My Science School

Often, during conversations pertaining to heredity, be it with respect to certain mannerisms or behaviour, you might have heard people allude to their DNA. This is because we now know that deoxyribonucleic acid, or DNA, holds the key to heredity to all forms of life and carries genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses.

First isolated by Swiss physician Friedrich Miescher in 1869, DNA’s role as the carrier of life’s hereditary data wasn’t known for nearly a century. For, it was only in 1952 that it was firmly established that DNA was the substance that transmits genetic information. That was done through the Hershey-Chase experiment, also often referred to as the blender experiment.

Born in Michigan, the U.S. in 1908, Alfred Day Hershey attended public schools before going on to study B.S. in Bacteriology and doing a Ph.D. in Chemistry. He was drawn towards bacteriology and the biochemistry of life as a graduate student and even his doctoral thesis was on the chemistry of a bacteria. After receiving his Ph.D., Hershey moved into a career of research and teaching.

DNA or protein?

The foundation for the field of molecular biology was laid in the 1940s and the 1950s through research on bacteriophages. Bacteriophages, or simply phages, were known to be viruses – consisting only of DNA surrounded by a protein shell – that infect bacteria.

One of the key questions that was haunting the field was to find out which was the genetic material. The prevalent notion at the time was that it must be a protein, as its structure was complex enough to hold such data. Even though there was some research that pointed at DNA as the possible genetic material, most chemists, physicists and geneticists still held on to the then popular assumption.

Hershey, whose research on phages had provided him with a number of discoveries, set out to conclusively prove that the genetic material in phages was DNA. Along with his assistant Martha Chase, who had recently graduated, Hershey found a way to figure out the role played in replication by each of the phage components.

In experiments conducted in 1951-52, Hershey and Chase used radioactive phosphorus to tag the phage DNA and radioactive sulphur to tag the protein. These tagged phages were then allowed to infect a bacterial culture and begin the process of replication.

Role of blender

This process was interrupted at a crucial moment when the scientists whirled the culture in a blender. This was because Hershey and Chase had been able to determine that a blender produced the right shearing force to tear the phage particles from the bacterial walls, without damaging the bacteria.

Upon examination, it was clear that while the phage DNA had entered the bacterium and forced it to replicate phage particles, the phage protein was still outside, attached to the cell wall. In short, they were able to show that it was DNA, and not protein, that was responsible for communicating genetic information necessary for producing the next generation of phages.

Stimulates research

Hershey and Chase published their results on September 20, 1952. The Hershey-Chase experiment came to be popularly referred to as the blender experiment because of the fact that a simple blender had been used to achieve their test results. These results stimulated research into DNA, and within months, molecular biologists James Watson and Francis Crick published their work establishing the double helix structure of the DNA molecule. In fact, Watson wrote in a 1997 memoriam that the Hershey-Chase experiment “made me ever more certain that finding the three-dimensional structure of DNA was biology’s next important objective”. It certainly turned out to be right.

Small in size, big prize

Alfred Hershey shared the Nobel Prize in Physiology or Medicine in 1969 with Max Delbruck, a physicist who did research in the U.S. after fleeing Nazi Germany in 1937, and Salvador Edward Luria, a biologist and physician from Italy who fled to France in 1938 and immigrated to the U.S. in 1940. They received the Noble Prize for their contributions to molecular biology and their work on bacteriophages, which are viruses that infect bacteria.

Working independently, Hershey and Luria showed the occurrence of spontaneous mutation in bacteriophages and the host in 1945.

In the next year, Hershey and Delbruck separately discovered the occurrence of genetic recombination in phages. This showed that when different strains of phages infect the same bacterial cell, they can exchange or combine genetic material.

The three men turned out to be collaborators, despite the fact that they never worked together in the same laboratory.

They encouraged each other in their phage research by sharing results through correspondence and conversations.

 

Picture Credit : Google