purine
/ PYOOR-een /
The four DNA letters come in two sizes, like big and small puzzle pieces, and the purines are the big ones. Knowing which letters are 'big' is the secret to understanding why A pairs with T and G with C, and not some other way.
A purine is a nitrogenous base built from two fused rings — a six-membered ring stuck to a five-membered ring — making it the larger, double-ringed kind of base. There are two purines in our nucleic acids: adenine (A) and guanine (G). Because they are double-ringed, they take up more room when they reach across the centre of the double helix. The trick of base pairing is that a big purine always pairs with a small single-ringed pyrimidine, so the rungs of the DNA ladder are all the same width and the helix stays smooth and even.
Purines matter far beyond spelling. The energy molecule ATP and the signalling molecule cyclic AMP are both adenine-based; GTP, which powers many cellular switches, is guanine-based; caffeine is a modified purine. And the human body's struggle to dispose of broken-down purines is exactly what causes gout, when uric acid crystals build up in joints.
A handy memory trick: 'Pure As Gold' — PUR-ines are Adenine and Guanine. Both are double-ringed and both pair across to a single-ringed pyrimidine.
Purines (A, G) are the big, double-ringed letters.
Adenine and guanine are both purines, but they are not interchangeable: A pairs only with T (two hydrogen bonds), G pairs only with C (three). 'Both purines' does not mean 'both pair the same way'.