Griffith’s experiment to demonstrate DNA as genetic material.

Griftth’s experiment demonstration transformation in Pneumococcus.

Transformation experiments were initially conducted by F. Griffith in 1928.

1. He injected a mixture of two strains of Pneumococcus (Diplococcus pneumoniae) into mice. One of these two strains S III as virulent and the other strain RII was non-virulent, i.e., causes no infection.

2. The S III type bacteria when injected into mice cause pneumonia and ultimately to death. The R II type bacteria when injected, no pneumonia occurred.

3. The SIII bacteria, prior to injection are killed by heating. It is then injected into the mice. It did not cause the disease.

4. Heat killed S III and RII (non-virulent) bacteria, when injected into mice, caused pneumonia. Finally death occurred.

This proved that DNA of S III type bacteria has transformed the DNA of R II type bacteria into virulent type S III. This phenomenon of transferring characters of one strain to another by using a DNA extract of former is called transformation.

Conclusion. Griffith concluded that virulence was transferred from S-Type dead cells to R-Type living cells in the form of capsule (some component of cell) rather than the whole cells.

Contribution of Avery, MacCleod and MacCarty

Avery, MacCleod and McCarty gave the proof that “transforming agent” is DNA They carried out the experiments with Diplococcus and showed transformation of type R-II to type S-III They gave proof that active component was DNA and not the RNA of Proteins or Polysaccharides.