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.
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.
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.
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.
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.
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.
1. What was Griffith's transforming principle?
Griffith was the one who initially conceived the idea of the transformative principle. The principle proved successful in converting a strain of non-pathogenic bacteria into a strain of pathogenic bacteria. Hereditary material is distinguished by a number of qualities, including the ability to undergo phenotypic change. Griffith referred to the component that was responsible for the altered phenotype as the transforming principle. It was determined through a series of studies carried out by Avery, McCartys and MacLeod that the hereditary material in question was DNA.
2. What is the semi-conservative DNA replication model?
The "semi-conservative DNA replication" model was proposed by Watson and Crick. The two DNA strands split apart in accordance with this theory. For the synthesis of a new strand, each strand serves as a template. Based on complementary base pairing with the template, the new strand is created. One parent strand and one freshly produced strand make up each new DNA molecule. This is how the single copy of the original DNA molecule is divided into two copies.
3. What is the biochemical nature of the transforming principle?
To find the transforming principle, bacteriologists did a number of experiments.
Alcohol precipitated the transforming principle. This demonstrated that it wasn't a carbohydrate.
Proteases were unable to eliminate the transforming principle. So, the protein was not the cause.
The lipases were unable to remove the transforming principle. This demonstrated that it wasn't a lipid.
Ribonuclease could not inactivate the transforming principle, hence RNA was not effective.
Deoxyribonuclease may be used to inactivate the transforming principle.
DNA was the transforming principle. As a result, DNA was the genetic material.