Identifying DNA as the
Genetic Material
History of DNA
8.1
Frederick Griffith - 1928
• Investigated two forms of the bacteria that causes pneumonia
• Smooth (S): known to be deadly
• Rough (R): not known to be deadly
• Injected bacterial strains into mice with interesting results
Weird!
• Living S bacteria are deadly
• Living R bacteria are
harmless
• Heat-killed S bacteria are
harmless
• BUT when heat-killed S and
living R are injected
together, it is deadly!
• It gets weirder…
• Blood samples from the
dead mice revealed living
S bacteria!
This baffled Griffith…
How could something
that was once harmless
become deadly?
How did living S end up
in the blood when he
didn’t inject it?
The Transforming Principle
• Griffith concluding that material from the dead S bacteria must have
been transferred to the living R bacteria
• This material had the power to turn a harmless bacteria into a deadly
one
• The big question:
• What is the material!?
• He named it the Transforming Principle
• (Spoiler alert: It’s actually DNA)
Oswald Avery Works to Identify Transforming
Principle
The protein vs DNA debate
• Avery and colleagues designed
experiments to determine
whether the transformation
principle was DNA or Protein
• Isolated special extract from S
bacteria
Tests conducted
• Qualitative
• Chemical Analysis
• Enzyme tests
1944 Oswald Avery Identifies Transforming Principle!
• Qualitative tests
• No protein present in S bacteria
extract
• Enzyme Tests
• When Dnases (enzymes that break
down DNA) were added, the
sample was destroyed
• When Proteases (enzymes that
break down protein) were added,
the sample was unharmed
• Chemical analysis
Not Convinced!
• Some scientists still had
trouble believing that
something as simple as DNA
could be the genetic material
• They wondered if DNA of
bacteria was the same DNA in
other organisms.
• Alfred Hershey and Martha
Chase conducted an
experiment to confirm it once
and for all
Hershey and Chase 8 years later…..
• Bacteriophages (think of King Candy in Wreck-it-Ralph—like a phage)
• Viruses that infects bacteria by injecting genetic material into host. It
takes over bacteria and directs it to make more viruses.
• Simple structure
• DNA surrounded by protein coat
• Question: What do they inject? The DNA or the protein?
Hershey and Chase: The Experiment
• Protein
• Contains sulfur, but little phosphorus
• DNA
• Contains phosphorus, but no sulfur
• Grew phages in cultures with radioactive isotopes of phosphorus and sulfur
• Radioactive “tag” that causes to glow
• Experiment 1
• Bacteria infected by phage with radioactive sulfur
• Examined bacteria
• No significant radioactivity
• Experiment 2
• Bacteria infected by phage with radioactive phosphorus
• Examined bacteria
• Lit up like a Christmas tree!
Review (Questions 1-5 page 228)