Question

'DNA is a genetic material.' Prove it with the help of Hershey - Chase experiment. Draw linear diagram of the experiment.

Answer 1

Formerly, scientists thought that proteins, which are found in chromosomes along with DNA could make an ideal genetic material. Proteins were known to have diverse amino acid sequences, while DNA was thought to be a repetitive polymer. However, it has been proved on the basis of experimental evidence that DNA is the genetic material.

Hershey-Chase experiment: A bacteriophage (T2 virus) infects the bacterium Escherichia coli. After infection, the virus multiplies and the T2 phages are released with the lysis of the bacterial cells. The phages were simple particles composed of protein and DNA, with the outer structures made of protein and the inner core consisting of DNA. To establish whether the phage injected DNA or protein into host bacteria, Hershey and Chase prepared two different batches of phage. In one preparation they made protein part radioactive in the presence of 35S and in the other preparation the DNA was made radioactive in the presence of 32P. Each batch of phage was used to infect a different culture of bacteria. After infection had taken place, each culture was whirled in a blender, removing any remaining phage and phage parts. Finally, the cultures were centrifuged, to separate the bacteria from the phage debris. The heavier pellets of infected bacterial cells settle down in the bottom of the tube and the lighter viral particles were found in the supernatant. It was found that when phage with DNA was used, the bacterial pellet was also radioactive. On the other hand, when phage with radioactive protein was used, the bacterial pellet had very little radioactivity and most of the radioactivity was found in the supernatant. This experiment proved that during infection with phage, it is DNA that actually entered the bacteria. This experiment conducted by Hershey and Chase has proved that it is the viral DNA and not protein that contains information for the production of more phage particles, hence DNA is the genetic material.