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Chapter 16 The Molecular Basis of Inheritance

Chapter 16 The Molecular Basis of Inheritance. Question?. Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? Two possibilities: Protein DNA. Qualifications. Protein: very complex. high specificity of function. DNA: simple.

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Chapter 16 The Molecular Basis of Inheritance

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  1. Chapter 16 The Molecular Basis of Inheritance

  2. Question? • Traits are inherited on chromosomes, but what in the chromosomes is the genetic material? • Two possibilities: • Protein • DNA

  3. Qualifications • Protein: • very complex. • high specificity of function. • DNA: • simple. • not much known about it (early 1900’s).

  4. For testing: • Name(s) of experimenters • Outline of the experiment • Result of the experiment and the importance of the result

  5. Griffith - 1928 • Pneumonia in mice. • Two strains: • S - pathogenic • R - harmless

  6. Griffith’s Experiment

  7. Result • Something turned the R cells into S cells. • Transformation - the assimilation of external genetic material by a cell.

  8. Problem • Griffith used heat. • Heat denatures proteins. • So could proteins be the genetic material? • DNA - heat stable. • Griffith’s results contrary to accepted views.

  9. Avery, McCarty and MacLeod - 1944 • Repeated Griffith’s experiments, but added specific fractions of S cells. • Result - only DNA transformed R cells into S cells. • Result - not believed.

  10. Hershey & Chase -1952 • Genetic information of a virus or phage. • Phage - virus that attacks bacteria and reprograms host to produce more viruses.

  11. Bacteria with Phages

  12. Phage Components • Two main chemicals: • Protein • DNA • Which material is transferred to the host?

  13. Used Tracers • Protein - CHONS, can trace with 35S. • DNA - CHONP, can trace with 32P.

  14. Experiment • Used phages labeled with one tracer or the other and looked to see which tracer entered the bacteria cells.

  15. Result • DNA enters the host cell, but the protein did not. • Therefore: DNA is the genetic material.

  16. Picture Proof

  17. Chargaff - 1947 • Studied the chemical composition of DNA. • Found that the nucleotides were in certain ratios.

  18. Chargaff’s Rule • A = T • G = C • Example: in humans, A = 30.9% T = 29.4% G = 19.9% C = 19.8%

  19. Why? • Not known until Watson and Crick worked out the structure of DNA.

  20. Watson and Crick - 1953 • Used X-ray crystallography data (from Rosalind Franklin) • Used model building. • Result - Double Helix Model of DNA structure. • (One page paper, 1953).

  21. Rosalind Franklin

  22. Book & Movies • “The Double Helix” by James Watson- His account of the discovery of the shape of DNA • Movie – The Double Helix

  23. DNA Composition • Deoxyribose Sugar (5-C) • Phosphate • Nitrogen Bases: • Purines • Pyrimidines

  24. DNA Backbone • Polymer of sugar-phosphate. • 2 backbones present.

  25. Nitrogen Bases • Bridge the backbones together. • Purine + Pyrimidine = 3 rings. • Constant distance between the 2 backbones. • Held together by H-bonds.

  26. Chargaff’s Rule • Explained by double helix model. • A = T, 3 ring distance. • G = C, 3 ring distance.

  27. Watson and Crick • Published a second paper (1954) that speculated on the way DNA replicates. • Proof of replication given by others.

  28. Replication • The process of making more DNA from DNA. • Problem: when cells replicate, the genome must be copied exactly. • How is this done?

  29. Models for DNA Replication • Conservative - one old strand, one new strand. • Semiconservative - each strand is 1/2 old, 1/2 new. • Dispersive - strands are mixtures of old and new.

  30. Replication Models

  31. Meselson – Stahl, late 1950’s • Grew bacteria on two isotopes of N. • Started on 15N, switched to 14N. • Looked at weight of DNA after one, then 2 rounds of replication.

  32. Results • Confirmed the Semiconservative Model of DNA replication.

  33. Replication - Preview • DNA splits by breaking the H-bonds between the backbones. • Then DNA builds the missing backbone using the bases on the old backbone as a template.

  34. Origins of Replication • Specific sites on the DNA molecule that starts replication. • Recognized by a specific DNA base sequence.

  35. Prokaryotic • Circular DNA. • 1 origin site. • Replication runs in both directions from the origin site.

  36. Eukaryotic Cells • Many origin sites. • Replication bubbles fuse to form new DNA strands.

  37. DNA Elongation • By DNA Polymerases such as DNA pol III • Adds DNA triphosphate monomers to the growing replication strand. • Matches A to T and G to C.

  38. Energy for Replication • From the triphosphate monomers. • Loses two phosphates as each monomer is added.

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