cytosines

Cytosine is one of the four main nucleobases in the nucleic acids DNA and RNA. It is a pyrimidine base that pairs with guanine through Watson-Crick hydrogen bonding, typically forming three hydrogen bonds in DNA and in RNA duplexes. In genetic notation, cytosine is abbreviated as C.

In nucleic acids, cytosine is attached to a sugar to form cytidine or deoxycytidine, and these nucleosides become the phosphorylated nucleotides CMP, CDP, and CTP (or their deoxy forms) that serve as building blocks for RNA and DNA, respectively. Cytosine is synthesized in cells via the pyrimidine biosynthesis pathway and can also be salvaged from degradation products.

Epigenetics plays a major role for cytosine through modification at the C5 position to form 5-methylcytosine. This epigenetic mark is involved in regulating gene expression and maintaining genomic imprinting in many organisms. 5-methylcytosine is prone to deamination, producing thymine and potentially causing C:G to T:A transition mutations if unrepaired.

Chemical damage and repair: Cytosine can undergo spontaneous deamination to uracil, which creates U:G mismatches that are recognized and corrected by base-excision repair in DNA. In RNA, cytosine can also be deaminated to uracil, affecting RNA stability and function in some contexts.

Distribution and role: Cytosine is ubiquitous in both DNA and RNA and contributes to genetic coding and regulatory processes. The balance of cytosine and guanine bases influences the stability of nucleic acid duplexes and the organization of genomes.