What DNA-copying model did the 1958 Meselson–Stahl experiment demonstrate?

The 1958 Meselson–Stahl experiment demonstrated that DNA replication is semiconservative.

In semiconservative replication, each daughter DNA molecule contains one original strand and one newly synthesized strand. Matthew Meselson and Franklin Stahl tested competing models by growing Escherichia coli in nitrogen-15, a heavy isotope, and then transferring the bacteria to nitrogen-14.

After one generation, the DNA formed a band of intermediate density, exactly as the semiconservative model predicted. After two generations, both intermediate-density and light DNA appeared. Conservative replication would have produced separate heavy and light bands after the first generation, while dispersive replication predicts a different pattern of DNA density.

James Watson and Francis Crick had proposed semiconservative replication in 1953, based on DNA’s paired-strand structure. The experiment did not discover DNA replication itself; it provided decisive experimental evidence for how the two strands are inherited.