Chapter 12 DNA & RNA

1. Chapter 12 DNA & RNA The genetic code is the way in which the cell stores information that it passes on from generation to generation.

2. GRIFFITH & TRANSFORMATION Griffith studied ways in which types of bacteria cause pneumonia. His main concern was to make sure that pneumonia causing bacteria did not produce toxins. He worked with 2 slightly different strains of bacteria.

3. Both strains of bacteria grew well in his lab but only one of them actually caused the disease. *The disease causing bacteria grew into SMOOTH colonies on the culture plates…Page 288 Figure 12-2

4. The harmless strain grew into rough TEXTURED colonies. • GRIFFITH’S EXPERIMENT: • He injected mice with the SMOOTH bacteria & they developed pnuemonia and died. • He injected mice with the ROUGH non-disease causing bacteria and the mice LIVED.

7. ??Do disease causing bacteria produce a poison??? 3. Griffith heat killed the disease causing bacteria and injected them into the mice…the mice survived! ??Maybe Pneumonia is not caused by a toxin released by the disease causing bacteria??

8. 4. In his next experiment Griffith injected a mixture of harmless bacteria & heat-killed bacteria into the mice…the mice developed pneumonia. When mixed, the heat-killed harmful strain had passed on its disease causing ability to the harmless rough bacteria.

9. Due to this surprise Griffith recovered the bacterial strains from the animals that had developed pneumonia & grew them on culture plates…the colonies that grew were SMOOTH! CONCLUSION: One strain of bacteria had been transformed into another strain. Griffith referred to this as TRANSFORMATION.

10. Video Classroom Clip: Griffith’s Experiment

11. The Transforming Factor GRIFFITH’S HYPOTHESIS: When live harmless bacteria & heat-killed bacteria were mixed, a factor was transformed from the heat-killed cells to the live cells.

12. In 1944 a group of scientists lead by Oswald Avery (@Rockefeller Institute, NY) repeated Griffith’s experiment & identified DNA as the transforming factor.

13. Video Segment 5 minutes:Oswald Avery, Colin MacLeod and Maclyn McCardy: Confirming DNA is the Transform

14. DNA- deoxyribonucleic acid The nucleic acid that stores & transmits the genetic information from one generation of an organism to the next…DNA CARRIES THE GENETIC CODE!

15. Alfred Hershey & Martha Chase studied the viruses that infect bacteria. Viruses are non-living particles smaller than a cell that can infect living organisms. BACTERIOPHAGE- “bacteria-eater”…a virus that infects bacteria.

16. Video Segment: Hershey Chase

17. Bacteriophages are composed of : • A DNA Core • A Protein Coat (Capsid) • The DNA core & Protein Coat make up the head of the bacteriophage. • The bacteriophage also has a tail.

20. How a Bacteriophage infects a bacterial cell: 1-The bacteriophage attaches to the bacterial cell & injects a material into the bacterium. 2. The injected material reproduces using the DNA of the bacterial cell…more bacteriophages are made.

21. 3. The bacterial cell becomes so filled with the bacteriophages that it LYSES (breaks open) & infects other cells. Hershey & Chase wanted to find out what part of the infecting virus was the material that entered the bacterium…was it the protein coat, the DNA or both?

24. Hershey-Chase Experiment Radioactive Tagging (Page 290) Two groups of bacteriophages were prepared: one had radioactive sulfur-35 added to it & the other had radioactive phosphorus-32 added.

26. The radioactive isotopes were used to trace the pathway of certain materials….this was a way of “labeling” the viruses. The protein coat was labeled with sulfur-35 & the DNA was labeled with Phosphorus-32.

27. Proteins contain little or no phosphorus & DNA does not contain sulfur… If sulfur was found in the bacteria this would mean that the viruses protein coat was injected into the cell.

28. If phosphorus was found, then it was the DNA that had been injected. RESULTS: Nearly all of the radioactive sulfur remained with the viruses…nearly all of the radioactive phosphorus had entered the bacteria.

29. CONCLUSION: The virus’s DNA enters a bacteria & the protein coat remains behind on the outside of the bacteria…This was convincing evidence that DNA is the molecule that contains genetic code!

30. THE STRUCTURE OF DNA Page 291 How can DNA store information AND duplicate itself? DNA is a polymer found in units called NUCLEOTIDES.

31. P.A. Levene: DNA Composition of 4 Nitrogen Bases and Deoxyribose Sugar (01:38)

32. NUCLEOTIDE- a molecule made up of three basic parts: 1-A 5 carbon sugar = Deoxyribose 2-A Phosphate Group 3-A nitrogen containing base

33. Nucleotide

34. DNA contains 4 Nitrogen Bases: • Adenine & 2.Guanine • *Both belong to a group of compounds known as purines. • 3. Cytosine & 4. Thymine • *Belong to pyrimidine group;

35. Individual nucleotides are joined together to form a long chain. The sugars & phosphate groups form the BACKBONE of the chain. The nitrogen bases form the MIDDLE PART of the chain.

36. X-RAY EVIDENCE Rosalind Franklin, in the early 1900’s purified a sample of DNA, stretched it out & made X-ray diffraction pictures of the molecule. See page 292 Time line…

37. VIDEO CLIP:Rosalind Franklin and Maurice Wilkins: X-Ray Crystallography of DNA

39. (It was noted that the fibers that make up the DNA molecule are twisted like strands of a rope…see pg 292…the X near the center) **It was also noted that large groups of molecules in the fiber are spaced out at regular intervals along the length of the fiber.

40. *The X-ray did not allow us to determine the structure of DNA…it was more like a fingerprint to give scientists evidence to work with.

41. BUILDING A MODEL OF DNA Francis Crick (British physicist) & James Watson (25 year old American biochemist) tried to determine the structure of DNA by building a 3-D models of the atomic groups in DNA.

42. After viewing Franklin’s x-ray pattern…they realized something important about the pattern…& within weeks Watson & Crick determined the structure of DNA.

43. THE DOUBLE HELIX The shape that Watson & Crick used for their model was a helix similar to a spiral but it was twisted. Franklin’s idea of DNA containing 2 strands was also useful in determining the structure.

44. Watson & Crick decided that the structure would be a double helix. Characteristics Described by the Watson & Crick Model: 1. The nitrogen bases on each of the strands of DNA were positioned exactly opposite each other.

45. 2. Nitrogen base positioning allows weak Hydrogen bonds to form between the nitrogen bases ADENINE & THYMINE…and between CYTOSINE & GUANINE.

46. Erwin Chargaff observed that in any sample of DNA, the # of adenine molecules was equal to the # of thymine molecules. This was also true for cytosine and guanine.

47. Video clip:Erwin Chargaff: DNA is Not Equal For All Species and Ratio of Bases Varies Amon (01:24)

48. This observation allowed Watson & Crick to determine that adenine bonds only to thymine and cytosine bonds only to guanine. BASE PAIRING- the attraction of one base for the other. Base pairing holds the 2 strands of a DNA double helix together.

49. Waston & Crick’s model was quickly accepted because it also showed how DNA could replicate. (The model explained much about the structure of DNA, including placement of the nitrogen bases & the formation of hydrogen bonds!)

50. In 1962 Watson, Crick, & Maurice Wilkins were awarded the Nobel Prize for medicine/physiology…for their discoveries concerning the DNA molecule. Rosalind Franklin was not included because she had already died.