Frederick Griffith in 1928, carried out a series of experiments with Streptococcus pneumoniae (a bacterium that causes pneumonia).
He observed that when these bacteria (Streptococcus pneumoniae) are grown on a culture plate, some of them produce smooth and shiny colonies (S-type) whereas, the others produce rough colonies (R-type). This difference in character (smooth/rough) is due to a mucoid (polysaccharide) coat present in the S-strain bacteria, which is not present in the R-strain.
In his experiments, he first infected two separate groups of mice. The mice that were infected with the S-strain die from pneumonia. .S. strains are the virulent strains causing pneumonia. The mice that were infected with the R-strain did not develop pneumonia and survived.
S-strain (virulent strain) `to` Injected into mice `to` Mice die R-strain (non-virulent strain) `to` Injected into mice `to` Mice live
In the next set of experiments, Griffith killed the bacteria by heating them. The mice that were injected with heat-killed S-strain bacteria did not die and lived. Whereas on injecting a mixture of heat-killed S-strain and live R-strain bacteria. The mice died. Moreover living S-bacteria were recovered from the dead mice.
S-strain (heat killed) `to` Injected into mice `to` Mice live S-strain (heat killed) + R-strain (live) `to` Injected into mice `to` Mice die
Griffith concluded that the live R-strain bacteria, had been transformed by the heat-killed S-strain bacteria.
He proved that there was some .transforming principle. that was transferred from the heat-killed S-strain, which helped the R-strain bacteria to synthesise a smooth polysaccharide coat and thus, became virulent. This must be due to the transfer of the genetic material. However, he was not able to .define the biochemical nature of genetic material from his experiments,
Biochemical Characterisation of Transforming Principle
Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44) worked to determine the biochemical nature of .transforming principle. in Griffith.s experiment in an in vitro system.
From the heat-killed S-strain bacteria. they purified bimolecules like proteins, DNA, RNA, etc., to observe, which molecules could transform live R-cells into S-cells. They discovered that DNA alone from heat-killed S-strain bacteria caused the transformation of ·non-virulent R-strain bacteria into S-strain virulent bacteria. This difference in character (smooth/rough) is due to a smooth (polysaccharide) coat present in the S-strain bacteria, which is absent in the R-strain bacteria.
They observed that neither protein digesting enzymes (proteases) nor RNA digesting enzymes (RNase) could inhibit transformation. However when DNA-digesting enzymes (DNase) were applied to the mixture, transformation was inhibited implying that DNA caused the transformation. Thus, these scientists came to the conclusion that DNA is the hereditary material.
