Frederick Griffith conducted experiments in 1928 using two strains of Streptococcus pneumoniae bacteria - R strain and S strain. The R strain was non-virulent while the S strain was virulent. Griffith found that when he injected a mixture of heat-killed S bacteria and live R bacteria into mice, the mice became sick, showing that something from the S bacteria had transformed the R bacteria. Later experiments by Avery, McCarty, and MacLeod in 1944 identified this transforming substance as DNA. Further experiments by Hershey and Chase in the 1950s using bacteriophage viruses provided more evidence that DNA, not protein, was the genetic material being transferred from viruses to bacteria to cause infection.

1. EXPERIMENTS:
DNAAS THE
GENETIC MATERIAL
Prasanna R Kovath
Assistant Professor
Department of Biotechnology

2. Frederick Griffith: Bacterial transformation
• In 1928, British bacteriologist Frederick Griffith conducted a series of
experiments using Streptococcus pneumoniae bacteria and mice.
• Trying to develop a vaccine against
pneumonia.
• Griffith used two related strains of bacteria
• R Strain
• S Strain

3. • The R bacteria were nonvirulent, meaning that they did not cause
sickness when injected into a mouse.
• The polysaccharide, coat protected the S bacteria from the mouse
immune system, making them virulent (capable of causing disease).
Mice injected with live S bacteria developed pneumonia and died.
• Griffith also injected mice with S-strain bacteria that had been killed
by heat. As expected, the killed bacteria did not harm the mice.
• R bacteria were combined with harmless heat-killed S bacteria and
injected into a mouse. mouse develop pnenumonia and die

5. • Experimental Results : a substance could be transferred to harmless
bacteria and make them deadly.
• R-strain bacteria must have taken up what he called a "transforming
principle" from the heat-killed S bacteria, which allowed them to
"transform" into smooth-coated bacteria and become virulent.

6. Avery, McCarty, and MacLeod Experiment
• In 1944, three Canadian and American researchers, Oswald Avery,
Maclyn McCarty, and Colin MacLeod, set out to identify Griffith's
"transforming principle.“
• 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. This proved that the
‘transforming substance’ was neither the protein no RNA.
• 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.

8. Hershey and Chase Experiment
• In the 1950s, Alfred Hershey and Martha Chase did experiments
with viruses and bacteria.
• Hershey and Chase studied bacteriophage,: viruses that infect
bacteria- the bacteriophage was composed of only DNA and protein-
they wanted to determine which of these molecules is the genetic
material that enters into the bacteria.
• The virus doesn’t have their own mechanism of reproduction but they
depend on a host for the same. Once they attach to the host cell, their
genetic material is transferred to the host. Here in case of
bacteriophages, bacteria are their host. The infected bacteria are
manipulated by the bacteriophages such that bacterial cells start to
replicate the viral genetic material.

9. • The experiment began with the culturing of viruses in two types of
medium. One set of viruses (A) was cultured in a medium of
radioactive phosphorus whereas another set (B) was cultured in a
medium of radioactive sulfur.
• They observed that the first set of viruses (A) consisted of radioactive
DNA but not radioactive proteins. This is because DNA is a
phosphorus-based compound while protein is not. The latter set of
viruses (B) consisted of radioactive protein but not radioactive DNA.
• The host for infection was E.coli bacteria. The viruses were allowed to
infect bacteria by removing the viral coats through a number of
blending and centrifugation.