Pure As Gold: purines are Adenine and Guanine; the rest are pyrimidines
Mnemonic
“PURe As Gold” and “CUT the Py”:
- Purines are Adenine and Guanine, and they have two rings
- Pyrimidines are Cytosine, Uracil and Thymine, and they have one ring
“Pyrimidines are CUT from one piece; purines are pure gold, two rings.”
Base pairing follows: A with T (or U) by two hydrogen bonds, G with C by three, which is why GC rich DNA has a higher melting temperature.
Synthesis differs in a way worth remembering: purines are built onto ribose from the start, whereas pyrimidines are built as a free ring and then attached, which is why orotic acid accumulates in pyrimidine pathway defects.
Expansion
- Purines (two rings): Adenine and Guanine. Pure As Gold
- Pyrimidines (one ring): Cytosine, Thymine (DNA) and Uracil (RNA). CUT the pie
Base pairing: A pairs with T (or U in RNA) by two hydrogen bonds; G pairs with C by three. This is why GC-rich sequences have a higher melting temperature and are harder to separate, which matters in PCR design.
Degradation differs importantly:
- Purines are degraded to uric acid, which is poorly soluble. Overproduction or underexcretion causes gout, and tumour lysis after chemotherapy releases a large purine load
- Pyrimidines are degraded to soluble products (beta-alanine, ammonia and carbon dioxide), so they cause no equivalent clinical problem
Nucleoside is base plus sugar; nucleotide adds one or more phosphates.
Drugs exploit these pathways: allopurinol inhibits xanthine oxidase; 5-fluorouracil blocks thymidylate synthase; methotrexate blocks folate-dependent purine and thymidine synthesis; and many antivirals are nucleoside analogues.