First of all we will discuss about the Frederick Griffith's experiment then we will discuss the Avery, Macleod and McCarthy experiment.
Frederick Griffith's Experiment:
Frederick Griffith who was a bacteriologist, working on Streptococcus pneumoniae that how it causes infection. In 1928, he took two strains of Streptococcus pneumoniae called the S and R strains. The S strains was the virulent due to polysaccharide coat which causes the disease and R strains are non-virulent. On culture of these both strains, the S strain shows smooth shiny colonies while the R strain shows rough colonies.
Then, Griffith did experiment on 4 mice. He made four different solutions and injected those solutions with each mice. The first solution contains live R strain organisms, second solution included R strain organisms, third solution had heat killed S strain organisms and fourth solution was mixture of live R strain and heat killed S strain organisms which are injected in four mice separately. In the result what was happened that the first and fourth solution injected mice died due to infection but the other two ones survived. When he extracted the infectious agents from both dead organisms, he found the S strain organisms.
Here was the interesting thing which Griffith explored that heat killed S strain organisms transformed the R strain organisms into S strain organisms that's why fourth mice died.
This experiment of Griffith's known as transformation experiment.
Avery, Macleod and McCarty's Experiment:
Griffith's experiment was not clear that which factor of heat killed S strain was responsible for transformation of R strain into S strain organisms. To further explore this, in 1944, 16 year later, Oswald Avery, Colin Macleod and MacLynn McCarty resolved this issue.
They started their experiment by dividing the heat killed S strain organisms into five different batches. The divided the organisms in batches like these;
1st Batch: In this batch they destroyed the bacteria's polysaccharide coat.
2nd Batch: Lipid content of bacteria was destroyed.
3rd Batch: In this batch, RNA of bacteria destroyed.
4th Batch: They destroyed the proteins of bacteria.
5th Batch: In final batch they destroyed the DNA of bacteria.
In the result what happen was that except last mouse (fifth batch) all other four mice died. S train organisms was extracted from all died mice this means that DNA was the main factor to transform the R strain organisms into S strain organisms because when there DNA was destroyed that bacteria don't have the ability to transform R strain to S strain bacteria. And one more reason is that while destroying the other components of bacteria such as polysaccharide, RNA, proteins, and lipids still the transformation took place. This also means that polysaccharide is the virulent factor but it was not involved in the transformation of any genetic material.
This was all about the Avery, Macleod and McCarty experiment.
Explanation:
