found in DNA. It carries a methyl group at the carbon-5 position of the cytosine ring and functions as a key epigenetic modification that helps regulate gene expression and genome stability.

Biosynthesis and distribution: In mammals, 5mC is produced by DNA methyltransferases using S-adenosylmethionine as the methyl donor. DNMT1 primarily maintains methylation patterns after DNA replication, while DNMT3A and DNMT3B establish new methylation marks (de novo methylation). In plants, DRM2 performs de novo methylation, with maintenance methylation carried out by MET1 and CMT family enzymes; methylation occurs in CG, CHG, and CHH contexts. 5mC can be modified by the TET family of dioxygenases to form 5-hydroxymethylcytosine (5hmC), and further to 5-formylcytosine and 5-carboxylcytosine, enabling active demethylation through base excision repair.

Biological roles: DNA methylation at promoter-associated CpG islands is often linked to transcriptional repression. 5mC participates in genomic imprinting, X-chromosome inactivation, and silencing of transposable elements, contributing to proper development and genome integrity. Methylation patterns are dynamic during development and differentiation and are frequently perturbed in cancer and other diseases. 5mC can spontaneously deaminate to thymine, creating C to T mutations at CpG sites, a common mutational signature in tumors.

Detection and context: DNA methylation is commonly detected by bisulfite sequencing, which differentiates methylated from unmethylated cytosines; other methods include MeDIP-seq and MBD-seq. In mammals, methylation is predominantly at CpG dinucleotides, whereas non-CpG methylation (CHG/CHH) is more prevalent in certain cell types and plants. Note that RNA also contains a distinct modification known as m5C.