Frederick Griffith's experiments in 1928 showed that a transforming principle could pass from dead virulent pneumonia bacteria to live avirulent bacteria, making them deadly. Oswald Avery later purified this principle and through chemical and enzymatic tests determined it was DNA, not protein as previously believed. In 1952, Hershey and Chase used radioactive labeling to track the entry of DNA and proteins from bacteriophages into infected bacteria. They found that only the labeled DNA entered the bacterial cells, providing definitive evidence that DNA is the genetic material.

1. DNA as heredity material
Presented by: Usman Ghani

2. DNA was identified as the genetic
material through a series of experiments
• 1868 Miescher found material in nucleus half protein and half
something else
• 1890’s learned more about the unknown and named it
deoxyribonucleic acid or DNA
• 1902 Walter Sutton proposed hereditary material is in the
chromosomes
• Chromosome have equal parts of protein and DNA
• Scientists still had not determined whether DNA or protein was
the one that made up genes

3. Griffith finds a ‘transforming principle’
• Griffith experimented with the
bacteria that cause pneumonia.
• He used two forms: the S form
(deadly) and the R form (not
deadly).
• A transforming material passed
from dead S bacteria to live R
bacteria, making them deadly.

4. Genetic Material and Transformation
•Frederick Griffith- 1928 Frederick Griffith, British microbiologist, made a
series of unexpected observations while performing an experiment with the
disease-causing bacteria pneumococcus and laboratory mice. Griffith's
experiment dealt with two strains of the bacteria pneumococcus. One was a
virulent strain with a smooth polysaccharide coat necessary for infection and
colonies of this strain appear smooth. The other was a non-virulent strain
with a rough coat that could not cause infection and colonies of the strain
appear rough.
Griffith injected one group of mice with the smooth virulent strain and these
mice died after a few days. He then injected another group with the rough
non-virulent strain and these mice continued to be healthy. Griffith took a
heat-killed strain of the virulent bacteria and injected it into mice and
observed that they did not die. Griffith's fourth experiment was to inject heat
treated, killed, smooth virulent strain mixed with the non virulent rough
strain. He injected this mixture and found that after a few days the mice died.
The blood of the dead mice showed high levels of virulent pneumococcus.
Griffith theorized that some type of transformation takes place from the
virulent to the non-virulent strain for it to synthesize a new polysaccharide
coat. He did not know if it was DNA or protein that was being transferred.

5. Fredrick Griffith experiment

6. Avery identified DNA as the transforming
principle
• Avery isolated and purified Griffith’s transforming
principle.
• Avery performed three tests on the transforming
principle.
Qualitative tests showed DNA was present.
Chemical tests showed the chemical
makeup matched that of DNA.
Enzyme tests showed only DNA-degrading
enzymes stopped transformation.

7. Experiment
• Transformation- the process during which bacteria are changed by absorbing
genetic material from an outside source
• Oswald Avery, Canadian physician and bacteriologist, found that the agent
responsible for genetic transferring is the nucleic acid DNA and not protein as
most biochemists theorized at the time. In 1944 Avery and his coworkers,
McCarty and MacLeod, discovered the "transforming principle.“
• The Experiment
 First they treated the bacteria with centrafugation, which eliminates large
cellular pieces. The result: bacteria still transformed
 Added protease, which removes all proteins
The result: bacteria still transformed
 Treated the bacteria with deoxyribonuclease, which eliminates all DNA
The result: no transformation in the bacteria
 Conclusion: the trio concluded that DNA is the cause of transformation,
where in this experiment virulence is inherited.
• There was not enough evidence to convince everyone

8. Avery experiment

9. Hershey and Chase confirm that DNA is the
genetic material
Hershey and Chase studied viruses that
infect bacteria, or bacteriophages.
They tagged viral DNA with
radioactive phosphorus.
They tagged viral proteins with
radioactive sulfur.
Tagged DNA was found inside the
bacteria; tagged proteins were not.

10. DNA and Bacterial Viruses
• 1952: Geneticists Alfred Hershey and Martha Chase publish the findings of their so-
called blender experiments, which conclude that DNA is where life's hereditary
data is found.
• Prior to these experiments, so named because they were conducted using a regular
kitchen blender, it was generally believed that proteins -- not DNA -- were the
genetic stuff of life.
• Using the blender, Hershey and Chase separated the protein coating from the
nuclei of bacteriophages, the viruses that infect bacteria. Injecting nucleic acid into
the bacterial cell, they found that it was the acid itself, and not the protein, that
caused the transmission of genetic information.
• Their conclusion was that genes are made of the nucleic acid DNA.
• Hershey would subsequently share the 1969 Nobel Prize in Physiology or Medicine
for his work in discovering the properties of DNA. But Chase, who served as
Hershey's lab assistant during his experiments and whose name appears on the
paper, was snubbed

11. Hershey & chase experiment