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DNA: The genetic material

DNA: The genetic material. Mendel did not know about DNA or chromosomes!. Experiments that proved DNA is heritable material 1928: transformation (Griffiths). Avery, MacLeod, and McCarty:DNA is the transforming molecule (1944). Used same Streptococcus strains as Griffiths

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DNA: The genetic material

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  1. DNA: The genetic material Mendel did not know about DNA or chromosomes!

  2. Experiments that proved DNA is heritable material1928: transformation (Griffiths)

  3. Avery, MacLeod, and McCarty:DNA is the transforming molecule (1944) • Used same Streptococcus strains as Griffiths • Purified protein did not transform • Neither did lipids or carbohydrates • Proteases did not affect transformation • “Transforming element” had same density as DNA • DNAse destroyed transforming capacity

  4. Hershey and Chase used bacteriophage to identify genetic material: 1952

  5. What is DNA made of?

  6. Information about DNA was accumulating… (early 1950s) • Chargaff’s rules: A=T and G=C • Bases can have more than one form (keto and amino) • Franklin and Wilkins: DNA has a very regular structure • Franklin: “phosphophodiester backbone” is on the outside of the molecule

  7. Watson and Crick deduced the structure of DNA • Two strands held together by hydrogen bonds • A paired with T, and C paired with G • Strands are complementary and antiparallel

  8. The double helix and base pairing

  9. DNA replication is semiconservative: the Meselson-Stahl experiment, 1958

  10. Introduction to DNA replication:enzymes, a primer, and energy- in the 5’3’ direction

  11. DNA replication involves many enzymes • Necessary functions: • Unwind the double helix • Start replication with a primer • Relieve torque in the DNA molecule • Add the nucleotides to the growing strand • Fill the gaps • Join pieces of DNA together (“proofread”) • Correct errors

  12. E. Coli as a model for replication • Bacterial genome is circular • One origin of replication • Replication proceeds in both directions • Bacterial genome has one replicon; a eukaryotic genome would have many

  13. Three or more DNA polymerases in E. coli • Polymerase I discovered first, but Pol III does most of the synthesis • Pol I acts on lagging strand to remove primers • Pol II repairs • All have 3’5’ exonuclease activity

  14. Leading strand and lagging strand

  15. Continuous and discontinuous replication

  16. Model of the DNA replication complex

  17. Prooofreading and repair mechanisms are built into enzymes • Radiation, oxygen products, mutagens • Cells have multiple methods to repair DNA • Nucleotide addition happens very rapidly (50/sec in prokaryotes, 500/sec in eukaryotes • Polymerases have proofreading capacity

  18. Eukaryotic replication (on linear chromosomes) • Similarities to prokaryotic DNA replication • Helicase, primase, polymerase • Polymerase has several subunits • Replicating the ends requires additional activity

  19. Chromosome gradually gets shorter

  20. Action of telomerase • Forms “ends” of short repeated sequences (like aglet) • Enzyme uses RNA template • Telomerase is usually not active in most adult cells

  21. How is DNA organized?

  22. Organization, continued (it’s complex!)

  23. Summary • DNA is designed for stability • Double helix of antiparallel strands • Semiconservative replication • Multiple repair mechanisms • DNA is the library: how is it used?

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