The Hershey and Chase Experiment, conducted in 1952 by Alfred Hershey and Martha Chase, demonstrated that DNA contains genetic information. They accomplished this by investigating viruses that infect bacteria, known as bacteriophages. In these tests, scientists labelled the virus's DNA with a radioactive marker while labelling the protein coat independently with another marker.
When the viruses infected bacteria, researchers discovered that only the DNA identifier, not the protein marker, was passed along to the next generation of viruses. This helped to demonstrate that DNA, not protein, is the molecule that conveys genetic instructions. We will read about the Hershey and Chase Experiment in detail in this article.
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Hershey and Chase Experiment
In 1952, Alfred Hershey and Martha Chase investigated bacteriophage, a virus that destroys bacteria. Their research focused on T2 bacteriophage that infects the bacterium Escherichia coli (E. coli). Their goal was to determine whether the T2 phage's genetic instructions or information required for life, were stored in its DNA or protein coat. They wanted to show that the DNA, not the protein, contained this important genetic information.
There were three steps in the experiment:
- Infection
- Blending
- Centrifugation
Alfred Hershey and Martha Chase used two forms of radioactive material, phosphorus-32 (32P) and sulfur-35 (35S), to designate the bacteriophages differently. Phosphorus is a component of DNA, the genetic material, whereas sulphur is present in proteins but not DNA.
They inserted these radioactive isotopes into the bacteriophages DNA (genetic material) and protein coat (capsid) separately. This enabled them to determine which parts of the virus entered the bacterial cell during infection. They then allowed the labelled viruses to infect E. coli bacterial cells.
Following a brief time of infection, they mixed the liquid to separate the viral protein coatings from the bacteria. This blending phase ensured that any viral proteins that were not bound to bacterial cells were eliminated. They next centrifuged the mixture, causing the heavier bacterial cells to sink at the bottom of the tube while the lighter viral protein coatings (if present) remained in the liquid above, known as the supernatant.
The results demonstrated that bacterial cells infected with phages labelled with phosphorus-32 (32P) exhibited radioactivity. This suggested that the phages' DNA entered the cells during infection. In contrast, bacterial cells infected with phages labelled with sulfur-35 (35S) exhibited little to no radioactivity, indicating that the phages' protein coat (also known as capsid) did not enter the cells.
Based on these findings, Hershey and Chase concluded that DNA, rather than protein, serves as the genetic material transmitting bacteriophages' hereditary information. This experiment offered solid proof that DNA is the chemical responsible for conveying genetic information in living beings.
DNA as Genetic Material
Scientists discovered that DNA is the primary factor in defining the characteristics of most living organisms. However, some viruses use RNA instead. So, for something to be genetic material, it must:
- Be able to create clones of itself (self replicable).
- Be stable structurally and chemicaly.
- Allow for mutations, which can lead to evolution.
- Be able to pass on traits according to Mendel's inheritance principles.
Most other compounds, such as proteins, carbohydrates, and lipids, did not meet the previously listed criteria. While RNA could meet those requirements, DNA was favoured over RNA for genetic material for a number of reasons:
- DNA has more structural stability than RNA.
- DNA has higher chemical stability than RNA.
- DNA has a double-stranded structure that allows it to effectively repair replication faults.
- RNA is required for protein production because DNA cannot directly code for them.
Approximate content of DNA in few organisms is given below:
Species | DNA amount pg/per cell | No. of nucleotide pairs (in millions) |
|---|---|---|
Bacteriophage T4 | 0.0024 | 0.7 |
Bacteria | 0.002-0.06 | 2 |
0.02-0.17 | 20 | |
Sponges | 0.1 | 100 |
Molluscs | 1.2 | 1000 |
Crustaceans | 3 | 2800 |
Fishes | 2 | 2000 |
7 | 6500 | |
Birds | 2 | 2000 |
Reptiles | 5 | 4500 |
6 | 5500 |
What is the Pulse and Chase Experiment?
Pulse-Chase Analysis is similar to a time-lapse camera for investigating what happens inside cells. In this procedure, cells are first exposed to a labelled chemical (the "pulse") that identifies certain molecules. Then they are given an unlabeled chemical (the "chase") to observe what occurs. This allows scientists to track how molecules migrate and change over time.
Researchers have utilised this method to analyse a variety of proteins, including protein kinase C and ubiquitin, as well as to better understand processes such as Okazaki fragment production during DNA replication. For example, George Palade used pulse-chase with radioactive amino acids to study how cells release chemicals.
Conclusion -DNA as Genetic Material: Hershey And Chase Experiment
Alfred Hershey and Martha Chase conducted to confirm DNA as the genetic substance. The Hershey-Chase investigations was crucial then as at that time many scientists believed that proteins contained genetic information rather than DNA. Hershey and Chase discovered that when viruses called bacteriophages infect bacteria, a small amount of their protein enters the bacterial cell. This suggested that DNA, not protein, was responsible for carrying genetic instructions. These findings, coupled with previous and subsequent discoveries, strongly showed that DNA was the genetic material. Later they received the Nobel Prize in Physiology or Medicine for their contributions to genetics.
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