DNA as a genetic material

Features of Genetic Material:

Following are the basic requirements of genetic material:

• It must be stable
• It must be capable of expressing itself
  
• It must be capable of replication
• It must be inherited from parents to progenies

Griffith's Experiment - transforming principle

Frederick Griffith, in the year 1928, performed an experiment suggesting that bacteria are capable of transferring genetic information through a process known as transformation. 

The Experiment:

• The bacterium Streptococcus pneumoniae was used in the experiment, as it forms two different strains.
• S III strain, the virulent one have lipopolysaccharide capsule, forming the smooth and capsulated colonies.
• R II strain, the avirulent one don't have lipopolysaccharide capsule, forming the rough and non-capsulated colonies.
• In the step first, mouse was injected with S III strain and mouse died.
• In the second step, mouse was injected with R II strain and mouse survived.
• In the third step, mouse was injected with heat killed S III strain and survived.
• In the fourth step, mouse was injected with heat killed S III and live R II strain and the mouse died.

Griffith's conclusion:

Presence of heat killed S III bacteria must have caused living R II strain to transform into S strain.

Avery, MacLeod and McCarty Experiment:

• The Avery-MacLeod-McCarty experiment was performed in 1944 by Oswald Avery, Colin MacLeod and Maclyn McCarty. The experiment was based on transformation principle.
• Avery, MacLeod and McCarty, repeated Griffith's experiment of transformation using purified cell extracts and concluded.
• Removal of all protein from the transforming material did not destroy the ability to transform R strain cells.
• DNA-digesting enzymes destroyed all transforming ability. The transforming material is DNA.
  
  
  
• Cell free extract of S III strain bacterium was subjected to DNase, RNase and Protease.
• Each treated extract was mixed with R II and mixture injected to mouse to see transformation.
• In case of Protease and RNase transformation was recorded.
• In case of DNase no transformation was recorded.
• Avery et al in 1944, revealed the chemical nature of the transforming substance to be DNA. With the help of experiment they showed that DNA isolated from S III strain bacteria could confirms the pathogenic properties to R II strain bacteria.

Conclusion:

Two conclusion were derived-

• Active factor is DNA which can cause transformation.
• S III strain contain the active factor. 

Hershey & Chase experiment with T2 Bacteriophage (1952):

 Phages gets attached on the surface of the host bacteria and inject some substance which can either be DNA or protein. Than injected substance causes bacterial cell to produce lots of phage particles or the injected genetic material.

To prove the genetic material Hershey and Chase prepared two different batches of phages and in each batch, specific radioactive element was introduced.

• One batch of T2 was grown in the presence of ³⁵S, a radioactive isotope of sulfur. As sulfur is present in proteins not in DNA, so only phage proteins were radioactively labeled.
• The other batch of T2 was grown in the presence of ³²P, a radioactive isotope of phosphorous. As phosphorous is present in DNA not in proteins, so only phage DNA was labeled.

• Events which take place is lifecycle of bacteriophage:
• Bacteriophage attaches itself to receptor on bacterial cell wall.
• Bacterial cell wall dissolves.
• Phage DNA enters inside the bacterial cell, while leaving the protein coat outside.
• DNA of phage starts replicating and forming many copies.
• Phage DNA assembles into new protein shells.
• Phage particles breaks/lyses the host cell and gets released.
• Both batches of Bacteriophage were allowed to infect bacteria.
• Once infection was done, the solution was subjected to blending and centrifugation.
• Infected bacteria was observed for radioactivity.
• The radioactive phosphorous was found in the bacterial cells.
• Radioactive Sulphur was only found in ghosts. 
• Only the Bacteriophage DNA (as indicated by the radioactive Phosphorous) entered in the bacteria and was used to produce more bacteriophage.

Conclusion:

DNA is the genetic material.