Boiling bacteria couldn't destroy the molecule that passed on their traits
In 1944, Oswald Avery, Colin MacLeod and Maclyn McCarty purified the mysterious substance that let dead, heat-killed bacteria transform living ones into their own type — a phenomenon Frederick Griffith had observed sixteen years earlier without explaining it. By degrading the extract with different enzymes and watching which one destroyed its transforming power, they showed the agent was DNA, not protein, years before Watson and Crick described its structure.