Griffith’s Experiment | Transformation Principle | MBOI by NRM Sir

Griffith’s Experiment | Transformation Principle | Class 12 Biology | Molecular Basis of Inheritance

In 1928, British bacteriologist Frederick Griffith performed a ground breaking experiment that introduced the world to the concept of bacterial transformation. Working with two strains of Streptococcus pneumoniae — the virulent smooth (S) strain with a polysaccharide capsule and the non-virulent rough (R) strain without a capsule — Griffith injected different combinations into mice and observed the outcomes:

1. Live S strain → Mouse died


2. Live R strain → Mouse lived


3. Heat-killed S strain → Mouse lived


4. Live R strain + Heat-killed S strain → Mouse died



Surprisingly, in the last case, live S strain bacteria were recovered from the dead mouse. This led Griffith to conclude that some "transforming principle" from the dead S strain had converted the non-virulent R strain into a virulent form. Although he didn’t identify the exact molecule, his experiment laid the foundation for discovering DNA as the genetic material in later years by Avery, MacLeod, and McCarty.


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Key Concepts Covered:

Transformation in bacteria

Virulent and non-virulent strains

Foundation for DNA discovery

Molecular basis of inheritance



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Questions for You:

1. What was the role of the heat-killed S strain in Griffith’s experiment?


2. Why did the combination of heat-killed S and live R strain kill the mouse?


3. What is meant by the “transforming principle”?


4. How did Griffith’s work pave the way for future discoveries about DNA?


5. Who later identified the transforming substance as DNA?




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