What are the search of genetic material?
Genetic material is a hereditary unit of living organisms which is carried out to next generation with genetic identity. DNA and RNA are the prime genetic material in all the life forms.
GRIFFITH/HERSHEY/CHASE: DNA IS THE GENETIC MATERIAL:
In 1928, Frederick Griffith was able to transform avirulent bacteria into virulent one with an active principle that Oswald Avery proved to be DNA in 1944.
In 1952, Martha Chase and Alfred Hershey used radioactively labelled virus DNA to infect bacteria to prove that DNA is the genetic material. These important experiments established that DNA is the genetic material.
Griffith’s Experiment:
The first unambiguous evidence that DNA was the hereditary material came from Frederick Griffith’s experiment in 1928. Griffith used chemical mutagens to isolate a non-virulent form of the bacterium Diplococcus pneumoniae which causes pneumonia. Virulence required the presence of a polysaccharide capsule around the bacterium (D. pneumoniae).
Non-virulent form (non-pathogenic): mutants did not have polysaccharide capsule. Colonies these bacteria appeared rough and were designated R.
Virulent form (pathogenic): have polysaccharide capsule and produced colonies that appeared smooth, so it was designated S. Several virulent forms were known, each with a characteristic polysaccharide capsule (called IS, IIS, IIIS, etc.), which is genetically inherited and is immunologically distinct from other forms.
A smooth bacterium of a particular capsule type (say IIS) can mutate to a non-encapsulated, non-virulent form (IIR). This transformation occurs at a very low frequency (in less than one in a million cells), but when it happens, it is inherited to the progeny. Similarly, the IIR cell can mutate back to the IIS virulent form at low frequency. However, the IIR cell line cannot mutate to an IIIS virulent form. This property provides the key to the experiment.
In his experiment Griffith mixed Pneumococcus type IIR with IIS cells that had been killed and rendered non-virulent by heating them to 65°C, and he injected them into a host mice.
Neither strains (virulent/avirulent) when injected alone produced disease, and no disease was expected from the mixed injections, as neither strain was virulent. However, many of the mice given mixed injections suffered from pneumonia and died. When analyzed, they all contained living virulent type IIIS cells! These cells could not have arisen from the type IIR cells by mutations (they would have produced type IIS cells), and the type IIIS cells were demonstrably dead (injected alone they caused no disease). Some factor must have passed from the dead IIIS cells to the live IIR ones, endowing them with the ability to make a capsule of the III type.
Griffith called the factor “transforming principle” and the process genetic transformation. The transforming principle could be isolated as a cell-free extract and was fully active. The stability of the principle’s transforming activity to heat treatment at 65°C suggested that it was not a protein (such high temperatures denature most proteins). In 1944, Oswald Avery, C. M. MacLeod, and M. J. McCarty succeeded in isolating a highly purified preparation of DNA from the type IIIS bacteria.
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