Question

The transforming principle in Griffith's experiment was detected _______.

• A. When heat killed S-strain of S. pneumoniae were mixed up with the R-strain.
• B. When the live S-strain of S. pneumoniae were mixed up with the R-strain.
• C. When live R-strain of S. pneumoniae were mixed up with the S-strain.
• D. When heat killed R-strain of S. pneumoniae were mixed up with the S-strain.

Hint:

Griffith's mice died only when heat-killed virulent bacteria were mixed with live non-virulent bacteria.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
Griffith worked with Streptococcus pneumoniae. The S strain is virulent and the R strain is non-virulent.

Step 2: The experiment
Live R alone and heat-killed S alone did not kill mice.
When heat-killed S cells were mixed with live R cells, the mice died and live S bacteria were recovered.

Step 3: Conclusion
Some factor from the dead S cells changed R cells into S cells. This factor was the transforming principle, later shown to be DNA.

Final Answer:
The transforming principle was detected when heat-killed S-strain was mixed with live R-strain. \[ \boxed{\text{(A) }\text{When heat killed S-strain of S. pneumoniae were mixed up with the R-strain}} \]