Cytosine can be found as part of DNA, as part of RNA, or as a part of a nucleotide. As cytidine triphosphate (CTP), it can act as a co-factor to enzymes, and can transfer a phosphate to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP). In DNA and RNA, cytosine is paired with guanine.
The purine bases adenine and guanine and pyrimidine base cytosine occur in both DNA and RNA, while the pyrimidine bases thymine (in DNA) and uracil (in RNA) in just one. Adenine forms a base pair with thymine with two hydrogen bonds, while guanine pairs with cytosine with three hydrogen bonds.
Nucleotides are the basic monomer building block units in the nucleic acids. A nucleotide consists of a phosphate, pentose sugar, and a heterocyclic amine. Adenosine 5'-monophosphate (AMP): The phosphoric acid forms a phosphate-ester bond with the alcohol on carbon #5 in the pentose.
There are four different DNA nucleotides, each with one of the four nitrogen bases (adenine, thymine, cytosine, and guanine). The first letter of each of these four bases is often used to symbolize the respective nucleotide (A for adenine nucleotide, for example).