Watson and Crick’s accomplishment, which is ranked as one of science’s major intellectual achievements, was based in part on two pieces of evidence in addition to Chargaff’s rules: the correct tautomeric forms of the bases and indications that DNA is a helical molecule. The purine and pyrimidine bases of nucleic acids can assume different tautomeric forms (tautomers are easily converted isomers that differ only in hydrogen positions; Fig. 3-4). X-Ray, nuclear magnetic resonance (NMR), and spectroscopic investigations have firmly established that the nucleic acid bases are overwhelmingly in the keto tautomeric forms