Question

(a) Describe the various steps of Griffith's experiment that led to the conclusion of the 'Transforming Principle'.
(b) How did the chemical nature of the 'Transforming Principle' get established?

Answer 1

Hint: The presence of transforming principles was proved in bacteria. He was the first to prove that the bacteria are capable of transferring genetic information by the process of transformation. The organism used in his experiment was virulent and non-virulent Streptococcus pneumoniae.

Complete answer:
a) Griffith proved the principle of transformation in 1928. He showed that the bacteria are capable of changing the function and form through transformation.
- He performed his experiment using strains of Streptococcus pneumoniae. He injected the mixture of virulent (SIII) and non-virulent (RII) strains into mice.
- The heat killed the SIII strain and did not cause any kind of infection.
- A mixture of heat killed SIII and living RII stains caused infection.
- This proved that there is some transforming substance present which was transferred from the heat killed strain to non-virulent strain which transformed non-virulent strain to virulent.
- The transfer of genetic material took place without any direct physical contact between the two strains.
- So, Griffith called this transforming principle as the genetic material.

b) The chemical nature of the transforming principle was given by Avery, Mcleod and McCarty, Harshey and chase experiment.
- They purified proteins, DNA and RNA from heat killed Strain.
- Then they are treated with protease, RNAse and DNAse respectively.
- The digestion with protease and RNA did not affect transformation.
- The digestion with DNAse inhibited transformation.
- So, they concluded that DNA is the transforming principle responsible for causing transformation.

Note:
- Bacterial transformation can be defined as the process of horizontal gene transfer in which the bacterial cells are able to take up the foreign DNA from the environment.
- Proteases are the proteolytic enzymes which destroy proteins.
- RNAse are the class of enzymes which are used to hydrolyse or break RNA.
- DNAse are the class of enzymes which are used to hydrolyse or break DNA.