Watson and Crick guessed how DNA copies itself. This experiment proved it.
DNA's double helix suggested how it might replicate, but not which way it actually did. In 1958, Matthew Meselson and Franklin Stahl grew bacteria in heavy nitrogen, switched them to light nitrogen, and tracked the DNA's weight across generations. The result confirmed each new DNA molecule keeps one original strand and builds one fresh one — often called the most beautiful experiment in biology.