Monday, July 27, 2020

Molecular Basis of Inheritance - Notes | Class 12 | Part 2: The Search for Genetic Material

6. MOLECULAR BASIS OF INHERITANCE


THE SEARCH FOR GENETIC MATERIAL

1. Griffith’s Transforming Principle experiment (1928)

Frederick Griffith used mice & Streptococcus pneumoniae.

Streptococcus pneumoniae has 2 strains:


   Smooth (S) strain (Virulent): Has polysaccharide mucus coat. Cause pneumonia.

   Rough (R) strain (Non-virulent): No mucus coat. Do not cause Pneumonia.

Experiment:

      • S-strain → Inject into mice → Mice die
      • R-strain → Inject into mice → Mice live
      • S-strain (Heat killed) → Inject into mice → Mice live
      • S-strain (Heat killed) + R-strain (live) → Inject into mice → Mice die

He concluded that some ‘transforming principle’ transferred from heat-killed S-strain to R-strain. It enabled R-strain to synthesize smooth polysaccharide coat and become virulent. This must be due to the transfer of genetic material.


2. Biochemical characterization of transforming principle 


-  Oswald Avery, Colin MacLeod & Maclyn McCarty worked to determine the biochemical nature of ‘transforming principle’ in Griffith’s experiment.

- They purified biochemicals (proteins, DNA, RNA etc.) from heat killed S cells using suitable enzymes.

-  They discovered that

  • Digestion of protein and RNA (using Proteases and RNases) did not affect transformation. It means that the transforming substance was not a protein or RNA.
  • Digestion of DNA with DNase inhibited transformation. It means that DNA caused transformation of R cells to S cells. It proves that DNA was the transforming principle.

3. Hershey-Chase Experiment (Blender Experiment)-1952

  • Hershey & Chase grew some bacteriophage viruses on a medium containing radioactive phosphorus (P32) and some others on medium containing radioactive sulphur (S35).
  • Viruses grown in P32 got radioactive DNA because only DNA contains phosphorus. Viruses grown in S35 got radioactive protein because protein contains sulphur.
  • These preparations were used separately to infect E. coli.
  • After infection, the E. coli cells were gently agitated in a blender to remove the virus particles from the bacteria.
  • Then the culture was centrifuged to separate lighter virus particles from heavier bacterial cells.
  • Bacteria infected with viruses having radioactive DNA were radioactive. i.e., DNA had passed from the virus to bacteria. Bacteria infected with viruses having radioactive proteins were not radioactive. i.e., proteins did not enter the bacteria from the viruses. This proves that DNA is the genetic material.


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