The Griffith and Avery Experiments - The Discovery of Hereditary Material
In 1928, Frederick Griffith made a startling observation during his experiments with the bacterium Streptococcus pneumoniae. His observations from this experiment suggested that the bacterium had capabilities that allowed it to transfer genetic information. However, Griffith actually discovered an important transformation agent found in all living things.
The Griffith experiment had two major components: the S and R strains of the Streptococcus pneumoniae and live mice. The Streptococcus pneumoniae is a bacterium that causes pneumonia is humans. However, the bacterium is lethal when injected into mice. During his experiments, Griffith used two strains that are differentiated by their appearance. The S-strain is the normal virulent strain. Because of the polysaccharide capsule that surrounds the strain, the S-strain has a smooth appearance. The R-strain is the rough strain since it is not encased in a polysaccharide capsule. The R-strain was the mutant non-virulent strain. This strain has evolved to be less virulent, thus not as deadly.
There are actually four major parts to the Griffith experiment. In the first part, the mouse is injected with the live virulent S-strain and the mouse dies. This shows that the live S-strain was deadly to the mouse. In the second part, the mouse is injected with the non-virulent R-strain and the mouse lives.
The injection of the live S-strain and the non-virulent R-strain are the controls of the experiment. It allows one to see the effects of only the S and the R strains. Griffith then goes and killed some of the virulent S-strain by boiling them. When the mouse was injected with the heat-killed S-strain, the mouse actually lives. The mouse’s survival shows that the carcasses of the heat-killed cells are not fatal to the mouse. However, when Griffith combines both the heat-killed S-strain and the non-virulent R-strain, the mouse dies. When blood samples were taken from the dead mouse, live virulent S-strain cells were found within the blood. Somehow, a transformation process has taken place in which the heat-killed S-strain altered the live R-strain into live S-strains. But, what is this transformation agent that caused the change from R-cells to S-cells?
It wasn’t until 1944 when Oswald Avery alongside his two colleagues, Colin MacLeod and Maclyn McCarty, were able to determine the identity of the transformation agent. Because this transformation agent was able to convert the genotype of the R-strain to that of the S-strain, the agent was considered to be a hereditary material. In order to extract the transformation substance, Avery went on to chemically destroy different candidates for this substance. For example, since the S-strain was covered in a polysaccharide coat, Avery went to chemically destroy all the polysaccharides in the S-strain and inject it into the mouse alongside the R-strain; however, the mouse still dies.
They went on to destroy lipids, proteins, and RNA; yet they were still unable to find the transforming agent. It was only when they added DNase, a deoxyribonuclease enzyme that breaks up DNA, to the mixture which prevented the R-strain from transforming, thus allowing the mouse to live. This shows that DNA was the transformation agent. DNA from virulent strain must have replaced those of the non-virulent strain in order to perform such transformation.
It was because of Frederick Griffith’s along with Oswald Avery’s experiments and observations that lead to the discovery of DNA as not only a transforming substance but also as hereditary and genetic material.