Question

1. In 1928 Frederick Griffith conducted an experiment which provided us one of the major steps toward understanding DNA as genetic material. Describe his experiments, his overall finding, and what conclusions he made from his experiment.

Answer 1

Griffith's experiment : ( also known as transforming principle)
Griffith cultured Streptococcus pneumoniae bacteria which showed two patterns of growth. One plate consisten of IIIS strain colonies that is smooth shiny colonies and the other consisted of IIR strain colonies that is rough strain. The S strain bacteria hadd mucous coat whereas R strain bacteria lacked the mucous coat.
Major difference between these two types; the III-S strain has a smooth polysaccharide coat and the host is unable to kill this strain and it causes pneumonia thus killingg the host due to resistance to the immune system of host.
Whereas the II-R strain lacked this coat and so was destroyed by the immune system of the host..
In his experiment:
Griffith injected both IIIS and IIR strains to mice.
The one which was injected with S strain tended to stay alive whereas the one injected with the R strain died.
In next step, Griffith heat - killed the IIIS strain and injected it in bacteria and the bacteria stayed alive.
In further step, He mixed Heat killed IIIS strain and IIR sdtrain and then injected it into mice and the mice died, and A living IIIS strain was found to be present in Dead mice.
In short:
Living IIIS strain(virulent) + Mice = Mice dies
Living IIR strain(non-virulent) + Mice = Mice lives
Heat killed III S strain + mice= Mice lives
Living IIR strain + Heat kiled IIIS strain + mice = Mice dies.
So , Griffith Concluded based on his observations that:
R strain bacteria must have been transformed by the IIIS strain bacteria. , The IIR strain must have inherited some transforming principle from heat killed IIIS  strain that made the bacteria virulent lelading to the death of mice.Griffith assumed this trasforming principle to be the Genetic principle , However it could not explain the biochemistry of genetic material.