sytosiini

Cytosine, known in Finnish as sytosiini, is one of the four canonical nucleobases in DNA and RNA. Its chemical formula is C4H5N3O, and its molecular weight is about 111.1 g/mol. It is a pyrimidine base.

In structure, cytosine is a pyrimidine ring bearing an exocyclic amino group at position 4 and a carbonyl group at position 2. The base is polar and can exist in several tautomeric forms, though the amino-oxopyrimidine form predominates in biology.

Role in nucleic acids: In DNA cytosine pairs with guanine through three hydrogen bonds, contributing to helix stability. In RNA it also pairs with guanine. As a nucleotide, cytosine is present as deoxycytidine in DNA and cytidine in RNA.

Metabolism and biosynthesis: Cytosine is formed in cells through the pyrimidine metabolic pathway and can be salvaged from cytosine-containing nucleotides. It is incorporated into nucleotides via phosphorylation, producing CMP or dCMP for RNA and DNA, respectively.

Epigenetics and mutations: 5-methylcytosine is a common epigenetic modification in many organisms. Deamination of 5-methylcytosine yields thymine, creating C:G to T:A transitions; cellular processes include active and passive demethylation to restore cytosine.

Damage and repair: Spontaneous cytosine deamination to uracil can occur; cells repair uracil in DNA by base excision repair pathways to maintain genetic fidelity.

Cytosine is central to genetics and molecular biology, underpinning genetic information storage and expression, epigenetic regulation, and numerous biotechnological applications.