1. Griffith’s Mouse Experiment
Fredrick Griffith

2. This experiment demonstrates the power of genetic material.

3. In his first experiment, Griffith wished to determine the pathogenicity (disease –causing capability) of his rough strain (Type IIR) of Streptococcus bacteria

4. To begin, Griffith injected cultures of his rough strain into mice.

5. Two weeks after injection, Griffith found that the mice survived the introduction of the rough strain into their systems.

6. How would you interpret these results? Select from the choices above.

8. Smooth
In the second experiment, Griffith wished to determine the pathogenicity (disease-causing capability) of his smooth (Type IIS) strain of Streptococcus bacteria.

9. To begin, Griffith injected living cultures of his smooth strain into mice.

10. Two weeks after injection, Griffith found that the mice were killed as a result of the introduction of the smooth bacteria into their systems.

11. How would you interpret the results? Select from the choices above.

13. In his third experiment, Griffith wished to determine whether the viability of the smooth strain was required for pathogenicity (disease-causing capability). To do this he first needed to kill these bacteria by boiling them for a short period of time.

14. Now that the smooth bacteria were dead, Griffith could test whether or not they could cause disease in that state.

15. Griffith injected the heat killed bacteria into the mouse.

16. Two weeks after injection, Griffith found that the mice survived the introduction of the heat-killed, smooth bacteria.

17. How would you interpret these results. Select from the choices above.

19. In his final experiment, Griffith wished to determine whether the factor present in the living smooth bacteria that causes disease could be transferred to nonpathogenic material.

20. Griffith first boiled pathogenic, smooth bacteria, heat killing them .

21. In the next step, Griffith needed to mix his heat-killed, smooth bacteria, with living, rough bacteria.

22. After mixing the two strains, Griffith injected a mouse with the mixture. Neither the rough bacteria nor the heat-killed smooth bacteria were capable of causing disease on their own.

23. The mixture was injected into mice, and the mice were incubated for two weeks.

24. After two weeks, Griffith found that the mice died.

25. How would you interpret these results? Select from the choices above.

27. The evidence for Griffith’s hypothesis came from an investigation of the dead mice.

28. When bacteria were recovered from the dead mice, Griffith cultured them and found living, smooth bacteria. Griffith reasoned that the only way for this to have occurred was if living bacteria (rough, in this case) were instructed to become smooth. Griffith proposed the following explanation.

29. Griffith proposed that when the smooth bacterial culture was heat killed…

30. …components present inside the smooth bacteria that caused the bacteria to be pathogenic might have been released into the media after death of the bacteria.

31. Therefore, when nonpathogenic, rough bacteria were introduced into the culture.

32. …the cellular components from the killed, smooth bacteria were able to enter the living, rough bacterial cells.

33. Once inside, the cellular components then transformed the living rough bacteria cells into living, smooth cells. Griffith therefore determined the cellular components transforming factors. At that time, the exact molecule, (or molecules) that make up the transforming factor were not known.

34. Fig. 2.2: Frederick Griffith’s Transformation Experiment - 1928
“transforming principle” demonstrated with Streptococcus pneumoniae
Griffith hypothesized that the transforming agent was a “IIIS” protein. But this was only a guess, and Griffith turned out to be wrong.

35. SUMMARY

37. Oswald T. Avery’s Transformation Experiment - 1944
Determined that “IIIS” DNA was the genetic material responsible for Griffith’s results (not RNA).
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

38. Hershey-Chase Bacteriophage Experiment - 1953
Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery.
Structure of T2 phage
Bacteriophages
are composed of
DNA & protein

39. Life cycle of virulent T2 phage:

40. http://highered. mcgraw-hill. com/olcweb/cgi/pluginpop. cgi

41. Hershey-Chase Bacteriophage Experiment - 1953
T2 bacteriophage is composed of DNA and proteins:
Set-up two replicates:
Label DNA with 32P
Label Protein with 35S
3. Infected E. coli bacteria with two types of labeled T2
4. 32P is discovered within the bacteria and progeny phages, whereas 35S is not found within the bacteria but released with phage ghosts.

42. Conclusions about these early experiments: Griffith 1928 & Avery 1944:
DNA (not RNA) is transforming agent.
Hershey-Chase 1953:
DNA (not protein) is the genetic material.
Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957:
RNA (not protein) is genetic material of some viruses, but no known prokaryotes or eukaryotes use RNA as their genetic material.
Alfred Hershey
Nobel Prize in Physiology or Medicine
1969

43. Structure of DNA
James D. Watson/Francis H. Crick 1953 proposed the Double Helix Model based on two sources of information:
Base composition studies of Erwin Chargaff
indicated double-stranded DNA consists of ~50% purines (A,G) and ~50% pyrimidines (T, C)
amount of A = amount of T and amount of G = amount of C
(Chargraff’s rules)
%GC content varies from organism to organism
Examples: %A %T %G %C %GC
Homo sapiens
Zea mays
Drosophila
Aythya americana

44. Structure of DNA
James D. Watson/Francis H. Crick 1953 proposed the Double Helix Model based on two sources of information:
X-ray diffraction studies by Rosalind Franklin & Maurice Wilkins
Conclusion-DNA is a helical structure with
distinctive regularities, 0.34 nm & 3.4 nm.