Matthew Meselson and Franklin Stahl grew bacteria in a heavy nitrogen isotope so every DNA strand in the population was labeled heavy, then switched the bacteria to ordinary nitrogen and tracked the weight of their DNA across further generations by spinning it in a centrifuge. Three competing ideas about how DNA copies itself predicted three different weight patterns, and what they measured matched only one: each new DNA molecule after one generation contained exactly one original heavy strand and one newly built light strand. That result, still often called the most beautiful experiment in biology, proved DNA replication is semiconservative, confirming a prediction that followed directly from Watson and Crick's double helix model.
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YearSourced to the subject's own account MethodSourced to the subject's own accountMeselson and Stahl grew E. coli in medium built on the heavy nitrogen isotope 15N, transferred the culture to ordinary 14N medium, and used density-gradient centrifugation to track the density of DNA extracted after each round of replication. 1 Key FindingSourced to the subject's own accountAfter one replication DNA showed a single intermediate density, ruling out conservative replication, and after two replications DNA split into equal amounts of two densities, confirming Watson and Crick's semiconservative replication model. 1 Cross-Tradition Connections
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