The transforming principal in Griffith's experiment was established

Options

A. When live S-strain of S. Pneumoniae were injected in mice .
B. When live R-strain of S. Pneumoniae were injected in mice .
C. When heat-killed R-strain of s.pneumonia were injected in mice.
D. When heat-killed S-strain of s.pneumonia were injected with R-strain in mice

Answer

### Step-by-Step Solution:

1. **Understanding Griffith's Experiment**:
- Frederick Griffith conducted an experiment in 1928 to understand how certain bacteria cause disease. He worked with two strains of *Streptococcus pneumoniae*: the virulent (S strain) and the non-virulent (R strain).

2. **Setup of the Experiment**:
- Griffith injected mice with different combinations of these bacterial strains:
- Live S strain (virulent)
- Live R strain (non-virulent)
- Heat-killed S strain (virulent)
- A mixture of heat-killed S strain and live R strain.

3. **Observations**:
- Mice injected with live S strain died, confirming that this strain was virulent.
- Mice injected with live R strain survived, confirming that this strain was non-virulent.
- Mice injected with heat-killed S strain also survived, indicating that the heat-killed bacteria could not cause disease.
- However, when mice were injected with a mixture of heat-killed S strain and live R strain, they died.

4. **Conclusion**:
- The key observation was that the living R strain transformed into a virulent strain after being mixed with heat-killed S strain. This indicated the presence of a "transforming principle" that allowed the non-virulent R strain to acquire virulence.
- The transforming principle was later identified as DNA.

5. **Identifying the Correct Option**:
- The question asks which scenario established the transforming principle. The correct scenario is when the heat-killed S strain was injected along with the live R strain, leading to the transformation of R strain into a virulent form.
- Therefore, the correct answer is option 4: "When heat killed S-strain of *Streptococcus pneumoniae* was injected with R-strain in mice."