Purines and Pyrimidines

"The numbering system for the heterocyclic base is arguably the most interesting, as it covers the two types of heterocyclic bases: purine bases (A, & G), and pyrimidine bases (C, T, & U). In this numbering system, all nitrogen atoms have odd numbers, 1,3,7, and 9 (the last two only apply to purine bases)" While the numbering may appear arbitrary, IUPAC developed a defined numbering system to reduce ambiguity. Key to the numbering system is where to start, and for a ring system, numbering starts on the largest ring and at the first substitution of carbon. Numbering continues in a way that enables the lowest numbers for substitutions. "The pyrimidine ring numbering system starts by assigning number “1” to the Nitrogen atom bonded to the pentose sugar (N-glycosidic bond), before counting clockwise to complete the numbering assignments (Figure 1-A). For purine, there are two fused rings: a six-membered ring (pyrimidine), and a five-membered ring (imidazole). In adenine, the numbering system for the pyrimidine follows a counterclockwise direction, starting from the Nitrogen atom “1” to the carbon atom “6” bonded to the exocyclic amine. " https://www.glenresearch.com/reports/gr33-15

Pyrimidine

Single aromatic ring

Purine

Double aromatic ring

Adenine

in DNA and RNA

Carbon 6 has an amino group attached

Purine

Cytosine

Found in both DNA and RNA

Pyrimidine

Thymine

Found in both DNA and some RNA

methyl group on carbon 5

Pyrimidine

Uracil

Found only in RNA

Purine

Guanine

Found in both DNA and RNA

Purine

Hypoxanthine

Purine

Carbon 6 has a carbonyl group

N6-Dimethyladenine

Purine

5-Methylcytosine

Pyrimidine

Amino of carbon 4

5-6 Dihydrouracil

Pyrimidine

Inosine

uncommon nucleoside

when a hypoxanthine is bonded to sugar

Purine

Cytidine

A ribonucleoside

Deoxyguanosine

A deoxyribonucleoside

Adenosine 5’ monophosphate

Ribonucleotides

Guanosine 5’ monophosphate

Ribonucleotides

Uridine 5’ monophosphate

Ribonucleotides

Cytidine 5’ monophosphate

Ribonucleotides