Pyrimidine

A pyrimidine is an organic molecule with a six-membered ring containing two nitrogen atoms. The term also names a class of nucleobases that includes cytosine, thymine and uracil, which carry information in DNA and RNA.

Pyrimidines have a single ring, whereas the purines adenine and guanine have two fused rings. In base pairing, each purine pairs with a pyrimidine so that the width of the double helix stays constant: adenine with thymine (or uracil in RNA) and guanine with cytosine. Cytosine and thymine occur in DNA, while uracil replaces thymine in RNA.

Cells build the pyrimidine ring first and attach the sugar later, which is the opposite of purines. The pathway begins with carbamoyl phosphate and the amino acid aspartate, forms the ring of orotate, and then adds ribose phosphate to make the nucleotide UMP. Other pyrimidine nucleotides are made from UMP. The pathway is regulated by feedback, a typical case of allosteric regulation.

Neighboring thymines in DNA easily form dimers when hit by ultraviolet light, a common kind of damage fixed by DNA repair. Because cancer cells and viruses copy their genes quickly, drugs that mimic pyrimidines, such as 5-fluorouracil and many antiviral nucleoside analogs, interfere with their DNA or RNA synthesis. Related building blocks are described in nucleic acids.

• Nucleobase — the class of bases they belong to
• Cytosine — a pyrimidine base found in DNA and RNA
• Nucleic Acids — the molecules that contain them