Briefly describe the Meselson and Stahl experiment that indicated that DNA replication is ...

Meselson and Stahl grew bacteria in the presence of 15N and then replaced the 15N media with the lighter 14N media. DNA was extracted at various intervals to collect several successive generations. Using high-speed CsCl density-gradient ultracentrifugation, they observed that after the first generation, the heavy DNA (15N/15N) was not present, and the DNA was intermediate between light and heavy (14N/15N). After the second generation, there were two bands with equal amounts of intermediate and light bands (14N/15N and 14N/14N). These results validated the semiconservative model, in which one strand of the parent DNA is present in each daughter strand of replicated DNA.
If the conservative model were correct, they would have expected to observe DNA molecules with two distinct densities after generation 1 (15N/15N and 14N/14N). Their first-generation results excluded this model.