| Description | Cytosine, also known as C, belongs to the class of organic compounds known as pyrimidones. Pyrimidones are compounds that contain a pyrimidine ring, which bears a ketone. Pyrimidine is a 6-membered ring consisting of four carbon atoms and two nitrogen centers at the 1- and 3- ring positions. Cytosine is also classified as 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). Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). The nucleoside of cytosine is cytidine. In Watson-Crick base pairing, cytosine forms three hydrogen bonds with guanine. Cytosine was discovered and named by Albrecht Kossel and Albert Neumann in 1894 when it was hydrolyzed from calf thymus tissues. Cytosine exists in all living species, ranging from bacteria to plants to humans. Within cells, cytosine can undergo several enzymatic reactions. It can be methylated into 5-methylcytosine by an enzyme called DNA methyltransferase (DNMT) or be methylated and hydroxylated to make 5-hydroxymethylcytosine. The DNA methyltransferase (DNMT) family of enzymes transfer a methyl group from S-adenosyl-l-methionine (SAM) to the 5’ carbon of cytosine in a molecule of DNA. High levels of cytosine can be found in the urine of individuals with severe combined immunodeficiency syndrome (SCID). Cytosine concentrations as high as (23-160 mmol/mol creatinine) were detected in SCID patients compared to normal levels of <2 mmol/mol creatinine (PMID: 262183 ). |
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