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Question

Describe Frederick Griffin's experiment on Steptococcus pnemoniae. Discuss the conclusion he arrived at.

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Solution

For this study, Griffith used two strains of Pneumococcus bacteria, type III-S and type II-R.
There is one major difference between these two types; the III-S strain has a smooth polysaccharide coat which makes it resistant to the immune system of mice, whereas the II-R strain lacks this coat and so will be destroyed by the immune system of the host.
For the first stage of the transforming principle experiment, Griffith showed that mice injected with III-S died but when injected with II-R lived and showed few symptoms.
The next stage showed that if the mice were injected with type III-S that had been killed by heat, the mice all lived, indicating that the bacteria had been rendered ineffective.
The interesting results came with the third part of the experiment, where mice were injected with a mixture of heat-killed III-S and live II-R.
Interestingly enough, the mice all died, indicating that some sort of information had been passed from the dead type III-S to the live type II-R. Blood sampling showed that the blood of the dead mice contained both live type III-S and live type II-R bacteria.
Somehow the type III-S had been transformed into the type III-R strain, a process he christened the transforming principle.
Conclusion: Griffith concluded that the type II-R had been "transformed" into the lethal III-S strain by a "transforming principle" that was somehow part of the dead III-S strain bacteria. Today, we know that the "transforming principle" Griffith observed was the DNA of the III-s strain bacteria.


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