Explanation

Transformation was first discovered by Griffith (1928), in Pneumococcus (Streptococcus pneumoniae), that causes pneumonia. Griffith injected a group of mice with non-encapsulated, rough (R), pneumococci; a second group with heat-killed encapsulated pneumococci cells and a third group a mixture consisting of a few living non-encapsulated, rough pneumococci derived from a type S culture, and heat-killed encapsulated cells (S type). Griffith observed that the mice in the first two groups were not infected, and the mice in the third group died within a few days. The mice of the third group should have survived as the organisms which could kill them had been killed and the cell of R type were incapable of causing disease. However, the mice died and living virulent encapsulated cells of the type S were recovered from their dead bodies. It was observed by Griffith, that killed encapsulated pneumococci had liberated some substance which favoured non-capsulated cells (R type) to produce a capsular substance. This substance in later experiments was proved to be DNA. These experiments showed that DNA is the genetic material.