Identifying the Genetic Material A.Griffith’s Experiment (1928) -Frederick Griffith was trying to find a vaccine against pneumonia. -Pneumonia is caused by a type of bacteria called Streptococcus pneumoniae -Two bacterial strains exist: TYPE S: TYPE R: Has a capsule and causes pneumonia No capsule and doesn’t cause pneumonia

EXPERIMENT 1. When mice were injected with live TYPE S, the mice__________. 2. When mice were injected with live TYPE R, the mice_________. 3. When mice were injected with heat killed TYPE S, the mice_________. 4. When the mice were injected with a mixture of killed TYPE S and live TYPE R, the mice_____________ and found that live TYPE R had _________________________________. died lived died Acquired capsules…became TYPE S

-Remember that dead TYPE S do not cause pneumonia and neither do live TYPE R. But when brought together, they do cause the disease. -CONCLUSION: Some factor from dead TYPE S bacteria had transformed TYPE R into TYPE S. The transformed bacteria acquired the ability to make capsules and cause pneumonia.

B. Avery’s Experiments (1944) -Avery identified the transforming material in Griffith’s experiments by performing the following: -EXPERIMENT: 1. Live TYPE R placed into 2 test tubes. 2. DNA from dead TYPE S placed into test tube Protein from dead TYPE S place into test tube Results: Test tube 1 contained TYPE S & tube 2 still contained TYPE R -CONCLUSION: Live R 1 DNA from dead TYPE S transformed TYPE R…got new genetic traits. 2

C. Alfred Hershey and Martha Chase (1952) -Many scientists remained unconvinced and thought protein was the carrier of genetic traits. Hershey & Chase resolved the debate. -They made use of viruses called bacteriophages which infect bacteria. -A bacteriophage, phage for short, consists of a core of DNA and a protein coat -When phages infect bacteria, they issue instructions that make the bacterial cell create more phage viruses.

-Question: How does the phage reprogram the bacterial cell to make more viruses? Does the phage DNA, the protein or whole phage issue the instructions??

-EXPERIMENT: 1. DNA contains___________. Grew phage in radioactive ___________ Radioactive Phosphorous became part of phage’s DNA 2. Phage infects bacteria 3. Results: Bacteria show… radioactivity 4. Conclusion: DNA from phage enters bacterial cell and issues instructions 1.Protein contains _______ Grew phage in radioactive ____________ Radioactive Sulfur became part of phage’s protein coat. 2. Phage infects bacteria. 3. Results: Bacteria show… no radioactivity 4. Conclusion: Protein coat from phage remains outside Phosphorous Sulfur Phosphorous ( 32 P) Sulfur ( 35 S)

Summary Hershey & Chase Experiments