Meselson-Stahl experiment

The Meselson-Stahl experiment tested how parental strands are distributed during DNA replication. Escherichia coli grown with heavy nitrogen were transferred to light-nitrogen medium. Extracted DNA was separated by buoyant density in a caesium-chloride gradient using analytical ultracentrifugation; ultraviolet imaging located the bands. ^[ref]

After one generation, the main DNA band had intermediate density, contrary to the separate heavy and light populations predicted by a conservative-copying model. After two generations, intermediate and light bands supported semiconservative replication over a uniformly dispersive model. Under ideal synchronous doubling, the fraction of duplexes retaining an original heavy strand after $n\ge 1$ generations is $2^{1-n}$. The experiment inferred strand inheritance from density distributions; it did not directly photograph separately coloured strands.