Frederick Griffith discovered the principle of transformation through experiments with Streptococcus pneumoniae bacteria. He found that heat-killed virulent S-strain bacteria could make non-virulent R-strain bacteria transform and become virulent. Later, Oswald Avery, Colin MacLeod and Maclyn McCarty discovered that DNA alone from S bacteria could transform R bacteria. Alfred Hershey and Martha Chase provided conclusive evidence that DNA, not protein, is the genetic material through experiments using bacteriophages infecting E. coli bacteria. They found that radioactive DNA, but not protein, passed from the bacteriophages into the bacterial cells.

1. Transforming Principle
Frederick Griffith,
• The genetic material is DNA..
• Alfred Hershey and Martha Chase (1952).

2. The search for genetic material
• Previous discoveries by Gregor Mendel, Walter
Sutton, Thomas Hunt Morgan concluded that
chromosomes located in the nucleus of most cells.
• But the question of what is actually the genetic material,
DNA or RNA or PROTEINS?

3. Transforming Principle
• In 1928, Frederick Griffith, in a series of experiments
with Streptococcus pneumoniae.
• Streptococcus pneumoniae responsible for pneumonia.

4. What is transformation ?What is Transformation:
• Transfer of genetic material form one bacterial to other
bacteria is called Transformation.

5. Two type of strains in Streptococcus pneumoniae
R –Strain and S-Strain..

6. • Streptococcus pneumoniae (pneumococcus) bacteria are
grown on a culture plate, some produce smooth shiny
colonies (S) others produce rough colonies (R).
• S strain bacteria have a mucous (polysaccharide) coat,
while R strain does not.
•

7. • Mice infected with the S strain (virulent) die from
pneumonia infection
• But the mice infected with the R strain (Avirulent) do
not develop pneumonia.

10. • When he injected a mixture of heat-killed S and live R
bacteria, the mice died.
• Moreover, he recovered living S bacteria from the dead
mice.

12.  He concluded that the R strain bacteria transformed by
the heat-killed S strain bacteria.
 Some ‘transforming principle’, transferred from the
heat-killed S strain, to R strain.
 R strain to synthesise a smooth polysaccharide coat and
become virulent.

13.  Transformation of R strain into S strain due to the
transfer of the genetic material.
 However, the biochemical nature of genetic material was
not defined from his experiments.

14. Biochemical Characterisation of Transforming
Principle
• Oswald Avery, Colin MacLeod and Maclyn McCarty
(1933-44),

15. • Initially Oswald Avery, Colin MacLeod and Maclyn
McCarty (1933-44), the genetic material was thought to
be a protein.
• They discovered that DNA alone from “S” bacteria
caused “R” bacteria to become transformed

16. • They purified biochemicals (Proteins, DNA, RNA, etc.)
from the heat-killed S cells.
• They also discovered that protein-digesting enzymes
(proteases) and RNA-digesting enzymes (RNases) did
not affect transformation,

17. • Enzyme DNase did inhibit transformation.
• They concluded that DNA is the hereditary material, but
not all biologists were convinced.

19. The Genetic Material is DNA
• The unequivocal proof that DNA is the genetic material
came from the experiments of Alfred Hershey and
Martha Chase (1952).
• They worked with viruses that infect bacteria called
bacteriophages..
• And E.coli bacteria.

20. • The bacteriophage attaches to the bacteria and its genetic
material then enters the bacterial cell.
• The bacterial cell treats the viral genetic material is its
own and subsequently manufactures more virus
particles.

21.  Bacteriophage (or) T2 Bacteriophage:
 The Genetic material present in Bacteriophage is dsDNA.

22. • Hershey and Chase worked to discover whether it was
protein or DNA from the viruses that entered the
bacteria.

23. • Methionine, cysteine sulfur-containing amino acids..
• Sulphur present in Proteins absent in Nucleic acids
(DNA, RNA).
• Phosphorus present in DNA &RNA absent in proteins.

24. • They grew some viruses on a medium that contained
radioactive phosphorus and some others on medium that
contained radioactive sulfur.
Medium
contain
Radio active
Sulphur
35S
Medium
contain
Radio active
Phosphorous
32P

25. • Viruses grown in the presence of radioactive phosphorus
contained radioactive DNA but not radioactive protein.
• Viruses grown on radioactive sulfur contained
radioactive protein but not radioactive DNA because

26. Three steps present in Hershey and Chase
experiment
1. Infection
2. Blending
3. Centrifugation

28. • Infection: Radioactive Bacteriophages were allowed to
attach to E. coli bacteria.
• Blending: The viral coats were removed from the
bacteria by agitating them in a blender.
• Centrifugation: The virus particles were separated from
the bacteria by spinning them in a centrifuge.

30. • Bacteria which was infected with viruses that had
radioactive DNA were radioactive.
• Indicating that DNA was the material that passed from
the virus to the bacteria.

31. • Bacteria that were infected with viruses that had
radioactive proteins were not radioactive.
• This indicates that proteins did not enter the bacteria
from the viruses.
• DNA is therefore the genetic material that is passed from
virus to bacteria

32. Dr. HarinathaReddy Aswartha
Assistant professor
Department of Microbiology
ANDHRAPRADESH
INDIA