Transforming principle:

  • Given by Frederick Griffith in 1928.
  • His experiment based on Streptococcus pneumoniae (caused pneumonia).
  • There is change in physical form of bacteria.
  • There are two colonies of bacteria:
    • Smooth shiny colonies called S strain.
    • Rough colonies called R strain.
  • S-strain bacteria have a mucous (polysaccharide) coat.
  • R-strain does not have mucous coat.
  • S-strain is virulent and caused pneumonia in mice and died when infected.
  • R-strain is non-virulent and dose caused pneumonia in mice when infected.
  • Heat killed S-Strain is non-virulent and dose not causes pneumonia.
  • The heat killed S-Strain mixed with live R-Strain injected into mice; the mice developed pneumonia and died.
  • He recovered live S-Strain bacteria form the dead mice.

Conclusion of experiment:

  • R – Strain bacteria had some how been transformed by the heat killed S-Strain bacteria.
  • Some ‘transforming principle’, transferred from heat killed S-Strain bacteria, had enabled the R-Strain to synthesize smooth polysaccharide coat and become virulent (S Strain).
  • The transformation of R-Strain to S-Strain is due to transfer of Genetic material.
  • However the biochemical nature of genetic material was not defined from his experiment.

Biochemical characterization of transforming principle:

  • Biochemical nature of transforming principle was discovered by Oswald Avery, Colin Macleod and Maclyn McCarty. (1933-44)
  • Prior to their work genetic material was thought to be protein.
  • They worked to determine the biochemical nature of the ‘transforming principle’ of Griffith’s experiment.
  • They purified biomolecules (proteins, DNA and RNA) from the heat killed S cells to see which one could transform live R cells to S cells.
  • Heat killed S-Strain + protease + Live R-Strain →transformation.
  • Heat killed S-Strain + RNase + Live R-Strain →transformation.
  • Heat killed S-Strain + DNase + Live R-Strain →transformation.

Conclusion of the experiments:

  • Protein of heat killed S-Strain is not the genetic material
  • RNA of heat killed S-Strain is not the genetic material.
  • DNA of heat killed S-Strain is the genetic material, because DNA digested with DNase mixed with R-strain unable to transform R-Strain to S-Strain.

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Articles


DNA fingerprinting is the technique where DNA sequences of two individuals are compared. It is widely used in the identification of paternity.


Genetic code is a triplet sequences of bases that codes for particular amino acids that further helps in protein synthesis. Mutations are sudden change; it can be point mutation or frame shift mutation.


Ribonucleic acid and deoxyribonucleic acid are nucleic acids which are differ from each other in the presence of thymine in DNA and uracil in RNA.


Transcription is a process where DNA is converted into RNA by the help of RNA polymerase. Information of DNA is copied into mRNA for the synthesis of protein. After RNA synthesis it is modified further by the process of post transcriptional modification.


Translation is a process of synthesis or formation of protein from mRNA. Three process like initiation, elongation and termination are involve in translational process.


Replication is a process of copying of DNA. Messelson and stahl done an experiment to prove that DNA is of semi-conservative in nature where one strand is antisense or template strand and other strand is sense or leading strand.


DNA is tightly packed in the nucleus of every cell. DNA wraps around special proteins called histones, which form loops of DNA called nucleosomes.


Expression of genes can be regulated by lac operon model which was proposed by Jacob and Monad. The regulation of lac operon by repressor is known as negative regulation.


Feel Free To Email Us .At: