Discoveries regarding genetic marerial:

 

Transforming Principle

Frederick Friffith, conducted experiments with Streptococcus pneumonia (bacteria that causes pneumonia). He observed two strains of this bacterium—one forming smooth shiny colonies (S-type) with capsule, while other forming rough colonies (R-type) without capsule.

1. When live S-type cells were injected into mice, they died due to pneumonia.
2. When live R-type cells were injected into mice, they survived.
3. When heat-killed S-type cells were injected into mice, they survived and there were no symptoms of pneumonia.
4. When heat-killed S-type cells were mixed with live R-type cells and injected into mice, they died due to unexpected symptoms of pneumonia and live S-type cells were obtained from mice.

Conclusion: He concluded that heat-killed S-type bacteria caused a transformation of the R-type bacteria into S-type bacteria but he was not able to understand the cause of this bacterial transformation.

Biochemical Characterisation of Transforming Principle

1. Oswald Avery, Colin MacLeod and McCarty repeated Griffith’s experiment in an in vitro system in order to determine biochemical nature of transforming principle.
2. They reported that DNA from the heat-killed S-type bacteria caused the transformation of non-virulent R-type bacteria into virulent S-type bacteria.
3. They also discovered that proteases and RNases did not affect transformation while DNases inhibited the process.

Conclusion: They concluded that DNA is the hereditary material.

Proof for DNA as the Genetic Material

Hershey and Chase conducted experiments on bacteriophage to prove that that DNA is the genetic material.

Procedure:

1.  Some bacteriophage virus were grown on a medium that contained radioactive phosphorus (³²P) and some in another medium with radioactive sulphur (³⁵S).
2.  Viruses grown in the presence of radioactive phosphorus (³²P) contained radioactive DNA.
3.  Similar viruses grown in presence of radioactive sulphur (³⁵S) contained radioactive protein.
4.  Both the radioactive virus types were allowed to infect E. coli separately.
5. Soon after infection, the bacterial cells were gently agitated in blender to remove viral coats from the bacteria.
6.  The culture was also centrifuged to separate the viral particle from the bacterial cell.

 Conclusions:  (i) Only radioactive ³²P was found to be associated with the bacterial cell, whereas radioactive ³⁵S was only found in surrounding medium and not in the bacterial cell.

(ii) This indicates that only DNA and not protein coat entered the bacterial cell.

(iii) This proves that DNA is the genetic material which is passed from virus to bacteria and not protein.