Watson and Crick’s theoretical model was confirmed with one of the most beautiful experiments in biology: Matthew Meselson and Frank Stahl fed Escherichia coli heavy nitrogen (with an extra neutron) and discovered that its DNA became heavier than that of bacteria fed normal nitrogen. They then fed the heavy DNA bacteria normal nitrogen and verified that in the first generation, the DNA weight was halfway between that of the heavy and normal bacteria. This demonstrated that DNA is indeed composed of two parts (the two helices): one is the original, made up of heavy nitrogenous bases, and the other is the copy, made up of light nitrogenous bases.
