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Saturday, 24 July 2021

Transforming Principle ll Griffith's Experiment (NOTES)

Transforming Principle

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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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