Question

Oswald Avery and others have continued Griffth's transforming principle to prove DNA as genetic material substantiate.

Answer 1

• In 1928, Frederick Griffith conducted a series of experiment with Streptococcus pneumoniae, a bacterium that commonly cause pneumonia in mammals including humans. He studied two strains of bacteria called S-strain and R-strain.
  
• S-strain was virulent. They have a mucopolysaccharide coat and produce smooth shiny colonies.
  
• R-strain was non-virulent. They do not possess coat and produces rough colonies when grown on culture plate.
  
• Griffith conducted a series of experiment on mice as shown in the figure. He observed that when a mixture of heat killed S-strain and live R bacteria are injected, the mice did not died. Further, the living S-strains can be recovered from the dead mice. He concluded that there was some factor in heat killed S-strain which transformed the live R-strains to S-strains and made the S-strain virulent. However, the biochemical nature of genetic material couldn't be revealed from his experiments.
  
• The first convincing evidence of the role of DNA in heredity came from the experiments of Avery, Macleod and McCarty, who showed that the genetic characteristics in bacteria would be altered from one type to another by treatment with purified DNA. In studies of Streptococcus pneumoniae, they transformed mutant cells which are unable to cause pneumonia into cell that could do so by treating with pure DNA of disease forms. From their experiment as showed in the figure, they concluded transformation cannot occur unless DNA is present thus, the molecule that carries heritable information is DNA.
  
• Hence, in this way Oswald Avery and others have continued Griffith's transforming principle to prove DNA as genetic material substantiate.