How did people find out that DNA is the genetic material

Genetic material is the core factor that determine the character of an organism. So to prove that DNA is the genetic material, it should be proved that DNA is responsible for the changes in an organism.

In 1928 Griffith did a simple experiment with pneumococci bacteria to prove the above. He selected a smooth strain (virulent) and a rough strain (non-virulent).

First He injected smooth strain to mice and all the mice died.

Then he injected rough strain to mice and all the mice tolerated it well.

Then he heat killed smooth strain and injected it to mice. Mice lived fine. When treated with heat, proteins get denatured and they loose their three dimensional shape and function. This step proves that the factor immediately responsible for virulency of the smooth strain is proteins.

Then he mixed heat killed smooth strain and live rough strain and cultured it in a new medium.

Then he extracted a sample from the culture and injected it to the mice. mice died. This Step shows that the core factor that was responsible for the virulency of smooth strain has some how transferred to the rough strain.
Now the character of rough strain or the virulency of rough strain has changed. Griffith termed this phenomenon as TRANSFORMATION.

When the DNA is extracted from the heat killed smooth strain and injected to mice, mice died. When extracted DNA is mixed with rough strain, the virulency of rough strain increased. This results proves that DNA is the component responsible for characteristics (in this case, virulency).

Hershay and Chase helped to establish the fact. They proved that only when phage DNA enter the host bacterial cells, new phage particles are formed.