Question

What was revealed by the Griffith experiment, the Avery experiment, and the Hershey Chase experiment as to the DNA of the gene itself. Explain in order the points that these experiments revealed.

Answer 1

The Griffith's experiment revealed that there is some component in the bacteria which is transferred from one bacterial strain to another bacterial strain. This substance transformed the non-pathogenic rough strain of the Pneumococus into the pathogenic smooth strain of the Pneumococus. The Griffiths experiment also made it clear the transforming molecule is very stable as it was not destroyed by the heat, when he killed the bacteria. This experiment of Griffith is called Transforming Principle. This was done in the year 1928.

Avery-MacLeod-MacCarty carried out an experiment in the year 1944 to check the molecule which was responsible for the transforming principle of Griffith's experiment. They treated the heat killed bacteria with proteinase enzyme, then checked whether it will lead to transformation or not. It was found that the proteinase treated heat killed bacteria was able to transform the non-pathogenic bacteria into pathogenic bacteria. This clearly proved that the molecule responsible for the transformation is not a protein as proteinase destroyed the protein.

After this, they treated the heat killed bacteria with DNase to destroy the DNA of the heat killed bacteria.This time the heat killed pathogenic bacteria was not able to transform the non-pathogenic bacteria. This clearly showed that it is the DNA portion of the pathogenic bacteria which is passed from the heat killed pathogenic bacteria into the non-pathogenic bacteria and transformed it into pathogenic smooth strain.

Alferd Hershey and Maratha Chase carried out an experiment in the year 1952.

This experiment proved beyond doubt that it is the DNA which acts as the genetic material.

They grew bacteriophage in a medium containing phosphorus-32 and sulphur-35. As we know that phosphorus occurs in the nucleic acids and sulphur occurs in proteins but never in nucleic acids.

The bacteriophage proteins thus contain sulphur-35 and their DNA contain phosphorus-32. Both these isotopes are radioactive.

They then infected bacteria with these bacteriophages. They found that the bacteriophages which were grown on the radioactive phosphorus medium transferred radioactivity to the host bacteria, which clearly indicated that it is the nucleic acid DNA which entered into the bacteria.

The bacteria which were treated with phages grown on the radioactive sulphur did not show radioactivity, which indicated that proteins don't enter into the bacteria.

This experiment then proved that it is the DNA which acts as the genetic materisl.