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DNA Roles of DNA Timeline of DNA Discovery and Structure DNA Structure DNA Replication Roles of DNA • Store Information DNA is STABLE!! Chemically & physically Stays in nucleus Roles of DNA • Copy Information Two complementary strands for replication and crosscheck for error Roles of DNA • Transmit Information Unique base sequences code for all the proteins that make up an organism These codes are passed from generation to generation “Pre-history” of DNA as heredity unit 1866 Mendel - heredity “factors” are segregated, have dominance and are independently sorted; used pea plants 1905 Bateson and Punnett – some “factors” are linked; used pea plants 1910 Morgan – chromosome theory, linkage maps; used fruit flies General thought: PROTEINS must be the heredity factor! DNA is just a structural molecule for the proteins. WHY was this thought? Timeline of DNA Discovery 1928 Griffith – “transforming principle” ; used bacteria and mice 1944 Avery, Macleod, McCarty – proposed DNA is ‘transforming principle’; used bacteria and enzymes 1950 Chargaff – rules of DNA base ratios; used paper chromatography and spectrophotometry 1952 Hershey and Chase – confirmed DNA as molecule of heredity; used isotopes and bacteriophage YAY for DNA! Griffith’s Experiment Avery, MacLeod, McCarty Chargaff’s Results Hershey-Chase Experiment Hershey-Chase Experiment Why it’s called the “Blender Experiment” Timeline of DNA Structure and Replication 1953 Wilkins and Franklin – image of DNA crystals; used X-ray crystallography 1953 Watson and Crick – the famous double helix model; used models 1955 Kornberg – purified DNA polymerase; used bacterial extracts 1958 Meselson - Stahl – semi-conservative replication; used isotopes and density gradient centrifugation Wilkins and Franklin Watson and Crick The Double Helix Model “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson and Crick (1953) Kornberg Proposed Models for DNA Replication • Semi-conservative - One “old”, one “new” strand • Conservative – “Old” double DNA directs a “new” DNA • Dispersive – “Old” and “new” get interspersed Meselson – Stahl Experiment Meselson – Stahl Results DNA Structure • Building Block – Nucleotide Phosphate-Deoxyribose-Nitrogenous Base • Sugar-Phosphate Backbone – Anti-Parallel • Base Pairing: A-T G-C via Hydrogen Bonds DNA Structure Basic Building Block - Nucleotide Nitrogenous Bases – A T G C Sugar-Phosphate Backbone • Covalent bonds Anti-Parallel Strands Double Helix A – T and G – C • Hydrogen Bonds DNA Replication During the S phase of the cell cycle, DNA replicates. DNA has to unwind to create a replication fork Enzyme: Helicase DNA Replication DNA Replication DNA Polymerase adds bases 5’ 3’ ONLY This results in two types of replication: Leading Strand: Add nucleotides 5’ 3’ sequentially in direction of replication fork; continuous Lagging Strand: Add nucleotides 5’ 3’ in sections in opposite direction which get connected later; discontinuous DNA Replication Prokaryote vs Eukaryote Prokaryote Eukaryote Location Cytoplasm Nucleus Chromosome 1 Circular 1 Many – non-circular Many DNA Polymerases DNA Polymerases AT G C Complementary Anti-Parallel AT G C Complementary Anti-Parallel Origin Enzymes DNA DNA Replication - Prokaryote DNA Replication - Eukaryote On to Transcription and Translation ….