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DNA: The Carrier of Genetic Information

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  1. DNA: The Carrier of Genetic Information Chapter 12

  2. Learning Objective 1 • What evidence was accumulated during the 1940s and early 1950s demonstrating that DNA is the genetic material?

  3. The Mystery of Genes • Many early geneticists thought genes were proteins • Proteins are complex and variable • Nucleic acids are simple molecules

  4. Evidence for DNA • DNA (deoxyribonucleic acid) • Transformation experiments • DNA of one strain of bacteria can transfer genetic characteristics to related bacteria

  5. Bacteriophage Experiments • Bacteriophage (virus) infects bacterium • only DNA from virus enters the cell • virus reproduces and forms new viral particles from DNA alone

  6. KEY CONCEPTS • Beginning in the 1920s, evidence began to accumulate that DNA is the hereditary material

  7. Learning Objective 2 • What questions did these classic experiments address? • Griffith’s transformation experiment • Avery’s contribution to Griffith’s work • Hershey–Chase experiments

  8. Griffith’s Transformation Experiment • Can a genetic trait be transmitted from one bacterial strain to another? • Answer: Yes

  9. Griffith’s Transformation Experiment

  10. Experiment 1 Experiment 2 Experiment 3 Experiment 4 R cells injected S cells injected Heat-killed S cells injected R cells and heat-killed S cells injected Mouse dies Mouse lives Mouse lives Mouse dies Fig. 12-1, p. 261

  11. Animation: Griffith’s Experiment CLICKTO PLAY

  12. Avery’s Experiments • What molecule is responsible for bacterial transformation? • Answer: DNA

  13. Hershey–Chase Experiments • Is DNA or protein the genetic material in bacterial viruses (phages)? • Answer: DNA

  14. Hershey–Chase Experiments

  15. 1 35S 32 P Bacterial viruses grown in 35S to label protein coat or 32P to label DNA 2 Viruses infect bacteria Fig. 12-2, p. 262

  16. 3 Agitate cells in blender Agitate cells in blender 4 Separate by centrifugation Separate by centrifugation 32 P 35S 5 Bacteria in pellet contain 32P-labeled DNA 35S-labeled protein in supernatant Fig. 12-2, p. 262

  17. 6 Viral reproduction inside bacterial cells from pellet 7 Cell lysis 32P 5 6 7 Fig. 12-2, p. 262

  18. Learning Objective 3 • How do nucleotide subunits link to form a single DNA strand?

  19. Watson and Crick • DNA Model • Demonstrated • how information is stored in molecule’s structure • how DNA molecules are templates for their own replication

  20. Nucleotides • DNA is a polymer of nucleotides • Each nucleotide subunit contains • a nitrogenous base • purines (adenine or guanine) • pyrimidines (thymine or cytosine) • a pentose sugar (deoxyribose) • a phosphate group

  21. Forming DNA Chains • Backbone • alternating sugar and phosphate groups • joined by covalent phosphodiester linkages • Phosphate group attaches to • 5′ carbon of one deoxyribose • 3′ carbon of the next deoxyribose

  22. DNA Nucleotides

  23. Thymine Nucleotide Adenine Cytosine Phosphate group Guanine Phosphodiester linkage Deoxyribose (sugar) Fig. 12-3, p. 264

  24. Animation: Subunits of DNA CLICKTO PLAY

  25. KEY CONCEPTS • The DNA building blocks consist of four nucleotide subunits: T, C, A, and G

  26. Learning Objective 4 • How are the two strands of DNA oriented with respect to each other?

  27. DNA Molecule • 2 polynucleotide chains • associated as double helix

  28. DNA Molecule

  29. Sugar–phosphate backbone Minor groove Major groove 3.4 nm 0.34 nm 2.0 nm = hydrogen = carbon = oxygen = atoms in base pairs = phosphorus Fig. 12-5, p. 266

  30. Double Helix • Antiparallel • chains run in opposite directions • 5′ end • phosphate attached to 5′ deoxyribose carbon • 3′ end • hydroxyl attached to 3′ deoxyribose carbon

  31. KEY CONCEPTS • The DNA molecule consists of two strands that wrap around each other to form a double helix • The order of its building blocks stores genetic information

  32. Animation: DNA Close Up CLICKTO PLAY

  33. Learning Objective 5 • What are the base-pairing rules for DNA? • How do complementary bases bind to each other?

  34. Base Pairs • Hydrogen bonding • between specific base pairs • binds two chains of helix • Adenine (A) with thymine (T) • forms two hydrogen bonds • Guanine (G) with cytosine (C) • forms three hydrogen bonds

  35. Base Pairs and Hydrogen Bonds

  36. Fig. 12-6a, p. 267

  37. Adenine Thymine Deoxyribose Deoxyribose Guanine Cytosine Deoxyribose Deoxyribose Fig. 12-6b, p. 267

  38. Chargaff’s Rules • Complementary base pairing • between A and T; G and C • therefore A = T; G = C • If base sequence of 1 strand is known • base sequence of other strand can be predicted

  39. KEY CONCEPTS • Nucleotide subunits pair, based on precise pairing rules: T pairs with A, and C pairs with G • Hydrogen bonding between base pairs holds two strands of DNA together

  40. Learning Objective 6 • What evidence from Meselson and Stahl’s experiment enabled scientists to differentiate between semiconservative replication of DNA and alternative models?

  41. Models of DNA Replication

  42. (a) Hypothesis 1: Semiconservative replication Parental DNA First generation Second generation Fig. 12-7a, p. 268

  43. (b) Hypothesis 2: Conservative replication Parental DNA First generation Second generation Fig. 12-7b, p. 268

  44. (c) Hypothesis 3: Dispersive replication Parental DNA First generation Second generation Fig. 12-7c, p. 268

  45. Meselson-StahlExperiment • E. coli • grown in medium containing heavy nitrogen (15N) • incorporated 15N into DNA • Transferred from 15N to 14N medium • after one or two generations, DNA density supported semiconservative replication

  46. Meselson-StahlExperiment

  47. Some cells continue to grow in 14N medium. Some cells are transferred to 14N (light) medium. Bacteria are grown in 15N (heavy) medium. All DNA is heavy. First generation Second generation High density Low density Cesium chloride (CsCl) DNA The greater concentration of CsCl at the bottom of the tube is due to sedimentation under centrifigal force. DNA is mixed with CsCl solution, placed in an ultracentrifuge, and centrifuged at very high speed for about 48 hours. 15N (heavy) DNA 14N (light) DNA 14N – 15N hybrid DNA DNA molecules move to positions where their density equals that of the CsCl solution. Fig. 12-8a, p. 269

  48. 14N (light) DNA 14N – 15N hybrid DNA 14N – 15N hybrid DNA 15N (heavy) DNA Before transfer to 14N One cell generation after transfer to 14N Two cell generations after transfer to 14N The location of DNA molecules within the centrifuge tube can be determined by UV optics. DNA solutions absorb strongly at 260 nm. Fig. 12-8b, p. 269

  49. Semiconservative Replication • Each daughter double helix consists of • 1 original strand from parent molecule • 1 new complementary strand

  50. Learning Objective 7 • How does DNA replicate? • What are some unique features of the process?