Hershey-Chase Experiment Prove that DNA is the Genetic Material

Hershey-Chase Experiment:

In 1952, Alfred D. Hershey and Martha Chase performed a confirmatory experiment using T₂ bacteriophage to prove DNA as genetic material. Bacteriophage T₂ is a tailed virus of Escherichia coli.

Viruses are the simplest living organisms and their reproduction is totally dependent on the metabolic machinery of the host. The viruses infect the bacteria and are called as bacteriophage. Bacteriophage T₂ which infects the bacteria is composed of about 50% protein coat and 50% with an inner core of DNA. The bacteriophage T₂ attaches the host bacterium by means of a tail. It enters its DNA into the cytoplasm of bacteria and the protein coat remains outside the cell. After half an hour, the bacteria cell bursts and numerous phages come out of the bacterial cell. This is due to fact that phage DNA lyses the bacterial cell and uses its metabolic machinery for synthesizing more of their DNA copies and protein coats. Thus, the parental DNA of the virus carries the genetic information which directs the synthesis of both DNA molecules and the protein coats of the progeny.

Hershey and Chase allowed to grow both bacteriophage T₂ and E. coli cells in a medium supplemented with radioactive isotopic phosphorous P³² or radioactive sulfur S³⁵. E. coli growing on a medium containing either P³² or S³⁵ were infected by the T₂ phage. Then radio-labelled T₂ phages were recovered from the respective culture medium by centrifugation. The progeny phages contained S³⁵ protein and P³² labelled DNA. Both types of phages were allowed to infect E. coli cells separately growing on a fresh normal medium. After some times (20 minutes) when infection was over, the empty phage coat proteins were recovered from the infected bacterial cells by agitation using a kitchen blender. The phage coat proteins were separated from infected cells by low speed centrifugation. The phage particles are present in supernatant and bacterial cells settled as pellets. Radioactivity in these two fractions was measured in both sets of an experiment.

It was found that the bacterial cells showed radioactivity when P³² labelled phage infected E. coli cells. When S³⁵ labelled phage was used to infect E. coli, the empty phage showed radioactivity but not the bacterial cells. The progeny phage also contained P³² in DNA but not S³⁵ in coat protein.

Hershey and Chase concluded that T₂ phage delivered its DNA into bacterial cell leaving the coat protein outside the cell wall. Progeny phage particles were produced inside the cell and consisted of P₃₂ in their DNA and unlabelled protein coat. It showed that DNA acts as genetic material. That is why it synthesises a new protein coat and replicates DNA.