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Meselson and Stahl Experiment *Meselson and Stahl grew E. colj cells for many Lendtatibng ina medium in which the heavy isotope ¢ of nitroge n, 5N, had been substituted for the normal, light isotope, “IN ' N. itl! *The purine and pyrimidine bases in DNA contain nitrogen. *Thus, the DNA of cells grown on medium containing 15N will havea greater density (mass per unit volu e) than the DNA of cells grown on medium containing MN: *Molecules with different densities can e Nenad by equilibrium density-gradient centrifugation. By using this technique, Meselson and Stahl were able to — distinguish between the three possible modes of DNA! replication — by following the changes in the density of DNA from cells § grown on+5N mediumand then transferred | to iN medium for various = periods ¢ of time—so- called density f Han transfer experiments. ee 
