purinepyrimidine

Purinepyrimidine is a term used to describe the two principal classes of nitrogenous bases that form the building blocks of nucleic acids: purines and pyrimidines. The purine bases are adenine and guanine, while the pyrimidine bases are cytosine, thymine (in DNA), and uracil (in RNA). Purines are larger, with a bicyclic ring system, whereas pyrimidines are single-ring structures.

In nucleic acids, these bases pair through hydrogen bonding to enable complementary base pairing and the storage of genetic information. In DNA, adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds. In RNA, thymine is replaced by uracil, so adenine pairs with uracil. These specific pairings contribute to the structure of the double helix in DNA and to the various structures of RNA.

Nucleotides are formed when these bases are linked to a sugar molecule (deoxyribose in DNA, ribose in RNA) and one or more phosphate groups. The resulting nucleotides serve as the monomers for nucleic acids and also function as key energy carriers and coenzymes (for example, ATP, GTP, NAD, and FAD).

Biosynthesis of purine and pyrimidine nucleotides occurs via distinct pathways. Purine nucleotides are synthesized de novo through a ribose-5-phosphate pathway ultimately yielding inosine monophosphate (IMP), a common precursor for AMP and GMP. Pyrimidine nucleotides are assembled from carbamoyl phosphate and aspartate to produce UMP, which is further converted to UDP, UTP, and CTP. Both classes also have salvage pathways that recover bases for reuse.

Disruptions in purine or pyrimidine metabolism have clinical relevance, including gout and certain cancers, where antimetabolite drugs target these pathways.