In the 1950s, details of the genetic material, DNA, began to pour out of laboratories. First, James Watson and Francis Crick built a model for the double helical structure of DNA, showing that one strand consists of a sequence of bases that are complementary to the bases in the opposite strand. These scientists also suggested a model for the replication of DNA that would allow a cell to copy its genetic material and pass down exact replicas to daughter cells. Each old strand of the double helix would serve as a template to make a new strand. Although the simplicity of their replication model was compelling, no data yet existed to prove that it was correct.

A few years after Watson and Crick published their DNA structural model, the scientists Matthew Meselson and Franklin Stahl designed an elegant experiment to determine how DNA replicates.


The Meselson-Stahl experiment showed that DNA replicates by a semiconservative mechanism. The double helix separates so that each old strand serves as a template for a new strand. Two new double helices result, each containing one new strand and one old strand.

Although Meselson and Stahl performed their experiment a half-century ago, the experiment has a modern quality. Their experiment represented a new way of doing science that is, making predictions based on different models and then determining which prediction most closely aligns with the experimental data. The materials they used are also still in wide use today. Isotopes are used for tracking atoms and molecules, CsCl-based ultracentrifugation is used to form a collected band of DNA molecules, and E. coli remains an important experimental organism. More significantly, the simplicity and elegance of the experimental design make their experiment a timeless classic.

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Textbook Reference: Concept 9.2 DNA Replicates Semiconservatively