The experiment that was directed at chemical characterization of the genetic material was carried out in 1951 — 50, 7 years after every work was published.
By then experimental techniques had progressed and new approaches to problem solving in Genetics were available. in particular the use of bacteriophages as experimental tool for studying molecular Genetics had become fully established.
Bacteriophages or phages as they are commonly known are viruses that specifically infect bacterium. Phage T2 for example is one of several type of pages that are specific for the bacterium Escherichia Coli.
The Phage consists of a protein coat with surround a core of DNA. electron Micrograph has revealed that the protein code portion that consists the genetic material is hexagonal and the centre portion is called head.In addition there is a tail also.
Alfred hershey and Martha Chase in 1952 published the result of experiments designed to carry find the events leading to phage reproduction.
several of the experiments clearly established the independent function of Phage code protein and nucleic acid in the reproductive process associated with the bacterial cell.
From the existing data Hershey and chase new that
- T2 phage consists of approximately 50% protein and 50% DNA
- Infection is initiated by adsorption of the phage through its tail fiber to the bacterial cell.
- The Reproduction of new viruses occur within the bacterial cell
While working with this phage,it appeared to Hershey and Chase that some molecular components of the phage DNA and protein enter the bacterial cell the direct viral reproduction.
What could possibly this component being was the main question arising in their Minds. both any and protein contains elements most characteristics of living organisms carbon , hydrogen , oxygen and nitrogen.
They differ in one major aspect that is Sulphur occurs only in proteins not in DNA why Phosphorus occur only in DNA and not proteins.
Based on the information that the two micro molecules are different they designed their experiment. the differently radiolabelled the macromolecules (DNA or protein) with radioisotopes market to follow the molecular component of age during infection.
Both P32 and S35 Radioactive isotopes of phosphorus and sulphur were used respectively.
Hershey and chase begin the experiment by growing culture of phages T2 in suspension of e coli rich in one of two radioactive isotopes, Sulphur. — (S35) or Phosphorus — 32 (P32) . They then proceeded to identify the material injected into the cell by phages attached to the bacterial wall.
All the T2 drive from culture has had their DNA labelled with P32 Aur the protein coats label with S35 next cell and label culture of e coli were infected separately with S35 labelled and p35 label T2. In the case of the P32 label phages that were mixed with equally found that the P32 label enter the bacterial cell and the next generation of pages that was from the infected cell Carried a significant amount of P32 label.
In the S35 labelled phages mix with an label Equaliser research found that the Global state on the outside of the bacteria in phages coats.From this observation they concluded that the outer protein coat of a phage does not enter the bacterium it infects whereas the features in a material consisting of a DNA does enter the bacterial cell.
Since the DNA is responsible for the production of new page during infection process the DNA not the protein must be the genetic material.
Transfection experiments —
During the eight years following the publication of the hershey chase experiment for the research provided additional Prove the DNA is a genetic material.
These studies involve the same organisms as were used by hershey and chase.
In 1957 there were several report showing that naked cells of E coli could be obtained on treating the cells with the enzyme lysozyme. this enzyme dissolve the outer wall without destroying the bacterium.
Such naked structures are called protoplast or spheroplast. John Spizizen and Dean fresher independently reported that you by using Protoplasm they were able to initiate phage multiplication with disrupted T2 particles.
That is provided Protoplasm I used it is not necessary for virus to be intact in order for infection to occur.
Later in 1960 similar but refined experiment were reported by George Gutherie and Robert Sinsheimer.
The purified DNA from the bacteriophage X 174. It is small phage consisting of a single stranded, circular DNA molecule of some 5500 nucleotides.
When added to the E coli. protoplasts, the purified DNA resulted in the production of complete X 174 bacteriophage.
This process of infection by only the viral and nucleic acid is called transfection, Prove beyond doubt that the phage DNA along contains all the necessary information for production of mature viruses.
Thus, the evidence that DNA serves as a genetic material was for the strength and even the all direct evidence had been obtained from bacterial and viral studies.
