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 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.
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? If Hershey and Chase could figure out what the bacteriophages inject, they could confirm what the genetic material is
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!
Review (Questions 1-5 page 228)