Transforming Principle - Practice Questions & MCQ

• In 1869, Friedrich Miescher discovered the acidic substance present in the nucleus and identified it as nuclein.
• Even though the discovery of nuclein by Meischer and the proposition for principles of inheritance by Mendel were almost at the same time, the DNA acts as a genetic material took long to be discovered and proven.

Transforming Principle:

• Frederick Griffith in 1928, carried out a series of experiments with Streptococcus pneumoniae (a bacterium that causes pneumonia).
• He observed that when Streptococcus pneumonia are grown on a culture plate, some of them produce smooth, shiny colonies (S-type), whereas, the others produce rough colonies (R-type). 
• This difference in character (smooth/rough) is due to a mucous (polysaccharide) coat present in the S-strain bacteria, which is not present in the R-strain.
• Therefore, ‘S’ strains are the virulent strains causing pneumonia.
• In his experiments, he first infected two separate groups of mice. The mice that were infected with the S-strain die from pneumonia.
• The mice that were infected with the R-strain do not develop pneumonia and they live.

• In the next set of experiments, Griffith killed the bacteria by heating them. 
• The mice that were injected with heat-killed S-strain bacteria did not die and lived. 
• The mice that were injected with a mixture of heat-killed S-strain and live R-strain bacteria, died due to unexpected symptoms of pneumonia.

Conclusion:

• Griffith concluded that the live R-strain bacteria, were transformed by the heat-killed S-strain bacteria.
• He proved that there was some ‘transforming principle’ that was transferred from the heat-killed S-strain, which helped the R-strain bacteria to synthesise a smooth polysaccharide coat and thus, become virulent. 
• That was due to the transfer of the genetic material.
• However, he was not able to define the biochemical nature of genetic material from his experiments.
   

• Oswald Avery, Colin MacLeod and Maclyn McCarty (1933-44) worked to determine the biochemical nature of ‘transforming principle’ in Griffith’s experiment in an in vitro system.
• From the heat-killed S-cells, they purified biochemicals (proteins, DNA, RNA, etc.) to observe, that which biochemicals could transform live R-cells into S-cells.
• Protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not cause this transformation.
• DNA-digesting enzyme (DNase) caused inhibition of transformation, which suggests that the DNA caused the transformation. 
• Thus, these scientists came to the conclusion that DNA is the hereditary material.
• Therefore, they discovered that DNA alone from heat-killed S-type bacteria caused the transformation of non-virulent R-type bacteria into S-type virulent bacteria.