pyrimidines

Pyrimidines are a class of heterocyclic aromatic organic compounds that form a six-membered ring containing two nitrogen atoms at positions 1 and 3, classically described as 1,3-diazines. The parent ring is simply called pyrimidine, and the term also denotes derivatives in which various carbon atoms are substituted.

In biology, the primary pyrimidine bases are cytosine, thymine, and uracil. Thymine is found in DNA, while uracil is the RNA equivalent, with uracil replacing thymine in RNA. Cytosine occurs in both DNA and RNA. In DNA, cytosine pairs with guanine via three hydrogen bonds; in RNA, uracil pairs with adenine. These bases form the genetic code together with purine bases, namely adenine and guanine.

Biosynthesis and metabolism: organisms synthesize pyrimidines through pathways that assemble the ring before attaching a ribose phosphate, forming nucleotides such as UMP, CMP, and eventually UDP, UTP, CTP, and dNTPs for DNA synthesis. There are also salvage pathways that recycle nucleosides like cytidine, uridine, and thymidine. Pyrimidines play essential roles as components of nucleic acids and as activated donors in various biosynthetic processes (for example, CTP and UTP in RNA synthesis).

Derivatives of the pyrimidine ring are widespread in medicine and industry. Many pyrimidine analogs, including 5-fluorouracil and nucleoside analogs like cytarabine and gemcitabine, are used as anticancer agents or antivirals. The chemistry and biology of pyrimidines thus intersect with genetics, metabolism, and pharmacology.