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Frederick Griffith was one of the leading scientists looking at the genetic role of DNA. He was working on two strains of Streptococcus pneumoniae: The rough strain (R) did not cause pneumonia, but the smooth strain (S) did. When Griffith heat-killed the S strain and gave both the heat-killed S strain and the R strain to mice, the mice developed pneumonia and died. What happened in this experiment that caused the mice to develop pneumonia?The R strain mutated and thus was able to cause pneumonia.There was more of the R strain added than previously.The immune systems of the mice were unable to cope with both the S and R strains simultaneously.Even though the S strain had been heat-killed, it changed the R strain.

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BioWizard

The correct answer is: Even though the S strain had been heat-killed, it changed the R strain.

Griffith in his experiment used two strains of bacteria:

• The rough strain (R) which did not cause pneumonia, (nonvirulent)

• the smooth strain (S) which did caused pneumonia (virulent)

When Griffith heat-killed the S strain it did not caused mice to die.

But, when he gave both the heat-killed S strain and the R strain to mice, the mice developed pneumonia and died.

Griffith conclusion was that the R-strain bacteria took what he called a "transforming principle" from the heat-killed S bacteria which transformed the nonvirulent R strain.