MeselsonStahlexperiment

The Meselson–Stahl experiment, conducted by Matthew Meselson and Franklin Stahl in 1958, provided decisive evidence that DNA replication is semi-conservative. They used isotopic labeling with heavy nitrogen (15N) to distinguish old DNA strands from newly synthesized ones in Escherichia coli.

In their setup, E. coli cells were grown for several generations in medium containing 15N to label the entire DNA. The culture was then shifted to 14N-containing medium to allow new DNA synthesis. DNA was extracted at various time points and analyzed by density gradient centrifugation in caesium chloride, a method that separates molecules by buoyant density.

After one round of replication in 14N, the DNA formed a single intermediate-density band, indicating that each double helix consisted of one heavy strand and one light strand. After a second round of replication, two bands appeared: one light and one intermediate, consistent with each molecule containing one old and one newly synthesized strand in some proportion. This pattern effectively ruled out the conservative and dispersive models and supported the semi-conservative replication mechanism.

The experiment is regarded as a classic in molecular biology, illustrating how genetic information is faithfully transmitted during cell division. It also showcased the utility of isotopic labeling and density gradient centrifugation as powerful tools in studying the molecular basis of heredity.