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The Meselson-Stahl experiment
Matthew Meselson
"The most beautiful experiment in biology"
Cal Tech biochemists Matthew Meselson and Frank Stahl demonstrate that replication of the DNA molecule does not produce carbon copies. Instead, each individual strand of the double helix serves as a template for the synthesis of a new complementary strand. Meselson and Stahl draw their conclusion by examining the nitrogen content of DNA molecules. Nitrogen is a principal component of DNA, but it comes in two different isotopic varieties. One, 15N, is heavier than the other, 14N. Meselson and Stahl work with E. coli bacteria. They culture some of the bugs in an environment containing only 15N, and others in an environment containing only 14N. When ‘pure’ types of DNA are established, the two cultures are mixed together. Meselson and Stahl observe, not the reproduction of pure types, but rather the synthesis of mixed molecules that contain one 15N strand and one 14N strand. They have established decisively that the replication of DNA is partly, but not fully, conservative. British physicist-turned-molecular biologist John Cairns later calls the demonstration “the most beautiful experiment in biology.”