Table of Contents
- Alfred Hershey and Martha Chase did a series of tests in 1952 called the Hershey–Chase experiments. These tests helped prove that DNA is genetic material.
- Alfred Hershey, who was a scientist, and Martha Chase Biologists have known about DNA since 1869, but at the time, many scientists still thought that proteins carried the information for inheritance because DNA seemed to be a non-living molecule, and because it is in the nucleus, it was thought that its job was to store phosphorus.
- In their experiments, Hershey and Chase showed that when bacteriophages, which are made of DNA and protein, infect bacteria, their DNA enters the cell of the host bacteria but most of their protein does not. Hershey and Chase, as well as other discoveries, all showed that DNA is the material that is passed down.
- Hershey, Max Delbrück, and Salvador Luria all won the Nobel Prize in Physiology or Medicine in 1969 for their “discoveries about the genetic structure of viruses.”
- We know about Griffith’s experiment and the other experiments that followed to find out how organisms pass on their genes.
- Using Griffith’s experiment as a guide, Avery and his team were able to separate DNA and show that DNA is the genetic material. But not everyone agreed with it until Hershey and Chase published the results of their tests.
- Alfred Hershey and Martha Chase started looking for the genetic material in organisms in 1952.
- Their experiments proved without a doubt that DNA is made up of genetic material.
- Bacteriophages, which are viruses that kill bacteria, were the most important part of the experiment by Hershey and Chase.
- The virus doesn’t have its own way to make more copies, so it needs a host to do so.
- Once they attach to the host cell, their DNA moves to the host cell. In this case, bacteria are bacteriophages’ hosts.
- The bacteriophages change the way the infected bacteria work so that the bacterial cells start to copy the viral DNA.
- Hershey and Chase did an experiment to find out whether the genetic material that went into the bacteria was protein or DNA.
- In 1952, the Hershey-Chase experiment was done to prove that DNA is the material that makes up genes.
- In the tests that Hershey and Chase did, they used E. coli and the bacteriophage T2.
- The bacteriophage attaches itself to the bacteria and sends its DNA into the bacterial cell. It has a protein shell and DNA.
- Some T2 phages were grown in a medium with radioactive sulphur (35S), and the rest were grown in a medium with radioactive phosphorus (32P).
- The T2 phages in (32P) medium had radioactive DNA because the protein coat doesn’t have any phosphorus, but the T2 in (35S) medium had radioactive protein because the DNA didn’t have any sulphur.
- The radioactive phages joined the E. coli after that. As the illness got worse, the viruses were separated using centrifugation.
- Because the radioactive DNA in the E. coli that had been infected by the T2 phage was also radioactive, this suggests that DNA was the thing that the virus gave to the bacteria.
- The conclusion of the Hershey and Chase experiment was that the bacteria that had been infected by the virus and covered with a radioactive protein coat were not radioactive. This showed that DNA is the genetic material that is passed from a virus to a bacteria.
DNA as Genetic Material
- The E.coli bacteria that were infected by radioactive DNA viruses (A) were radioactive, but the ones that were infected by radioactive protein viruses (B) were not radioactive.
- Bacteria with radioactive DNA and bacteria without radioactive DNA show that viruses with radioactive DNA gave their DNA to the bacteria, but viruses with radioactive protein didn’t give their protein to the bacteria. So, DNA is the material that holds the genes and not the protein.
Why is DNA Considered a Genetic Material?
It was found that DNA was the most important part of most species’ genes. There were some important exceptions, such as viruses whose genes were made of RNA. But what makes DNA different from other molecules that can be used as genetic material, such as proteins, carbohydrates, etc.? Important things that make something genetic are:
- able to make copies of itself.
- Stable in both structure and chemistry.
- Allow for a change that might lead to evolution.
- Can talk to itself using “Mendelian Characters.”
Most of the other compounds, like proteins, carbohydrates, and lipids, did not meet the requirements above. Even though RNA could meet the requirements, DNA was still the preferred genetic material because:
- Structure-wise, RNA is less stable than DNA.
- Chemically, RNA is less stable than DNA.
- DNA is made up of two strands, which makes it easier to fix mistakes in replication.
- Proteins can’t be made without RNA because DNA can’t code for it directly.
Pulse Chase Experiment
The Pulse-Chase Analysis is a method used in biochemistry and genetics experiments to look at biological activity over time. It does this by exposing the cells to the same substance first in a labelled form (the pulse) and then in an unlabeled form (the second pulse) (chase).
This method can be used to keep track of what a cell does over a long period of time. Protein kinase C, ubiquitin and numerous other proteins have been studied using this technique. The technique was also used to show that Okazaki fragments exist and can be useful. George Palade used a pulse-chase of radioactive amino acids to figure out how secretion works.
Alfred Hershey and Martha Chase did a number of tests in 1952 that helped prove that DNA is the material that carries genes. The Hershey-Chase experiments are the name for these tests. Even though biologists have known about DNA since 1869, many scientists at the time thought that proteins held genetic information instead of DNA because DNA seemed less complicated than proteins.
In their experiments, Hershey and Chase showed that when bacteriophages, which are made of DNA and protein, infect bacteria, only a small amount of the protein from the bacteriophage actually gets into the cell of the bacteria that is being infected. Even though the results were not clear and Hershey and Chase were cautious in how they interpreted them, they all pointed to the fact that DNA is the material that is passed down. The 1969 Nobel Prize in Physiology or Medicine went to Max Delbruck, Salvador Luria, and Hershey for their work in genetics.