Since the knowledge of genetics develop, it was asked quite often, what is the genetic material and which molecules are responsible for transfer of genetic information from one generation to next generation.
The early work of Fred Griffith in 1928 on the transfer of virulence in the pathogen Streptococcus pneumoniaecommonly called as pneumococcus. Make it more clear that DNA is the genetic material and it is the molecule which carry information (characters) from one generation to next generation.
Griffith, while working on pneumococcus, found that if he boiled virulent bacteria and injected them into mice, the mice were not affected and no pneumococci could be recovered from animals. When he injected a combination of killed virulent bacteria and a living non virulent strain, the mice died, and he could recover the living virulent bacteria from the dead mice. Griffith named this change of non virulent bacteria into virulent pathogens transformation.
Oswald Avery and his colleagues then set out to discover which constituent in the heat-killed virulent pneumococci was responsible for Griffith’s transformation. These investigators selectively destroyed constituents in purified extracts of virulent pneumococci (S cells), using enzyme that would hydrolyze DNA, RNA, or protein. They then exposed non virulent pneumococcal strains (R strains) to the treated extracts. Transformation of non virulent bacteria was blocked only if the DNA was destroyed, suggesting that DNA was carrying the information required for transformation.
Hershey and Chase Experiment
Some year later, of Griffith’s experiment. Alfred Hershey and Martha Chase performed several experiments indicating that DNA was the genetic material in a bacterial virus called T2 bacteriophage.
Chase made the virus’s DNA radioactive with 32P or they labeled the its protein coat with 35S. they mixed radioactive bacteriophage with Escherichia coli and incubated the mixture for a few minutes. The suspension was then agitated violently in a blender to shear off any adsorbed bacteriophage particles. After centrifugation, radioactivity in the supernatant (where the virus remained) versus the bacterial cells in the pellet was determined. They found that most radioactive protein was released into the supernatant, whereas 32P DNA remained within the bacteria. Since genetic material was injected and T2 progeny were produced. DNA must have been carrying the genetic information for T2.