How did the Meselson-Stahl experiment prove DNA replication is semiconservative?
Bacteria were grown in heavy nitrogen (15N) so all their DNA was dense, then shifted to light 14N.
Bacteria were grown in heavy nitrogen (15N) so all their DNA was dense, then shifted to light 14N. After one round of replication every DNA molecule was of intermediate density -- one heavy old strand paired with one light new strand. After two rounds, half was intermediate and half fully light. A conservative model would have left some fully heavy DNA, which never appeared. So each daughter keeps one parental strand: semiconservative.
Key point
After one generation all DNA is intermediate in density -- a result only the semiconservative model predicts.
Common mistake
thinking replication makes one all-old and one all-new molecule (the conservative idea).
Memory tip
one replication round -> all 'hybrid' density = semiconservative proof.
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