(a). Frederick Griffith (1928) performed the experiments on Bacterial transformation with Streptococcus pneumoniae, the bacterium that causes pneumonia.
- He observed two strains of this bacterium, one forming smooth shiny colonies with capsule (S-type) and the other forming rough colonies without capsule (R-type).
- The S-type cells are virulent while the R-type cells are nonvirulent.
- When live S-type cells were injected into the mice, they suffered from pneumonia and died. S strain → Injected into mice → Mice died
- When live R-type cells were injected into the mice, the disease did not appear and the mice survived. R strain → Injected into mice → Mice lived
- When heat-killed S-type cells were injected, the disease did not appear.
S strain → Injected into mice → Mice lived (Heat-killed)
- When heat-killed S-type cells were mixed with live R-type cells and injected into the mice, the mice died of pneumonia and live S-type cells were isolated from the body of the mice.
- He concluded that the R-strain had somehow been transformed by the heat-killed S-strain bacteria, which must be due to the transfer of genetic material, the transforming principle.
(b). Avery, MacLeod and McCarty worked to determine the biochemical nature of transforming ‘principle’
- They purified biochemicals like proteins, DNA and RNA from the heat-killed S-cells.
- When these fractions were added individually to the culture of live R-cells, DNA was able to cause transformation of R-cells into S-cells.
- They also found that protein-digesting enzymes and RNAdigesting enzymes did not affect transformation, indicating that transforming substance is not a protein or RNA digestion with DNase did inhibit transformation; this suggests that the DNA caused transformation.
