What happened when heat-killed S cells along with live R cells were injected into mice in Griffith’s experiment?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Mice died and showed live S cells in the blood
Griffith worked with two strains of Streptococcus pneumoniae. The S strain has a smooth mucous coat and kills mice; the R strain has no coat and is harmless. Heat-killed S bacteria alone did not kill mice.
Option (1) Mice died and showed live R cells in the blood — Incorrect. The mice did die, but the bacteria recovered from them were live S bacteria. Live R cells alone cannot cause the disease.
Option (2) Mice survived and showed live S cells in the blood — Incorrect. Live S bacteria are virulent, so mice that carry them do not survive.
Option (3) Mice died and showed live S cells in the blood — Correct. Something from the dead S cells, the ‘transforming principle’, changed the live R cells into virulent S cells. It was later shown to be DNA.
Option (4) Mice survived and showed live R cells in the blood — Incorrect. This is the result of injecting live R cells alone, without the heat-killed S cells.
NCERT Extract:
The Search for Genetic Material – “… Transforming Principle In 1928, Frederick Griffith, in a series of experiments with Streptococcus pneumoniae (bacterium responsible for pneumonia), witnessed a miraculous transformation in the bacteria. During the course of his experiment, a living organism (bacteria) had changed in physical form. When Streptococcus pneumoniae (pneumococcus) bacteria are grown on a culture plate, some produce smooth shiny colonies (S) while others produce rough colonies (R). This is because the S strain bacteria have a mucous (polysaccharide) coat, while R strain does not. Mice infected with the S strain (virulent) die from pneumonia infection but mice infected with the R strain do not develop pneumonia. Griffith was able to kill bacteria by heating them. He observed that heat-killed S strain bacteria injected into mice did not kill them. When he injected a mixture of heat-killed S and live R bacteria, the mice died. Moreover, he recovered living S bacteria from the dead mice. He concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria.”