Embryo Project Encyclopedia Articles

Matthew Meselson, Franklin Stahl, and Jerome Vinograd, developed cesium chloride, or CsCl, density gradient centrifugation in the 1950s at the California Institute of Technology, or Caltech, in Pasadena, California. Density gradient centrifugation enables scientists to separate substances based on size, shape, and density. Meselson and Stahl invented a specific type of density gradient centrifugation, called isopycnic centrifugation that used a solution of cesium chloride to separate DNA molecules based on density alone. When Meselson and Stahl developed the technique in the mid-1950s, scientists had no other way to separate macromolecules that were of similar size but varied in density. Meselson and Stahl employed their method to determine how DNA replicates, became known as the Meselson-Stahl experiment. Density gradient centrifugation using cesium salts allowed scientists to isolate DNA and other macromolecules by density alone.

Between 1953 and 1957, before the Meselson-Stahl experiment verified semi-conservative replication of DNA, scientists debated how DNA replicated. In 1953, James Watson and Francis Crick proposed that DNA was composed of two helical strands that wound together in a coil. Their model suggested a replication mechanism, later termed semi-conservative replication, in which parental DNA strands separated and served as templates for the replication of new daughter strands. Many scientists, beginning with Max Delbrück, questioned Watson and Cricks’ model and suggested new theories for DNA replication. By 1957, three theories about DNA replication prevailed: semi-conservative, conservative, and dispersive replication. Then, Matthew Meselson and Franklin Stahl conducted the Meselson-Stahl experiment, which returned results that supported the semi-conservative theory of DNA replication. The collaboration among scientists that ultimately produced concrete evidence of the DNA replication mechanism furthered both theoretical and physical explanations of genetics and molecular biology, providing insight into how life develops, reproduces, and evolves.