Cytosine is one of the four main building blocks of DNA and RNA, which are essential molecules for life. It is a type of nitrogenous base, along with adenine, guanine, and thymine. In DNA, cytosine pairs with guanine to help form the structure of the double helix. This pairing is crucial for the accurate replication and transmission of genetic information. In addition to its role in DNA, cytosine is also found in RNA, where it pairs with uracil instead of thymine. Cytosine plays a vital role in various biological processes, including gene expression and protein synthesis. Its unique structure allows it to participate in important chemical reactions within cells.