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DNA Structure and Function

DNA Structure and Function. Chapter 13. Miescher Discovered DNA. 1868 Johann Miescher investigated the chemical composition of the nucleus Isolated an organic acid that was high in phosphorus He called it nuclein We call it DNA (deoxyribonucleic acid). Mystery of the Hereditary Material.

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DNA Structure and Function

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  1. DNA Structure and Function Chapter 13

  2. Miescher Discovered DNA • 1868 • Johann Miescher investigated the chemical composition of the nucleus • Isolated an organic acid that was high in phosphorus • He called it nuclein • We call it DNA (deoxyribonucleic acid)

  3. Mystery of the Hereditary Material • Originally believed to be an unknown class of proteins • Thinking was • Heritable traits are diverse • Molecules encoding traits must be diverse • Proteins are made of 20 amino acids and are structurally diverse

  4. Structure of the Hereditary Material • Experiments in the 1950s showed that DNA is the hereditary material • Scientists raced to determine the structure of DNA • 1953 - Watson and Crick proposed that DNA is a double helix Figure 13.2Page 217

  5. Griffith Discovers Transformation • 1928 • Attempting to develop a vaccine • Isolated two strains of Streptococcus pneumoniae • Rough strain was harmless • Smooth strain was pathogenic

  6. Griffith Discovers Transformation 1. Mice injected with live cells of harmless strain R. 2. Mice injected with live cells of killer strain S. 3. Mice injected with heat-killed S cells. 4. Mice injected with live R cells plus heat-killed S cells. Mice live. No live R cells in their blood. Mice die. Live S cells in their blood. Mice live. No live S cells in their blood. Mice die. Live S cells in their blood. Figure 13.3Page 218

  7. Transformation • What happened in the fourth experiment? • The harmless R cells had been transformed by material from the dead S cells • Descendents of the transformed cells were also pathogenic

  8. Oswald & Avery • What is the transforming material? • Cell extracts treated with protein-digesting enzymes could still transform bacteria • Cell extracts treated with DNA-digesting enzymes lost their transforming ability • Concluded that DNA, not protein, transforms bacteria

  9. Bacteriophages • Viruses that infect bacteria • Consist of protein and DNA • Inject their hereditary material into bacteria bacterial cell wall plasma membrane cytoplasm Figure 13.4bPage 219

  10. Hershey & Chase’s Experiments • Created labeled bacteriophages • Radioactive sulfur • Radioactive phosphorus • Allowed labeled viruses to infect bacteria • Asked: Where are the radioactive labels after infection?

  11. virus particle labeled with 35S virus particle labeled with 32P Hershey and Chase Results bacterial cell (cutaway view) label outside cell label inside cell Figure 13.5Page 219

  12. Information • Mon., 28 November • Chapter 14 and 16 highlights • Wed., 30 November • Final exam review – BRING YOUR QUESTIONS! • Instructor evaluations • Mon., 12 December, 2:15-4:15pm in C317 • Final Exam • kpeterson@oeb.harvard.edu • Exam 3 will be returned at the end of this class.

  13. Structure of Nucleotides in DNA • Each nucleotide consists of • Deoxyribose (5-carbon sugar) • Phosphate group • A nitrogen-containing base • Four bases • Adenine, Guanine, Thymine, Cytosine

  14. Nucleotide Bases ADENINE (A) GUANINE (G) phosphate group deoxyribose THYMINE (T) CYTOSINE (C) Figure 13.6Page 220

  15. Composition of DNA • Chargaff showed: • Amount of A relative to G differs among species • Always: A=T and G=C

  16. Rosalind Franklin’s Work • Expert in X-ray crystallography • Examined DNA fibers • Concluded that DNA was some sort of helix

  17. Watson-Crick Modelof DNA • 2 nucleotide strands • Run in opposite directions • Held together by H bonds between bases • A binds with T and C with G • Molecule is a double helix

  18. DNA Structure Helps Explain How It Duplicates • DNA is 2 nucleotide strands held together by H bonds • H bonds between 2 strands are easily broken • Each single strand then serves as template for new strand

  19. DNA Replication • Each parent strand remains intact • Every DNA molecule is half “old” and half “new” new old old new Figure 13.9Page 222

  20. Base Pairing during Replication Each old strand serves as the template for complementary new strand Figure 13.10Page 223

  21. Enzymes in Replication • Enzymes unwind the two strands • DNA polymerase attaches complementary nucleotides • DNA ligase fills in gaps • Enzymes wind two strands together

  22. A Closer Look at Strand Assembly Energy for strand assembly is provided by removal of two phosphate groups from free nucleotides newly forming DNA strand one parent DNA strand Figure 13.10Page 223

  23. Continuous and Discontinuous Assembly Strands can only be assembled in the 5’ to 3’ direction Figure 13.10Page 223

  24. DNA Repair • Mistakes can occur during replication • DNA polymerase can read correct sequence from complementary strand and, together with DNA ligase, can repair mistakes in incorrect strand

  25. Information • Mon., 28 November • Chapter 14 and 16 highlights • Wed., 30 November • Final exam review – BRING YOUR QUESTIONS! • Instructor evaluation • Mon., 12 December, 2:15-4:15pm in C317 • Final Exam • kpeterson@oeb.harvard.edu • Exam 3 will be returned ... Now!

  26. Exams

  27. Grades • A = 100-79 • B = 78-70 • C = 69-61 • D = 60-53 • Average = 63 • High = 99

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