Define transformation in Griffith's experiment. Discuss how it helps in the identification of $DNA$ as the genetic material.

(A) In $1928$,Frederick Griffith,in a series of experiments with $Streptococcus$ $pneumoniae$ (the bacterium responsible for pneumonia),witnessed a miraculous transformation in the bacteria. During his experiment,a living organism (bacteria) changed its physical form.
When $Streptococcus$ $pneumoniae$ bacteria are grown on a culture plate,some produce smooth shiny colonies $(S)$ while others produce rough colonies $(R)$.
This is because the $S$ strain bacteria have a mucous (polysaccharide) coat,while the $R$ strain does not. Mice infected with the $S$ strain (virulent) die from pneumonia,but mice infected with the $R$ strain do not.
$S$ strain $\rightarrow$ Inject into mice $\rightarrow$ Mice die
$R$ strain $\rightarrow$ Inject into mice $\rightarrow$ Mice live
Griffith killed the bacteria by heating them. He observed that heat-killed $S$ strain bacteria injected into mice did not kill them.
$S$ strain (heat-killed) $\rightarrow$ Inject into mice $\rightarrow$ Mice live
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.
$S$ strain (heat-killed) $+$ $R$ strain (live) $\rightarrow$ Inject into mice $\rightarrow$ Mice die
He concluded that the $R$ strain bacteria had been transformed by the heat-killed $S$ strain bacteria. Some 'transforming principle',transferred from the heat-killed $S$ strain,enabled the $R$ strain to synthesize a smooth polysaccharide coat and become virulent. This indicated the transfer of genetic material,although the biochemical nature of this material was not defined by his experiments.