The below mentioned Griffith’s and Avery’s transformation experiments provide evidence of DNA role as genetic material.
The first direct evidence that DNA rather than protein is the genetic material came from simple experiments and observations, with bacteria and viruses. The bacterium Diplococcus pneumoniae (popularly known as Pneumococcus) causes pneumonia in mammals.
In 1928, Frederick Griffith, a British Medical Officer, observed that there are two strains of D. pneumoniae, one that forms smooth colonies (S. strain) protected by a capsule and another that forms irregular or rough colonies (R-strain) without a capsule when grown on a suitable medium in petri-dish.
When injected into mice, only S-cells produced the disease (pathogenic) but not the R-cells (non-pathogenic). Curiously, when the heat killed S-cells (incapable of producing the disease) were mixed with the live R-cells and then injected, the disease was produced. From the blood of the infected mice, live S-cells could be isolated. Thus, some factor from the dead S-cells converted the live R-cells into live S-cells and the latter produced the disease (Fig. 6.1).
In 1944, Avery, Mc Carty and Mac Leod gave a molecular explanation for the observations of Griffith. They found that the DNA isolated from the heat-killed S-cells, when added to R-cells changed their surface character from rough to smooth and also made them pathogenic.
Thus, DNA was shown to be the genetic material responsible for inducing the smooth character of the cells and their virulence (disease-causing) property in mice. The phenomenon by which the DNA isolated from one type of cell, when introduced into another type, is able to give some of the properties of the former to the latter, is known as Transformation.
The above experiment conclusively proves that DNA is genetic material in all organisms, except some viruses. Evidence that DNA is the genetic material has since been obtained in several other experiments too, and the best supporting evidence has been provided by an experiment on bacteriophage performed by Hershey and Chase (1952).