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Chapter 14.

Chapter 14. DNA The Genetic Material Replication. Nature of DNA. Stores complete instructions for making proteins Passes instructions on to new cells Copies itself flawlessly Can put instructions into action Sometimes changes. Scientific History.

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Chapter 14.

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  1. Chapter 14. DNA The Genetic Material Replication

  2. Nature of DNA • Stores complete instructions for making proteins • Passes instructions on to new cells • Copies itself flawlessly • Can put instructions into action • Sometimes changes

  3. Scientific History • The march to understanding that DNA is the genetic material • T.H. Morgan (1908) • Frederick Griffith (1928) • Avery, McCarty & MacLeod (1944) • Hershey & Chase (1952) • Watson & Crick (1953) • Meselson & Stahl (1958)

  4. What’s so impressiveabout proteins?! 1908 | 1933 Genes are on chromosomes • T.H. Morgan • working with Drosophila (fruit flies) • genes are on chromosomes • but is it the protein or the DNA of the chromosomes that are the genes? • through 1940, proteins were thought to be genetic material… Why?

  5. 1928 The “Transforming Factor” • Frederick Griffith • Streptococcus pneumonia bacteria • was working to find cure for pneumonia • harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice • substance passed from dead bacteria to live bacteria = “Transforming Factor”

  6. The “Transforming Factor” mix heat-killed pathogenic & non-pathogenic bacteria live pathogenic strain of bacteria live non-pathogenic strain of bacteria heat-killed pathogenicbacteria A. B. D. C. mice die mice live mice live mice die Transformation? something in heat-killed bacteria could still transmit disease-causing properties

  7. 1944 DNA is the “Transforming Factor” • Avery, McCarty & MacLeod • purified both DNA & proteins from Streptococcus pneumonia bacteria • which will transform non-pathogenic bacteria? • injected protein into bacteria • no effect • injected DNA into bacteria • transformed harmless bacteria into virulent bacteria What’s the conclusion?

  8. Avery, McCarty & MacLeod Oswald Avery Colin MacLeod Maclyn McCarty

  9. Why useSulfurvs.Phosphorus? 1952 | 1969 Confirmation of DNA • Hershey & Chase • classic “blender” experiment • worked with bacteriophage • viruses that infect bacteria • grew phage viruses in 2 media, radioactively labeled with either • 35S in their proteins • 32P in their DNA • infected bacteria with labeled phages

  10. Hershey & Chase Martha Chase Alfred Hershey

  11. Protein coat labeled with 35S DNA labeled with 32P Hershey & Chase T2 bacteriophages are labeled with radioactive isotopes S vs. P bacteriophages infect bacterial cells bacterial cells are agitated to remove viral protein coats Which radioactive marker is found inside the cell? Which molecule carries viral genetic info? 32P radioactivity foundin the bacterial cells 35S radioactivity found in the medium

  12. Taaa-Daaa! Blender experiment • Radioactive phage & bacteria in blender • 35S phage • radioactive proteins stayed in supernatant • therefore protein did NOT enter bacteria • 32P phage • radioactive DNA stayed in pellet • therefore DNA did enter bacteria • Confirmed DNA is “transforming factor”

  13. What do you notice?! 1947 Chargaff • DNA composition: “Chargaff’s rules” • varies from species to species • all 4 bases not in equal quantity • bases present in characteristic ratio • humans: A = 30.9% T = 29.4% G = 19.9% C = 19.8%

  14. 1953 | 1962 Structure of DNA • Watson & Crick • developed double helix model of DNA • other scientists working on question: • Rosalind Franklin • Maurice Wilkins • Linus Pauling Franklin Wilkins Pauling

  15. Three-Dimensional Structure of DNA • Wilkins/Franklin X-ray diffraction suggested DNA had helical shape with a diameter of about 2 nanometers.

  16. Rosalind Franklin (1920-1958)

  17. Photo 51 http://www.pbs.org/wgbh/nova/body/DNA-photograph.html

  18. 1953 article in Nature Watson and Crick

  19. Chemical Nature of Nucleic Acids • DNA made up of nucleotides. • Five carbon sugar, phosphate group, and an organic base. • Purines - Large bases • Adenine (A) and Guanine (G) • Pyrimidines - Small bases • Cytosine (C) and Thymine (T)

  20. Double helix structure of DNA the structure of DNA suggested a mechanism for how DNA is copied by the cell

  21. This will beIMPORTANT!! Directionality of DNA • You need to number the carbons! • it matters! nucleotide PO4 N base 5 CH2 O 1 4 ribose 3 2 OH

  22. I mean it…This will beIMPORTANT!! 5 The DNA backbone PO4 • Putting the DNA backbone together • refer to the 3 and 5 ends of the DNA • the last trailing carbon base CH2 O C O P –O O O base CH2 O OH 3

  23. 3’ end of molecule has –OH group attached to the #3 carbon on the deoxyribose sugar. • 5’ end of molecule has phosphate group attached to the #5 carbon on the deoxyribose sugar.

  24. hydrogen bonds Bonding in DNA 5’ 3’ phosphodiester bonds 3’ 5’ ….strong or weak bonds? How do the bonds fit the mechanism for copying DNA?

  25. Base pairing in DNA • Purines • adenine (A) • guanine (G) • Pyrimidines • thymine (T) • cytosine (C) • Pairing • A : T • C : G

  26. Anti-parallel strands • Phosphate to sugar bond involves carbons in 3 & 5 positions • DNA molecule has “direction” • complementary strand runs in opposite direction “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick

  27. Copying DNA • Replication of DNA • base pairing allows each strand to serve as a pattern for a new strand

  28. verify throughexperiments… Models of DNA Replication • Alternative models • so how is DNA copied?

  29. make predictions… 1958 Semi-conservative replication? • Meselson & Stahl • label nucleotides of “parent” DNA strands with heavy nitrogen =15N • label new nucleotides with lighter isotope = 14N “The Most Beautiful Experiment in Biology” parent replication

  30. 1958 Semi-conservative replication • Make predictions… • 15N strands replicated in 14N medium • 1st round of replication? • 2nd round?

  31. let’s meetthe team… DNA Replication • Large team of enzymes coordinates replication

  32. Replication: 1st step • Unwind DNA • helicase enzyme • unwinds part of DNA helix • stabilized by single-stranded binding proteins single-stranded binding proteins

  33. But… We’re missing something! What? Where’s theENERGYfor the bonding! Replication: 2nd step • Bring in new nucleotides to match up to template strands • DNA Polymerase III bonds them in place

  34. YourememberATP! Is that theonly energymolecule? Energy of Replication • Where does the energy for the bonding come from? energy ATP GTP TTP CTP ADP AMP GMP TMP CMP

  35. Energy of Replication • The nucleotides arrive as nucleosides • DNA organic nitrogen bases coupled with P–P–P • DNA bases arrive with their own energy source for bonding • bonded by DNA polymerase III ATP GTP TTP CTP

  36. B.Y.O. ENERGY 5' 3' Replication energy DNA P III • Adding bases • can only add nucleotides to 3 end of a growing DNA strand • strand grow 5'3’ energy energy energy 3' 5' leading strand

  37. 5' 3' 5' 3' ligase energy 3' 3' leading strand 5' lagging strand 5'

  38. Leading & Lagging strands Leading strand - continuous synthesis Okazaki Lagging strand - Okazaki fragments - joined by ligase - “spot welder” enzyme

  39. Okazaki fragments

  40. Priming DNA synthesis • DNA polymerase III can only extend an existing DNA molecule • cannot start new one • cannot place first base • short RNA primer is built first by primase • starter sequences • DNA polymerase III can now add nucleotides to RNA primer

  41. Cleaning up primers DNA polymerase I removes sections of RNA primer and replaces with DNA nucleotides

  42. Replication fork DNA polymerase III lagging strand DNA polymerase I 3’ Okazaki fragments primase 5’ 5’ ligase SSB 3’ 5’ 3’ helicase DNA polymerase III 5’ leading strand 3’ direction of replication

  43. Recap… • Opening DNA Double Helix • Initiating replication • Unwinding duplex • Building a Primer • Assembling Complementary Strands • Removing the Primer • Joining Okazaki Fragments

  44. And in the end… • Ends of chromosomes are eroded with each replication • an issue in aging? • ends of chromosomes are protected by telomeres

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