History of DNA 81 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 ID: 678256
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Slide1
Identifying DNA as the Genetic Material
History of DNA
8.1Slide2
Frederick Griffith - 1928
Investigated two forms of the bacteria that causes pneumonia
Smooth (S): known to be deadly
Rough (R): not known to be deadlyInjected bacterial strains into mice with interesting resultsSlide3
Weird!
Living
S
bacteria are
deadly
Living R bacteria are harmlessHeat-killed S bacteria are harmlessBUT 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?Slide4
The Transforming Principle
Griffith concluding that
material
from the dead S bacteria must have been transferred to the living R bacteriaThis 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)Slide5
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 testsSlide6
Qualitative tests
No protein present in S bacteria extract
Enzyme Tests
When Dnases (enzymes that break down DNA) were added, the sample was destroyedWhen Proteases (enzymes that break down protein) were added, the sample was unharmed
1944 Oswald Avery Identifies Transforming Principle!
Chemical analysisSlide7
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 allSlide8
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 structureDNA surrounded by protein coat
Question: What do they inject? The DNA or the protein?Slide9
Hershey and Chase: The Experiment
Protein
Contains
sulfur, but little phosphorusDNAContains
phosphorus
, but no sulfur
Grew phages in cultures with radioactive isotopes of phosphorus and sulfur
Radioactive “tag” that causes to glowExperiment 1Bacteria infected by phage with radioactive sulfurExamined bacteriaNo significant radioactivityExperiment 2Bacteria infected by phage with radioactive phosphorusExamined bacteriaLit up like a Christmas tree!It’s DNA, folks!Slide10
Review (Questions 1-5 page 228)