Question

Avery, MacLeod and McCarthy’s experiment first determined that DNA was the “transforming principle” present in the cell extract of a pathogenic strain of bacteria. Describe the experiment and explain how the results demonstrated the presence of a “transforming principle” as well as identifying it as DNA.

Answer 1

Avery-MacLeod-McCarty Experiment

There are two strains of bacteria R- rough
S- smooth.
S are virulent or able to cause disease.
R are avirulent. They are unable to cause disease.
R are unable to cause disease but when injected with heat killed smooth bacteria, they were able to cause the disease.
Now it is need to be tested that which is the transforming element.
So, lipids, carbohydrates and proteins all are tested.
They prepared cultures containing the heat-killed S strain of the bacteria and then removed lipids and carbohydrates from the solution.

they treated the solutions with different digestive enzymes (DNase, RNase or protease) to destroy the targeted compound.

Finally, they introduced living R strain cells to the culture to see which cultures would develop transformed S strain bacteria

Only in the culture treated with DNase did the S strain bacteria fail to grow (i.e. no DNA = no transformation). This means that DNA was the transforming agent.

This indicated that DNA was the genetic component that was being transferred between cells.