A. Describe the series of experiments of F.Griffith. Comment on the significance of the results obtained.

B. State the contribution of Macleod, McCarty and Avery.

A. Frederick Griffith, in 1928, performed a series of experiments on Streptococcus pneumoniae and mice that laid the foundation of discovery of DNA is the genetic material. He was not interested in genetic material debate but rather, trying to develop a vaccine against pneumonia.

For his experiment, Griffith took two strains of Streptococcus, III-S and type II-R. The III-S strain has a smooth polysaccharide coat. The coat is resist to mice immune system. Hence, III-S stain causes pneumonia in mice. Whereas, the II-R strain does not have coat and so this stain is killed by the immune system of the mice. Hence, it does not cause disease.

Steps of the Griffith’s experiment:

Isolation of type II-S type bacteria from the dead mouse when he injected mixture of heat killed III-s and II-R, shows that the Type II R-strains had taken something from the heat killed III- S and changed into II-S.

B. Later, in 1944 Oswald Avery, Colin MacLeod and MacLynn McCarty did their transformations in a test tube. They demonstrated that the cell extracts prepared from S-strains could cause the transformation and it was the DNA that was responsible for the transformation.