Class 12 Biology Molecular Basis of Inheritance Griffiths Experiment

Griffith’s Experiment

Griffith experimented with Streptococcus pneumoniae bacteria. This bacteria causes Pneumonia.Two strains of this bacteria were used R-strain & S-strain.

  • S strain
    • Smooth mucous polysaccharide coat
    • Resistant to immune system
    • Virulent
  • R strain
    • Lacks the coat
    • Destroyed by immune system of the host
    • Non-virulent

Class_12_Molecular_Basis_Of_Inheritance_R-Strain_S-Strain

The experiment was performed in multiple steps.

  1. S-strain (virulent) was injected into mouse. It was found that the mouse died of pneumonia

 

 Class_12_Molecular_Basis_Of_Inheritance_S-Strain

 

  1. R-strain (non-virulent) was injected into mouse. It was found that the mouse remained alive

 Class_12_Molecular_Basis_Of_Inheritance_R-Strain

 

  1. Heat killed S-strain (S-strain bacteria were killed by heating) was injected into mouse. It was found that the mouse remained alive

 Class_12_Molecular_Basis_Of_Inheritance_S-Strain1

 

  1. Heat killed S-strain & live R-strain were injected into mouse. It was found that the mouse died of pneumonia

Class_12_Molecular_Basis_Of_Inheritance_S-Strain1

Griffith found that Live S-strain bacteria could be recovered from the dead mouse (Step 4).

Griffith thus arrived at the following conclusion:

  • Something caused bacteria to change from one type (R) to another type (S)
  • Some ‘Transforming principle’, transferred from heat-killed S strain to R strain and transformed it virulent.

However, the biochemical nature of the ‘Transforming principle’ was still unknown.

 

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