Definition of Transformation:
The process by which a part of the double-stranded DNA molecule released from the donor cell enters the recipient cell with the help of the extracellular medium and recombines with the DNA molecule of the recipient cell, initiating new characteristics, is called transformation.
The bacterium Diplococcus pneumoniae is responsible for a lung disease called pneumonia. There are two strains of this bacterium-
1. Type II R: The bacterium of this strain is called smooth because its capsule is not formed. When it enters the body, the white blood cells or WBC can easily engulf the bacteria of this strain, so this bacteria cannot cause disease. The bacteria of this strain are called mild or non-infectious.
2. Type III S: The bacterium of this strain is covered with a capsule, which makes it smooth. When it enters the animal body, the white blood cells cannot engulf them due to their smooth surface. As a result, they reproduce rapidly and cause symptoms. For this reason, the III S type is called highly infectious or virulent.
Oswald Avery, Colin MacLeod, MacLean McCarty explained the reason for the fourth experiment. According to their explanation, there is no possibility of III S bacteria surviving in the mixture of bacteria (live IIS + dead IIIS), but even though the virulent IIIS is dead, the live non-virulent IIR is transformed into virulent IIIS bacteria under its influence. To see which substance from the dead IIIS bacteria converts the live IIR into IIIS or virulent bacteria, three separate experiments were conducted-
1. Protease was added to the mixture of dead IIIS and live IIR, which dissolves or destroys the protein. It was found that in this case, the IIR bacteria were being transformed into IIIS, that is, the substance responsible for the transformation was not protein.
2. Even if RNase is applied to the said mixed bacteria, the transformation process occurs, that is, RNA is not responsible for the transformation.
3. DNase enzyme was applied to the mixture of dead IIIS and live IIR, which dissolves the DNA molecule if there is one in the bacterial culture medium. It was found that the application of DNase did not transform the live II R bacteria into III S bacteria. Through this experiment, scientists came to the conclusion that the DNA molecule of the dead III S bacteria is divided into different fragments as a result of the application of heat and the fragment that carries the capsule-producing gene enters the live II R bacteria and creates a capsule in the II R bacteria. Naturally, the II R bacteria transform into III S bacteria, that is, virulent bacteria.