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

(a) Griffith proved presence of transforming principle in bacteria that leads to genetic transformation. He infected mice with a mixture of virulent (SIII) and non virulent strains (RII) of Streptococcus pneumoniae. Heat killed SIII did not caused any infection but a mixture of heat killed SIII and living RII strains caused infection because a transforming substance was transferred from heat killed strain to non virulent strain causing non virulent strain to transform into virulent form. There was no direct physical contact between virulent and non virulent strains during transfer of genetic material.

(b) The followed up experiment by Avery, McLeod and McCarty and Hershey and Chase established that DNA is the transforming principle, not proteins as it was assumed to be.