Question

Explain the biological significance of Griffith's experiment with R and S strains of S. pneumoniae.

Answer 1

Rough (R) strains of S pneumonia has spiky rough appearance and non virulent. It do not cause disease when administered to a rat.
Smooth (S) strain has smooth appearance due to polysaccharide coat and are virulent. It causes disease and death when administered to a rat.
When S strains are heat killed it did not cause any disease and death. But when heat killed S strains is administered along with live R strain it caused death in rats. On analyzing the blood, S strains were isolated.

Significance : There is a transformation in the genotype of R strain when it obtained DNA from the S strain. The R strain got converted to S strain since the gene coding the polysaccharide coat from S strain is expressed in the R strain. Hence nonvirulence is converted to virulence strain. This is termed as transforming principle.