sitosin

Sitosin, known in English as cytosine, is one of the four main nucleobases found in DNA and RNA. It is a pyrimidine base characterized by a carbonyl group at position 2 and an exocyclic amino group at position 4, giving it the chemical designation 2(1H)-pyrimidinone with an amino substituent.

In genetic material, cytosine pairs with guanine through three hydrogen bonds, contributing to the stability of DNA and to the structure of RNA where cytosine also pairs with guanine. The GC content of a sequence influences the melting temperature and the stability of nucleic acids. Cytosine is widely distributed across organisms and is present in coding and noncoding regions of genomes, as well as in various RNA molecules.

Cytosine can undergo deamination to form uracil, which can lead to C:G to T:A transitions if not repaired. Therefore, DNA repair systems continually monitor and correct such lesions. A major area of cytosine biology concerns its cytosine modifications. The most common is 5-methylcytosine, formed by methylation at the 5-position, a key epigenetic mark in vertebrates that can regulate gene expression. Further oxidation products, such as 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine, are involved in active DNA demethylation pathways.

Cytosine is also present in RNA, where variations and modifications influence RNA stability, folding, and function. Overall, cytosine plays essential roles in genetic information storage, expression, and regulation across life.