Base Pairing Rules

Base-Pairing Rules in DNA

By playing with scale-models of the four DNA bases A C G & T, James Watson (1926 - ) & Francis Crick (1916 - 2004) realized that an A+T pair joined by two H-bonds was structurally identical to a G+C pair joined by three H-bonds. This would allow a series of such pairs to fit, in any order, into the space between the two sugar / phosphate backbones. This explains Chargaff's Rules, and incorporates Franklin's X-ray data for two helices, with the bases inside.