cytosine

Cytosine is one of the four main nucleobases found in DNA and RNA. It is a pyrimidine derivative with the chemical name 4-aminopyrimidin-2-one and a molecular formula of C4H5N3O. Structurally, cytosine features a six-membered pyrimidine ring bearing an exocyclic amino group at the 4-position and a carbonyl group at the 2-position.

In nucleic acids, cytosine pairs with guanine through three hydrogen bonds, contributing to the stability of the genetic double helix in DNA and to base-pairing interactions in RNA structures. Cytosine is incorporated into RNA alongside the other ribonucleotides and participates in the same C–G base-pairing rules when RNA strands form structures or duplexes.

Cytosine can undergo chemical modification. The most common modification in many organisms is methylation at the 5-position to form 5-methylcytosine, an epigenetic mark involved in gene regulation and development. Cytosine is also subject to spontaneous deamination, converting it to uracil. If this occurs in DNA, repair systems such as base-excision repair recognize the uracil and restore cytosine to prevent mutation; failure to repair can lead to C to T transitions over time. In addition, cytidine deaminases can catalyze deamination in other contexts, which is relevant in innate immunity and viral editing.

Cytosine is ubiquitous in genetic material across life and is essential for the encoding and expression of genetic information in both DNA and RNA.