How it was proved that $DNA$ is the Genetic material ?
 

Even though the discovery of nuclein by Meischer and the proposition for principles of inheritance by Mendel were almost at the same time but that the $DNA$ acts as a genetic-material took long to be discovered and proven.

By $1926$, the quest to determine the mechanism for genetic inheritance had reached the molecular level.

Previous discoveries by Gregor Mendel, Walter Sutton, Thomas Hunt Morgan an numerous other scientists had narrowed the search to the chromosomes located in the nucleus of most cells.

Transforming Principle : In $1928$, Frederick Griffith, in a series of experiment with streptococcus pneumoniae (bacterium responsible for pneumonia), witnessed a miraculous transformation in the bacteria.

During the course of his experiment, a living organism (bacteria) had changed in physical form. When streptococcus pneumoniae (pneumococcus) bacteria are grown on a culture plate some produced smooth shiny colonies $(S)$ while others produce rough colonies $(R$).

This is because the $'S'$ strain bacteria have a mucous (polysaccharide) coat, while $R$ strain does not.

Mice infected with the $S$ strain (virulent) die from pneumonia infection but mice infected with the

$R$ strain do not develop pneumonia.

$S$ strain $\longrightarrow$ Inject into mice $\longrightarrow$ mice die

$R$ strain $\longrightarrow$ Inject into mice $\longrightarrow$ mice live

Griffith was able to kill bacteria by heating them.

He observed that heat killed $S$ strain bacteria injected into mice did not kill them.

$S$ strain (heat killed) $\longrightarrow$ Inject into mice $\longrightarrow$ mice live

$S$ strain (heat killed) $+\mathrm{R}$ strain (live) $\longrightarrow$ Inject into mice $\longrightarrow$ mice die

When he injected a mixture of heat-killed $S$ and live $R$ bacteria the mice died.

Moreover, he recovered living $S$ bacteria from the dead mice.

He concluded that the $R$ strain bacteria had somehow been transformed by the heat killed $S$ strain bacteria.

Some 'transforming principle' transferred from the heat-killed $S$ strain, had enabled the $\mathrm{R}$ strain to synthesise a smooth polysaccharide coat and become virulent.

This must be due to the transfer of the genetic material.

However, the biochemical nature of genetic material was not defined from his experiments.