(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? (5)
(a) Various steps the Griffith's experiment that led to the conclusion of the 'Transforming Principle'.
Transformation was first studied by Frederick Griffith in 1928. He conducted experiments with streptococcus pneumoniae, the bacterium responsible for pneumonia.
1. He observed two strains of this baterium, some produce shiny colonies (S) while others produce rough colonies (R). Strain bacteria have a mucous (polysaccharide) coat, while R-strain does not have.
2. The S-strain bacteria are virulent while the R-strain bateria are non-virulent.
3. When live S-type bacteria were injected into mice, they died from pneumonia.
4. Mice injected with the R-strains, mice survived.
5. Heat killed S-strain bacteria were injected into mice, did not kill them.
6. S-strain (heat-killed) were mixed with R-strains (live) and then injected into mice, the mice died of pneumonia.
7. He isolated living S baterium from the dead mice. He concluded that the R-strain bacteria had somehow been transformed by the heat killed S-strain bateria. Some transforming principle transferred from the heat killed S-strain bacteria. It changed the R-bacteria into S-bacteria, so R-bacteria were able to produce a smooth polysaccharide coat and became virulent.
(b) Chemical nature of the 'Transforming Principle'. The three scientists were: Oswald Avery, Colin Macleod, Maclyn McCasty.
They conculded that DNA is a hereditary material.
(i) They purified bio-chemicals (DNA, RNA, Proteins etc.) from the heat killed S-cells.
(ii) These fractions were inserted individually alongwith live R-cells. DNA was able to transform R-cells into S-cells.
(iii) Also protein digesting enzymes and RNA digesting enzymes did not affect transformation, so transforming substance was not RNA or protein. Digestion with DNA which inhibits transformation suggested DNA causes transformation. (5)