1. Identifying DNA as the Genetic Material
History of DNA
8.1

2. Frederick Griffith
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

3. ??? 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?

4. 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)

5. Oswald Avery Works to Identify Transforming Principle
The protein vs DNA debate
Tests conducted
Avery and colleagues designed experiments to determine whether the transformation principle was DNA or Protein
Isolated special extract from S bacteria
Qualitative
Chemical Analysis
Enzyme tests
At this point in history scientists were trying to decide whether DNA or Protein was the genetic material
Protein seemed more logical because proteins can be so varied and complex while DNA is just composed of four different bases. Surely genetic material would be complex!.... Right?
No.

6. 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

7. 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

8. 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?
If Hershey and Chase could figure out what the bacteriophages inject, they could confirm what the genetic material is

9. 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
Lit up like a Christmas tree!
It’s DNA, folks!

10. Review (Questions 1-5 page 228)