Describe Griffith’s experiment to demonstrate that DNA is the basic genetic material. What explanation for Griffith’s observation was given by Avery, McCarty, and MacCleod?
Griffith’s experiment demonstrates DNA as genetic material. Transformation experiments were initially conducted by F. Griffith in 1928. He injected a mixture of two strains of Pneumococcus (Diplococcus pneumoniae) into mice. One of these two strains S III was virulent and the other strain All was non-virulent. 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. The S III bacteria, prior to injection are killed by heating. It is then injected into the mice. It did not cause the disease. Heat killed S III and RII (non-virulent) bacteria, when injected into mice, causing pneumonia. Finally, death occurred. This proved that the 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 the 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 a capsule (some component of a cell) rather than the whole cells. Contribution of Avery, MacCleod, and MacCarty: Avery, MacCleod, and McCarty gave the proof that the “transforming agent” is DNA. They carried out the experiments with Diplococcus and showed the transformation of type R-ll to type S-III. They found that proteases and RNases did not affect transformation but DNAses affect it. Thus they gave proof that the active component was DNA and not the RNA, proteins, or polysaccharides.
