Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

(1) In 1928, Frederick Griffith, carried out experiments with bacterium Streptococcus pneumoniae (which causes pneumonia in humans and other mammals).

(2) Griffith used two strains of Streptococcus pneumonia:

• S-type (Virulent, smooth, pathogenic and encapsulated).
• R-type (Non-virulent, rough, non- pathogenic and non-capsulated).

(3) Experiments carried out by E Griffith:

• Mice were injected with R-strain bacteria and they survived (no pneumonia).
• Mice injected with S-strain bacteria developed pneumonia and died.
• When heat-killed S-strain bacteria were injected in mice, the mice survived.
• On injecting a mixture of heat-killed S-bacteria and live R bacteria, the mice died.

(4) Griffith obtained live S-strain bacteria from the blood of the dead mice.

(5) He concluded that the live R-strain bacteria must have picked up something (which he called transforming principle) from the heat killed S bacterium, and got changed into S-type. Transforming principle allowed R-type to synthesize capsule and it became virulent.

(6) Thus, F. Griffith first demonstrated genetic transformations.