Question

In Fredrick Griffith's famous experiment, shown in the image above, mice were injected with combinations of two strains of bacteria. What was the most important concept demonstrated by this experiment?

A. The living bacteria was able to become transformed by a heritable substance from the dead bacteria.
B. Heat destroys the ability of the bacteria to kill the mouse.
C. Proteins are the primary source of genetic information.
D. Proteins are the primary source of genetic information.

Answer 1

Hint: The transforming principle experiments, conducted by Fredrick Griffith in 1928, are of great importance in establishing DNA as the genetic material. Pneumococcus bacteria cause disease only when a capsule is present.

Complete answer:
Fredrick Griffith in his famous experiment (Transforming principle), used two strains of bacteria Diplococcus pneumoniae :-
-Smooth strain (S Strain): These bacteria are virulent and cause pneumonia.
-Rough strain (R Strain): These bacteria are nonvirulent and do not cause pneumonia.

The experiment can be described in four parts:-
-In the first step of the experiment, Rough strain (R Strain) was injected into the mouse, the mouse lived and pneumonia was not produced.
-In the second step, Smooth Strain (S Strain) was injected into the mouse, the mouse died as a result of pneumonia caused by the bacteria.
-In the third step, Smooth Strain (S Strain) of the bacteria which normally causes disease was heat killed and then injected into the mouse. The mouse lived and pneumonia was not produced.
-In the last step, Rough Strain and heat killed Smooth Strain (both known to cause disease) were injected together into the mouse. The mouse died due to pneumonia.

The last step of the experiment indicates that some Rough Strain bacteria were transformed by the heat killing Smooth Strain bacteria. This occurred perhaps due to the absorption of some ‘transforming principle’ by Rough Strain bacteria from heat killed Smooth Strain bacteria. This transforming substance from Smooth Strain bacteria caused the synthesis of capsule which resulted in production of pneumonia and death in mice. Therefore, transforming principle appears to control genetic characters.

Note: In search for genetic material, Avery, Macleod and McCarthy (1994) repeated the same experiment in vitro to identify the transforming substance. They proved that the transforming substance is in fact DNA. Evidence that DNA is the genetic material was proved by experiment with bacteriophages involving the process of transduction done by Harshey and Chase (1952).