Question

Define transformation in Griffith's experiment. Discuss how it helps in the identification of DNA as the genetic material.

Answer 1

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 pneumonia (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.

S strain → Inject into mice → Mice die R strain → Inject into mice → Mice live

• Griffith was able to kill bacteria by heating them. He observed that heat-killed S strain bacteria injected into mice did not kill them.

S strain (heat-killed) → Inject into mice → Mice live

• 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.

S strain (heat-killed ) + → Inject into mice → Mice die R strain (live)

• He concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria. Some ‘transforming principle’, transferred from the heat-killed S strain, had enabled the R strain to synthesise a smooth polysaccharide coat and become virulent. This must be due to the transfer of the genetic material. However, the biochemical nature of genetic material was not defined from his experiments.