Actually he did not won any noble prize , his work was not accepted by biologists ............................................................Transforming Principle
In 1928, Frederick Griffith, in a series of experiments with Streptococcus
pneumoniae (bacterium responsible for pneumonia), witnessed a
miraculous transformation in the bacteria. During the course of his
experiment, a living organism (bacteria) had changed in physical form.......
When Streptococcus pneumoniae (pneumococcus) bacteria are grown
on a culture plate, some produce smooth shiny colonies (S) while others
produce rough colonies (R). This is because the S strain bacteria have a
mucous (polysaccharide) coat, while R strain does not. Mice infected with
the S strain (virulent) die from pneumonia infection but mice infected
with the R strain do not develop pneumonia.Griffith was able to kill bacteria by heating them. He observed that
heat-killed S strain bacteria injected into mice did not kill them. When he injected a mixture of heat-killed S and live R bacteria, the mice died.
Moreover, he recovered living S bacteria from the dead mice.
He concluded that the R strain bacteria had somehow been
transformed by the heat-killed S strain bacteria. Some ‘transforming
principle’, transferred from the heat-killed S strain, had enabled the
R strain to synthesise a smooth polysaccharide coat and become virulent.
This must be due to the transfer of the genetic material. However, the
biochemical nature of genetic material was not defined from his
Biochemical Characterisation of Transforming Principle
Prior to the work of Oswald Avery, Colin MacLeod and Maclyn McCarty
(1933-44), the genetic material was thought to be a protein. They worked
to determine the biochemical nature of ‘transforming principle’ in Griffith's experiment.
They purified biochemicals (proteins, DNA, RNA, etc.) from the
heat-killed S cells to see which ones could transform live R cells into
S cells. They discovered that DNA alone from S bacteria caused R bacteria
to become transformed.
They also discovered that protein-digesting enzymes (proteases) and
RNA-digesting enzymes (RNases) did not affect transformation, so the
transforming substance was not a protein or RNA. Digestion with DNase
did inhibit transformation, suggesting that the DNA caused the
transformation. They concluded that DNA is the hereditary material, but
not all biologists were convinced.
The unequivocal proof that DNA is the genetic material came from the
experiments of Alfred Hershey and Martha Chase (1952). They worked
with viruses that infect bacteria called bacteriophages.
The bacteriophage attaches to the bacteria and its genetic material
then enters the bacterial cell. The bacterial cell treats the viral genetic
material as if it was its own and subsequently manufactures more virus
particles. Hershey and Chase worked to discover whether it was protein
or DNA from the viruses that entered the bacteria.
They grew some viruses on a medium that contained radioactive
phosphorus and some others on medium that contained radioactive sulfur.
Viruses grown in the presence of radioactive phosphorus contained
radioactive DNA but not radioactive protein because DNA contains
phosphorus but protein does not. Similarly, viruses grown on radioactive
sulfur contained radioactive protein but not radioactive DNA because
DNA does not contain sulfur.
Radioactive phages were allowed to attach to E. coli bacteria. Then, as
the infection proceeded, the viral coats were removed from the bacteria by
agitating them in a blender. The virus particles were separated from the
bacteria by spinning them in a centrifuge.
Bacteria which was infected with viruses that had radioactive DNA
were radioactive, indicating that DNA was the material that passed from
the virus to the bacteria. Bacteria that were infected with viruses that had
radioactive proteins were not radioactive. This indicates that proteins did
not enter the bacteria from the viruses. DNA is therefore the genetic
material that is passed from virus to bacteria.