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· Transformation is a change in genetic material of an organism by obtaining genes from outside as from remains of its dead relatives.
· Frederick Griffith (1928) conducted as series of experiments with Streptococcus pneumoniae, (Pneumococcus), the bacterium causing pneumonia
· He observed, during the course of his experiment, a living organism (bacteria) had changed in physical form
· He observed two strains of this bacterium on culture plates:
-S strain : forms smooth shiny colonies; have a mucous (polysaccharide) coat; Virulent/pathogenic
- R strain : forms rough colonies; without a mucous (polysaccharide) coat; Non-virulent/non-pathogenic
· When live S strains (virulent) were injected into the mice, they produced pneumonia and the mice died
· Mice infected with R strain do not develop pneumonia
· When heat-killed S strains were injected into the mice, it did not kill the mice
· When heat-killed S strains were mixed with live R strains and injected into the mice, the mice died and he recovered live S strain from the dead body of the mice
· He concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria,
· He further explained that some ‘transforming principle’ transferred from the heat-killed S strain, had enabled the R strain to synthesise a smooth polysaccharide coat and become virulent;
· This must happened due to the transfer of genetic material; But, the biochemical nature of genetic material was not defined from his experiments
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· Before the work of Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44), the genetic material was thought to be a protein.
· They worked to determine the biochemical nature of ‘transforming principle’ in Griffith's experiment.
· They purified biochemicals (proteins, DNA, RNA, etc.) from the heat-killed S cells and checked which ones could transform live R cells into S cells.
· They found that DNA alone from S bacteria caused R bacteria to become transformed.
· They also observed that protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not affect transformation, so the transforming substance was not a protein or RNA.
· They observed that Digestion with DNase did inhibit transformation, so the DNA caused the transformation.
· They concluded that DNA is the hereditary material, but not all biologists were convinced.
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