Cytosine

Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto group at position 2). The nucleoside of cytosine is cytidine.

Structure and Properties

Cytosine is a crystalline, water-soluble compound and can be hydrolyzed to yield urea and cytosine arabinoside. In DNA and RNA, cytosine is paired with guanine. However, it can mutate into uracil (spontaneous deamination). This can lead to a point mutation if not repaired by the DNA repair enzymes.

Biological Functions

Cytosine plays a crucial role in the structure of the genetic code. It pairs with guanine through three hydrogen bonds, assisting in the stability of the DNA molecule. Cytosine methylation, a modification of the DNA molecule involving the addition of a methyl group to the cytosine residue, is an important mechanism in controlling gene expression.

Clinical Significance

Alterations in cytosine residues in DNA, such as mutations and abnormal methylation patterns, can lead to various diseases, including cancer. Cytosine arabinoside is a chemotherapy drug used to treat certain types of leukemia.

See Also

• Nucleotide
• Nucleoside
• Pyrimidine
• DNA
• RNA

References

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