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Cytosine is one of the four main nucleobases found in DNA and RNA. It is a pyrimidine derivative, meaning it has a two-ring structure. Cytosine pairs with guanine in DNA through three hydrogen bonds. In RNA, cytosine also pairs with guanine.

Cytosine was first identified in 1894 by Albrecht Kossel and Albert Neumann, who isolated it from calf thymus tissues. Its chemical synthesis was later achieved by Emil Fischer. Cytosine plays a crucial role in genetic information storage and transfer. It is also involved in the methylation of DNA, a process that affects gene expression.

The chemical formula for cytosine is C4H5N3O. It is a white crystalline solid. Cytosine can undergo tautomerization, existing in both keto and enol forms, though the keto form is predominant under physiological conditions. Its presence and accurate pairing with guanine are essential for the stability and integrity of the genetic code. Mutations involving cytosine, such as deamination, can lead to changes in genetic information.