What was the experiment Matthew Meselson and Franklin Stahl perform?
They cultured E. coli bacteria for several generations on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, N15. The bacteria incorporated the heavy nitrogen into their DNA. The scientists then transferred the bacteria to a medium with only N14, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be lighter than the parental DNA made in the N15 medium. They could distinguish DNA of different densities by centrifuging DNA extracted from the bacteria.