Hershey and Chase Experiment

There were many scientists who knew that the element essential for inheritance is found within the body of an organism, but they failed to discover it. Many experiments were performed to extract the chromosomal components, but the question of inheritance remains unanswered. However, with the advent of Griffith’s experiments, the path was opened for the discovery of genetic material.

Working off on Griffith's experiment, Avery and his colleagues successfully isolated DNA and demonstrated that DNA is the genetic material. However, until Hershey and Chase published their experimental data, not everyone agreed with this theory.

The Hershey and Chase Experiment

Hershey and Chase Experiment Diagram

• To establish that DNA serves as the genetic material, the Hershey-Chase experiment was carried out in 1952.

• E. coli and the bacteriophage T₂ were used in the tests conducted by Hershey and Chase.

• The bacteriophage binds to the bacteria and introduces its genetic material into the bacterial cell. It has DNA and a protein coat.

• Some T₂ phages were cultivated in radioactive sulphur (³⁵S) media, while the other T₂ phages were cultured in a radioactive phosphorus (³²P) medium.

• While the T₂ phages in (32P) medium contained radioactive DNA because the protein coat does not contain phosphorus, the T₂ in (³⁵S) medium contained radioactive protein due to the absence of sulphur in the DNA.

• After that, the radioactive phages joined the E. coli. As the illness grew worse, centrifugation was used to separate the viruses.

• The fact that the radioactive DNA in the T₂ phage-infected E. coli was similarly radioactive suggests that DNA was the substance that was transferred from the virus to the bacteria.

• Conclusion of Hershey and Chase Experiment: The bacteria that had been infected by the virus and coated with a radioactive protein coat were not radioactive, demonstrating that DNA is the genetic material transmitted from a virus to a bacteria.

Why is DNA Considered a Genetic Material?

It was discovered that DNA dominated the genetic makeup of the majority of species. There were notable exceptions, including certain viruses whose genetic makeup was RNA. But what distinguishes DNA from other molecules such as proteins, carbohydrates etc. as genetic material? Important requirements for being a genetic material are:

• Able to replicate itself.

• Structurally and chemically stable.

• Give room for a mutation that could result in evolution.

• Able to communicate itself with "Mendelian Characters".

The majority of other compounds, including proteins, carbohydrates and lipids, did not meet the aforementioned requirements. Although RNA could meet the requirements, DNA remained the favoured genetic material over RNA for the following reasons:

• RNA is less stable structurally than DNA.

• RNA is less stable chemically than DNA.

• Due to its double-stranded structure, DNA can more easily correct replication faults.

• RNA is required for protein synthesis because DNA can not code for it directly.

Pulse Chase Experiment

The Pulse-Chase Analysis is a technique used in Biochemistry and genetic experiments to look at the biological activity that is happening over time by exposing the cells to the same substance first in a labelled form (the pulse) and then in an unlabelled form (the second pulse) (chase).

This technique can be used to track a cell's activity over an extended period of time. Protein kinase C, ubiquitin and numerous other proteins have been studied using this technique. The technique was additionally employed to demonstrate the existence and utility of Okazaki fragments. To clarify the secretory process, George Palade used a pulse-chase of radioactive amino acids.

Conclusion

Alfred Hershey and Martha Chase carried out a series of tests in 1952 that helped to establish that DNA is the genetic material. These investigations are known as the Hershey-Chase experiments. Despite the fact that DNA has been known to biologists since 1869, many scientists at the time still believed that proteins contained genetic information because DNA seemed to be less complex than proteins.

In their tests, Hershey and Chase demonstrated that when bacteriophages, which are made up of DNA and protein, infect bacteria, only a small portion of their protein actually reaches the host bacterial cell. The prior, current and later discoveries all served to indicate that DNA is the hereditary material, even though the results were inconclusive and Hershey and Chase were circumspect in their interpretation. Max Delbruck, Salvador Luria and Hershey received the 1969 Nobel Prize in Physiology or Medicine for their discoveries relating to genetics.