citosin

Cytosine, sometimes referred to as citosina in some languages, is one of the four principal nucleobases in DNA and RNA. It is a pyrimidine base with the chemical formula C4H5N3O. In nucleic acids, cytosine participates in hydrogen bonding with guanine, forming the C–G base pair that helps stabilize the double helix in DNA and epithets of RNA duplexes.

In DNA, cytosine stores genetic information and participates in Watson–Crick base pairing with guanine. Cytosine can be enzymatically modified to 5-methylcytosine, a common epigenetic mark in many organisms. 5-methylcytosine influences gene expression, development, and cellular differentiation, and it can be demethylated to restore unmodified cytosine. These methylation dynamics are central to epigenetic regulation and are involved in processes such as imprinting and X-chromosome inactivation.

Cytosine is also present in ribonucleotides as cytidine and in deoxyribonucleotides as deoxycytidine, incorporated into RNA and DNA as CMP, CDP, CTP and their deoxy counterparts. It is synthesized in cells through de novo pyrimidine biosynthesis and can also be supplied by nucleotide salvage pathways that recycle existing nucleosides.

Chemical and biochemical properties of cytosine contribute to genome stability and mutation patterns. Spontaneous deamination of cytosine yields uracil, which, if unrepaired, can lead to C•G to T•A transition mutations. Methylated cytosine deamination produces thymine, contributing to common mutational signatures in genomes. Cytosine thus plays key roles in genetics, molecular biology, and epigenetics.