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DNA is described as the tape of information which encodes the messages for all the vital activities and then transmits it from one generation to the other. This concept, the one that states that DNA is a genetic material and transmits genetic information from one generation to the next, has been supported by two sets of experiments. They are :
1) An experiment conducted on bacteria (Bacterial transformation) by Griffith.
2) An experiment conducted on viruses by Hershey and Chase.
These experiments concluded that DNA, rather than proteins, is the genetic material.
Firstly, transformation is a process, by which the DNA isolated from one type of cell is introduced into the other cell, and is able to show properties of former to later. It is first shown in bacteria.
This experiment was performed by British doctor Frederick Griffith in 1928. This was done to provide evidence that DNA is the heredity material. While working onDiplococcus pneumonia (a bacteria, associated with pneumonia disease), Griffith identified two types of strains:-
I) Capsulated and virulent strain (S-type) - This strain was responsible for the cause of disease pneumonia. This strain of bacteria was covered by a polysaccharide coat, which made the bacterial colony smooth, hence referred as S-type.
II) Non-capsulated and non-virulent strain (R-type)- This strain was not responsible for the cause of disease pneumonia. It lacks a polysaccharide coat, and had a rough appearance, hence referred as R-type.
Griffith performed a series of experiments by injecting both strains of bacteria into mice and observed following result:
1) When R-type(non-virulent) strain was injected into mice, the mice didn't suffer from pneumonia and survived.
Living R-type→ Injected in mice→ mice survived.
2)When S-type(virulent) strain was injected into mice, the mice suffered from pneumonia and died.
Living S-type→ Injected in mice→ mice died.
3) When heat killed S-type(virulent) strain was injected into mice, the mice didn't suffer from pneumonia and survived.
Heat killed S-type→ Injected in mice→ mice survived.
4) When a mixture of heat-killed S-type(virulent) strain and R-type(non-virulent) strain was injected into mice, the mice suffered from pneumonia and died.
A mixture of heat-killed S-type and living R-type → Injected in mice → mice died.
In observation 4, both the R-type and heat killed S-type were non-virulent, so Griffith expected the mice to survive. But this didn't happen. When he observed the blood of the dead mice, he found living S-type bacteria. Therefore, Griffith concluded that there might be some factors in heat killed S-type cells which transferred the R-type bacteria into S-type. This phenomenon is known as "Griffith effect" or "Bacterial transformation".
Although this experiment was completed, Griffith did not prove the transference of material. Therefore, Avery, McCarty, and MacLeod made further studies in 1944. They isolated three components -polysaccharides, protein, and DNA from the heat killed S-type bacteria. Then they made a series of experiments, and obtained the following results:
1) The polysaccharide of S-type + R-type living cell → Injected in mice → mice survived. (R-type)
2) Protein of S-type + R-type living cell → Injected in mice→ mice survived. (R-type)
3) DNA of S-type + R-type living cell → Injected in mice → mice died. (Both R and S-type)
4) DNA of S-type + R-type living cell + DNAase → Injected in mice → mice survived. (R-type)
Thus, this experiment supported Griffith's experiment and furthermore gave molecular explanations to their experiments. They concluded that the DNA isolated from the heat killed S-type when added to the R-type cells changed their surface from rough to smooth, and hence made them virulent. Thus, from this experiment, it was shown that DNA was the genetic material as DNA component of the heat killed S-type transformed living R-type into virulent S-type cells.
A second evidence implicating DNA as the genetic material resulted from a study of the T2 virus conducted by Alfred Hersey and Martha Chase. T2is a virus (bacteriophage)that infects bacteriumEscherichia coliand then multiplies inside it. As T2 phage is made of DNA and protein coat, it is the most suitable material to determine whether DNA or protein contains information for the production of new virus particles. Hershey and Chase, in 1952, demonstrated that only DNA of the phage enters the bacterial cells and contains necessary genetic information for the assembly of new phage particles.
Hersey and Chase designed a series of experiments to determine whether the phage protein or the phage DNA was transmitted in phage reproduction. For that, they used isotopes(radioactive forms) of phosphorous(32P) and sulphur(35S). As DNA contains phosphorus but no sulphur, phage DNA was labelled with32P by growing bacteria infected with phages in culture medium containing32PO4.Similarly , the protein of phage contains sulphur but no phosphorous. Therefore, the phage protein coat was labelled with35S by growing bacteria infected with phages in another culture medium containing35SO4.
Both types of labelled phages were allowed to infect normally cultured bacteria in separate experiments. It was observed that the phage labelled with 35S infected the bacteria first. After placingE.coli cells in a blender and removing them, it was found that most of the isotope was separated with the protein ghosts and little 35S remained in the cell [Figure-(a)]. This result indicated that, although the protein component of a phage was necessary for infection, it didn't enter that cell and was not transmitted to the progeny phages,
In addition, when Hershey and Chase infected bacteria with32P labelled phages and removed the protein ghosts, the bacteria still contained32P-labelled DNA. This was most likely due to that after the cells lysed and new progeny phages emerged, many of these phases emitted radioactivity from32P [Figure-(b)]. Thus, this shows that DNA was passed from the infected phages to the progeny.
These results confirmed that DNA is the genetic material of phages and not protein. Plus, DNA is the infective part of the virus that carries all the genetic information.
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