Meselson–Stahl experiment
Meselson and Stahl found that DNA employs semi-conservative replication. They put the bacteria in an environment with a Nitrogen isotope. They first used N15. The bacteria then integrated this isotope into their DNA. Later they used an environment that contained N14. They then looked at which of the isotopes the bacterial DNA contained. They found that it contained both isotopes of nitrogen, which implies that conservative replication is not the correct conclusion. By having a closer look they could then also rule out disperse replication. Some helix strands were all v. Other helix strands from the time in the N15 environment had N14.