DNA160-page2 - But Mice injected with a mixture of...

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DNA: Structure and Replication - 2 Discovering the Genetic Molecule But not all were trying to find out how proteins could function as our genetic molecules, and the evidence for nucleic acids accumulated. Evidence #1 In 1928, the British researcher, Fred Griffith, was trying to find a vaccine to protect against a pneumonia-causing bacterium, Streptococcus pneumoniae . He isolated two strains of the bacterium. One had a polysaccharide capsule that gave it a smooth (S) appearance in culture. The other form appeared rough (R) in culture. The S form is a virulent form of the bacterium, since the capsule protects it from harmful things in its environment, which in this case is the immune system of the host. Griffith injected bacteria into mice, and observed what happened. Mice injected with S forms died. Mice injected wit R forms lived. Mice injected with heat-killed S forms lived.
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Unformatted text preview: But: Mice injected with a mixture of heat-killed S forms and live R forms died, and when necropsied, contained live S form bacteria. What did this mean? 1. The production of a capsule is an inheritable trait that distinguishes the R form from the S form of the bacterium. 2. Somehow, the heat which killed the S cells did not damage the material that had genetic instructions so that this material (instructions on how to make a capsule) could be incorporated into the living R cells (The R cells could pick up this material from the environment) transforming these R cells into virulent S forms. 3. Griffith called this the Transformation Principle. Today, transformation is defined as the process by which external DNA is assimilated into a cell changing its genotype and phenotype. Transformation is one of the processes used in DNA technologies....
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