Question

Describe the experiment of Hershey and Chase to prove that DNA is the genetic material. 

               Hershey and Chase Experiment

Answer 1

Hershey and Chase worked with viruses that infect bacteria i.e. bacteriophages, which are composed of DNA and protein. They used radioactive phosphorous ³²P in the medium for some viruses and radioactive sulphur ³⁵S for some others.

They grew some viruses on a medium that contained radioactive phosphorus and some others on a medium that contained radioactive sulphur. Viruses grown in the presence of radioactive phosphorus contain radioactive DNA (labelled DNA), but not radioactive proteins because DNA contains phosphorus (labelled DNA) but proteins do not. Similarly, viruses grown on radioactive sulphur contained radioactive protein but not radioactive DNA because DNA does not contain sulphur.

Radioactive phages were allowed to infect E.coli bacteria grown on the medium containing normal ‘P’ and ‘S’. Then, as the infection proceeded, the viral coats were removed with the help of a centrifuge. Bacteria which were infected by viruses with radioactive DNA, were radioactive, indicating that DNA was the material that passed from the viruses to the bacteria. Bacteria which were infected by viruses having radioactive sulphur (protein) were not radioactive. This indicates that proteins from the viruses did not enter the bacteria. DNA is, therefore, the genetic material that is passed from virus to bacteria.

In other words, sometime after infection, radioactivity for ‘P’ and ‘S’ was tested. Only radioactive ‘P’ was found inside the bacterial cell, indicating that DNA is the genetic material.