Question

describe the experiments of Hershey and chase and explain their significance

Answer 1

Alfred Hershey and Martha Chase conducted a series of experimentin 1952 that lead to the conformation that DNA is genetic material.

Though DNA was discovered, it was believed by several scientists that proteins are the heriditery elements .

In their experiment , Hershey and Chase used viruses or bacteriophages. Bacteriophage is composed of protein shell and DNA. They radioactively labeled the proteins and DNA separately using radiolabelled sulfur 35 and phosphorous 32. Since sulfur is present only in protein and never in DNA, the radioactive sulfur will be expressed in proteins alone. Likewise phosphorous ispresent only in DNA but not in proteins, therefore radioactive P32 will labeled in DNA of T phage alone.

In their experiment, Hershey and Chase allowed to grow the bacteria in a medium containing radiolabelled sulfur and phosphorous (S³⁵, P ³²). The bacteriophages infect the bacteria and produce proogeeniies of bacteriophages which conttaiin radiolabelled elements. These phages were again allowed to infect unlabelled bacteria and the progenies produced contain radioactive isotopes in their structures.

The experiment was repeated with bacteriophages with only sufur labeled and phages which were only phosphorous labeled. The sulfur labeled phages were allowed to infect bacteria . Similarly , phosphorous labeled phages were also allowed to infect bacteria. The bacterial cells were purely unlabelled. The phage coat remains outside and only genetic material enters the bacterial cell. The cultures were centrifuged or agitated to shake off the phage particle that remain outside the bacterial cells. The lighter viral paticles containing viral protein coat remain supernantant whereas the heavier bacterial cells found at bottom. Radioactivity of the pellet and supernatant were measured. In tubes were protein were labeled with S 32 , most of the radioactivity was at the supernatant. And in tubes with DNA laballed phages P 32, most of the radioactivity were found in the pellet with bacterial cells. When the bacteria with DNA labeled phages were returned into culture, the phage progeny released contain P32 in their DNA. This indicate that viral proteins remained outside and DNA injected into bacterial cells. The injected DNA of the phages produced additional viral particles and viral DNA. This indicate that nucleic acid rather than proteins are the hereditary material.

(The bacteriophage structure comprises of a protein head inside which the nucleic acid is present and also the tail. During the phage infection it attaches to the bacteria, inject the nucleic acid into the bacteria and protein coat remain outside. )