SEARCH OF GENETIC MATERIAL

THE SEARCH OF GENETIC MATERIAL:

Transforming principle:

  • Given by Frederick Griffith in 1928.
  • His experiment based on Streptococcus pneumoniae (caused pneumonia).
  • There is change in physical form of bacteria.
  • There are two colonies of bacteria:
    • Smooth shiny colonies called S strain.
    • Rough colonies called R strain.
  • S-strain bacteria have a mucous (polysaccharide) coat.
  • R-strain does not have mucous coat.
  • S-strain is virulent and caused pneumonia in mice and died when infected.
  • R-strain is non-virulent and dose caused pneumonia in mice when infected.
  • Heat killed S-Strain is non-virulent and dose not causes pneumonia.
  • The heat killed S-Strain mixed with live R-Strain injected into mice; the mice developed pneumonia and died.
  • He recovered live S-Strain bacteria form the dead mice.

Conclusion of experiment:

  • R – Strain bacteria had some how been transformed by the heat killed S-Strain bacteria.
  • Some ‘transforming principle’, transferred from heat killed S-Strain bacteria, had enabled the R-Strain to synthesize smooth polysaccharide coat and become virulent (S Strain).
  • The transformation of R-Strain to S-Strain is due to transfer of Genetic material.
  • However the biochemical nature of genetic material was not defined from his experiment.

Biochemical characterization of transforming principle:

  • Biochemical nature of transforming principle was discovered by Oswald Avery, Colin Macleod and Maclyn McCarty. (1933-44)
  • Prior to their work genetic material was thought to be protein.
  • They worked to determine the biochemical nature of the ‘transforming principle’ of Griffith’s experiment.
  • They purified biomolecules (proteins, DNA and RNA) from the heat killed S cells to see which one could transform live R cells to S cells.
  • Heat killed S-Strain + protease + Live R-Strain →transformation.
  • Heat killed S-Strain + RNase + Live R-Strain →transformation.
  • Heat killed S-Strain + DNase + Live R-Strain →transformation.

Conclusion of the experiments:

  • Protein of heat killed S-Strain is not the genetic material
  • RNA of heat killed S-Strain is not the genetic material.
  • DNA of heat killed S-Strain is the genetic material, because DNA digested with DNase mixed with R-strain unable to transform R-Strain to S-Strain.

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