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The Meselson - Stahl Experiment The three hypothesis of DNA replication were evaluated by Mathew Meselson and Franklin Stahl of the California Institute of Technology in 1958. They grew bacteria in a medium containing heavy isotope of nitrogen, “N, which became incorporated into the bases of the bacterial DNA. After several generations, the DNA of these bacteria was denser than that of bacteria grown in a medium containing the lighter isotope of nitrogen, N'*. Meselson and Stahl then transferred the bacteria from the N'° medium to the N'* medium and collected the DNA at various intervals. . They dissolved the DNA in cesium chloride and then spun it at a very high speed in an ultra-centrifuge. DNA strands of different densities got separated. The enormous centrifugal forces generated by the ultracentrifuge caused the cesium ions to migrate toward the bottom of the centrifuge tube, creating a gradient of CsCl, and thus of density. Each DNA floats or sinks in the gradient until it reaches the position where its density exactly matches the density of cesium there. Because N" strands are denser than N _ strands, they migrate farther down the tubes to a denser region of the cesium chloride gradient. Bottom of tube 
