1941

The first proof that DNA was the hereditary material was provided in 1944 by Oswald Avery and Colin MacLoed. Avery and MacLoed discovered that bacteria were able to transfer genetic material through a liquid to other bacteria. And thus the bacteria that recieved the genetic material had many of the same characteristics that the orginal bacteria had!
Avery and Macloed also took a substance from a bacterium with a smooth surface and introduced it into a rough-surfaced bacterium. When the rough-surfaced bacteria transformed into the smooth-surfaced type, they discovered the substance that thhey had taken contained the gene that coded for the smooth surface. Avery and Macloed purified this substance and found it was pure DNA!

Oswald Avery