Mice live + Mice die a.b.c.d. Live Virulent Strain of S. pneumoniae Live Nonvirulent Strain of S. pneumoniae Heat-killed Virulent Strain of S. pneumoniae.
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Mice live
+
Mice liveMice die
a. b. c. d.
Live VirulentStrain of S. pneumoniae
Live NonvirulentStrain of
S. pneumoniae
Heat-killed VirulentStrain of S. pneumoniae
Mixture of Heat-Killed Virulentand Live Nonvirulent
Result: When the experiment is done, only 32P makes it into the cell in any significant quantity.
Hypothesis: DNA is the genetic material in bacteriophage.
Prediction: The phage life cycle requires reprogramming the cell to make phage proteins. The information for this must be introduced into the cell during infection.
Test: DNA can be specifically labeled using radioactive phosphate (32P), and protein can be specifically labeled using radioactive sulfur (35S) . Phage are grown on either 35S or 32P, then used to infect cells in two experiments. The phage heads remain attached to the outside of the cell and can be removed by brief agitation in a blender. The cell suspension can be collected by centrifugation, leaving the phage heads in the supernatant.
Phage grown in radioactive 35S,which is incorporated into phage coat
Virus infectbacteria
Blender separatesphage coat from bacteria
Centrifuge formsbacterial pellet
35S in supernatant
35S-Labeled Bacteriophages
Phage grown in radioactive 32P.which is incorporated into phage DNA
Virus infectbacteria
Blender separatesphage coat from bacteria
Centrifuge formsbacterial pellet
32P in bacteria pellet
32P-Labeled Bacteriophages
Conclusion: Thus, DNA must be the molecule that is used to reprogram the cell.
Further Experiments: How does this experiment complement or extend the work of Avery on the identity of the transforming principle?