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DNA replication

This chapter explores the history of DNA, including the experiments conducted by Griffith, Hershey-Chase, and Franklin. It also delves into the process of DNA replication, highlighting the Meselson-Stahl experiment and the role of various enzymes involved in replication.

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DNA replication

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  1. DNA replication Chapter 16

  2. Figure 16.1

  3. History of DNA • Griffith • Mice & Strep • Transformation • External DNA taken in by cell

  4. Experiment Living R cells(nonpathogeniccontrol) Mixture of heat-killed S cells andliving R cells Living S cells(pathogeniccontrol) Heat-killed S cells(nonpathogeniccontrol) Results Mouse dies Mouse dies Mouse healthy Mouse healthy Living S cells

  5. History of DNA • Hershey-Chase • Bacteriophages • Supported heredity information was DNA

  6. Bacteriophages

  7. Figure 16.4 4 2 1 3 4 Experiment Batch 1: Radioactive sulfur (35S) in phage protein Labeled phagesinfect cells. Agitation frees outsidephage parts from cells. Centrifuged cellsform a pellet. Radioactivity(phage protein)found in liquid Radioactiveprotein Centrifuge Pellet Batch 2: Radioactive phosphorus (32P) in phage DNA RadioactiveDNA Centrifuge Radioactivity (phageDNA) found in pellet Pellet

  8. History of DNA • Franklin • X-ray diffraction • Double helix • Watson-Crick • Double helix model

  9. History of DNA Duplication • Meselson and Stahl • Bacteria • 14N and 15N • Semiconservative method.

  10. First replication Second replication Fig. 16-10 Parent cell (a) Conservative model (b) Semiconserva- tive model (c) Dispersive model

  11. Figure 16.11 Experiment 2 1 4 3 Bacteria cultured in medium with 15N(heavy isotope) Bacteria transferred to medium with 14N(lighter isotope) Results DNA samplecentrifugedafter firstreplication DNA samplecentrifugedafter secondreplication Less dense More dense Conclusion Predictions: Second replication First replication Conservativemodel Semiconservativemodel Dispersivemodel

  12. DNA structure

  13. Nucleic acid structure • DNA deoxyribonucleic acid • RNA ribonucleic acid • Nucleotides

  14. Nucleotide structure • 1. 5 carbon sugar (ribose) • 2. Phosphate • 3. Nitrogenous base

  15. Nucleotide structure

  16. Nitrogenous base • Purines (2 rings) • Adenine(A) & Guanine(G) • Pyrimidines (1 ring) • Cytosine (C), Thymine (T) DNA only • Uracil (U) RNA only

  17. Phosphodiester bondLinks 2 sugars (nucleotides)

  18. Nucleic acids • 5’ Phosphate group (5’C) at one end • 3’ Hydroxyl group (3’C) at the other end • Sequence of bases is expressed in the 5’ to 3’ direction • GTCCAT 5’pGpTpCpCpApT---OH 3’

  19. Double helix • Complementary • Sequence on one chain of DNA • Determines sequence of other chain • 5’-ATTGCAT-3’ • 3’-TAACGTA-5’

  20. Double Helix • Complementary • Purines pair with pyrimidines • Diameter of base pairs are the same • Adenine (A) forms 2 hydrogen bonds with Thymine (T) • Guanine (G) forms 3 hydrogen bonds with cytosine (C)

  21. Double Helix • Sugar-phosphates are the backbone • Complementary • Phosphodiester bonds • Strands are antiparrellel • Bases extend into interior of helix • Base-pairs form to join the two strands

  22. Fig. 16-7 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm 5 end (a) Key features of DNA structure (b) Partial chemical structure

  23. Duplication • DNA unzips-breaks hydrogen bonds • New strand forms based on existing strand • Old strand is saved • Compliment of new strand • New DNA-one old strand & one new strand • Semiconservative replication

  24. Fig. 16-9-3 A T A T A T A T C G C G C G C G A T A T A A T T T A T A T T A A C C G C G C G G (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand (b) Separation of strands (a) Parent molecule

  25. Duplication Enzymes • DNA helicase: • Enzyme opens helix starts duplication • Separates parental strands • Single-strand binding protein: • Binds to unpaired DNA • After separation • Stabilizes DNA

  26. Duplication Enzymes • DNA polymerases: • Help lengthen new strand of DNA • Adds new nucleotides strand • Synthesis occurs only one direction • 5’ to 3’ • Adding new nucleotides to the 3’OH

  27. Duplication Enzymes • Primer: • Section of RNA • Complementary to the parental DNA • Synthesis occurs only one direction • 5’ to 3’ • DNA primase: • Enzyme creates the primer

  28. Duplication Enzymes • Topoisomerase: • Relieves strain of unwinding DNA • DNA pol1: • Removes primers • Replaces with DNA nucleotides • DNA ligase: • Creates phosphodiester bonds between Okazaki fragments

  29. Table 16.1

  30. Duplication • OriC • Origins of replication • Starting point in DNA synthesis • Replication is bidirectional • Proceeds in both directions from origin • 5’to 3’direction

  31. Duplication • E coli (bacteria) • Circular DNA • One origin • Eurkaryotes • Multiple origins

  32. Origins of Replication

  33. Duplication • Replication bubble: • Separation of strands of DNA • Replication of DNA • Replication fork: • Y-shaped region • End of replication bubble • Site of active replication

  34. Duplication

  35. Duplication

  36. DNA Replication Overview

  37. Duplication • Leading strand: • DNA continuous 5’ to 3’ replication (towards fork) • Template is 3’ to 5’ • Lagging strand: • DNA duplicated in short segments (away from fork) • Okazaki fragments: • Short stretches of new DNA-lagging side

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