The Hershey-Chase Experiment

1. The Hershey-Chase Experiment • Alfred Hershey(1908-1997) was born • in Owosso, Michigan. He studied at the Michigan State College, where he obtained B. S. in 1930, and Ph. D. in 1934. In 1967 he got an honorary D. Sc. at the University of Chicago. • Martha Chase (1930-  ) recieved her bachelor's degree in biology from the University of Dayton and applied for a summer position, as an undergrad, in Hershey's lab. She later recieved her Ph. D in microbial physiology.

3. Escherichia coli Escherichia coli is a bacterium that is a common - but certainly not the most abundant - inhabitant of the human intestine. It also lives in the intestine of many other animals, wild as well as domestic.

4. T2 Bacteriophage • infects a bacterium & use the bacterium to produce more viruses. • contain 50% DNA(head) and 50% protein • tail fibers of the phage attach to the bacterial cell to initiate infection • new viruses are produced within the bacterial cell

5. T2 Bacteriophage

6. Discover of Bacteriophage • Felix d'Herelle, was studying the feces of patients who had recovered from a bacterial dysentery. • discovery of an organism capable of killing bacteria - so small it could pass through a filter. • a bacterial virus----is an extremely simple organism, composed only of protein and DNA • This discovery is useful in fighting disease

7. Which substance directed this takeover - DNA or protein? • In 1952, American biologists Alfred Hershey and Martha Chase set out to determine what composed the genetic material of a bacteriophage.

8. Bacteriophage attach a bacteriuminject genetic material (DNA) into bacteriumsynthesis ofDNA replicate & protein coat and tailform a new bacteriophage • .

9. Protein contains sulfur, but DNA doesn't. Protein contains a small amount of phosphorus; DNA contains a lot of phosphorus.

10. DNA is the genetic • Alfred Hershey and Martha Chase discovered that DNA is the genetic material of a phage known as T2

11. They knew that T2 • Is one of many phages to infect bacterium Escherichia coli (E. coli). • Like other viruses, is little more than DNA enclosed by a protein coat. • Can quickly reprogram an E. coli cell to produce T2 phages and release the viruses when the cell lyses. • They did not know is which viral component - DNA or protein - was responsible for reprogramming the host bacterial cell

12. Experiment • 1: Viral protein and DNA were tagged with different radioactive isotopes. Protein Tagging: T2 and E. coli were grown in media with radioactive sulfur (35S) which incorporated only into the phage protein. DNA Tagging: T2 and E. coli were grown in media containing radioactive phosphorus (32P) which was incorporated only into the phage DNA.

13. 2: Protein-labeled and DNA-labeled T2 phages were allowed to infect separate samples of nonradioactive E. coli cells. • 3: Cultures were agitated to shake loose phages that remained outside the bacterial cells.

14. 4: Mixtures were centrifuged forcing the heavier bacterial cells into a pellet on the bottom of the tubes. The lighter viruses remained in the supernatant. • 5 : Radioactivity in the pellet and supernatant was measured and compared

15. Results: • In tubes with E. coli infected with protein labeled T2, most of the radioactivity was in the supernatant with viruses. • In tubes with E. coli infected with DNA-labeled T2, most of the radioactivity was in the pellet with the bacterial cells. When the bacteria containing DNA-labeled phages were returned to culture medium, the bacteria released phage progeny which contained 32P in their DNA.

16. Hershey and Chase found that • When bacteriophages containing 35P (radioactive), were allowed to infect nonradioactive bacteria, all the infected cells became radioactive and, in fact, much of the radioactivity was passed on to the next generation of bacteriophages.

17. when the bacteria were infected with bacteriophages labeled with 35S and then the virus coats removed (by whirling them in an electric blender), practically no radioactivity could be detected in the infected cells.

18. Conclusions • Viral proteins remain outside the host cell. Viral DNA is injected into the host cell. Injected DNA molecules cause cells to produce additional viruses with more viral DNA and proteins. These data provided evidence that nucleic acids rather than proteins are the hereditary material.

19. From these experiments, it was clear that: • the DNA component of the bacteriophages is injected into the bacterial cell while the protein component remains outside. • However, it is the injected component - DNA - that is able to direct the formation of new virus particles complete with protein coats.

20. Additional evidence • These experiment provided evidence that DNA is the hereditary material in viruses. Additional evidence pointed to DNA as the genetic material in eukaryotes as well.

21. Some circumstantial evidence was: • A eukaryotic cell doubles its DNA content prior to mitosis. • During mitosis, the doubled DNA is equally divided between two daughter cells. • An organism's diploid cells have twice the DNA as its haploid gametes.