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Deoxyribonucleic Acid The Molecular Basis of Inheritance Who are these guys? AP Biology Who were the scientists? How did their work contribute to the discovery or understanding of DNA? AP Biology Griffith’s Transformation Experiment - 1928 Bacteria could get traits from other bacteria “transforming” their traits. AP Biology Avery, McCarty and MacLeod 1944 Refined Griffith’s experiment Proved transforming agent was nucleic acid. AP Biology Hershey and Chase – “Blender Experiment “ 1952 Further proved that DNA, not protein, is the hereditary material Bacteriophages! Viruses that infect bacteria AP Biology Maurice Wilkins and Rosalind Franklin Were using a technique called X-ray crystallography to study molecular structure Rosalind Franklin Produced a picture of the DNA molecule using this technique Why was this an important discovery? AP Biology Figure 16.6 a, b (a) Rosalind Franklin (b) Franklin’s X-ray diffraction Photograph of DNA Watson and Crick – 1953 Discovered the structure of DNA Nobel prize in 1962 (with Wilkins) Deduced that DNA was a double helix AP Biology Through observations of the X-ray crystallographic images of DNA from Rosalind Franklin Erwin Chargaff – 1950’s All organisms have the same bases just in different amounts. In any DNA: Base pairing is highly conserved through evolution AP Biology What is DNA? Primary source of genetic information RNA can be used in some cases Eukaryotic cells – multiple, linear chromosomes in nucleus Mitochondria/chloroplast have circular chromosome Prokaryotic cells – 1 circular chromosomes in cytosol Plasmids = separate extra-piece of circular DNA (only a few genes) AP Biology DNA Structure Monomers = nucleotides Nucleotide structure: Phosphate Sugar (deoxyribose) Nitrogen base Adenine, guanine, thymine, cytosine AP Biology DNA Structure Antiparallel - Nucleotides on one side run 5’ to 3’ and the opposing side runs from 3’ to 5’ Complementary strands – nucleotides of one side are bonded with their complement not identical. 5 3 3 5 EX: Reading strand from 5’ to 3’ left top down is the same as right bottom up. AP Biology Bonding in DNA 5 hydrogen bonds 3 covalent bonds 3 5 ….strong or weak bonds? AP Biology How do the bonds fit the mechanism for copying DNA? Nitrogen Bases and Pairing in DNA – Purines adenine (A) guanine (G) Pyrimidines thymine (T) cytosine (C) Base Pairing A:T 2 Hydrogen bonds C:G 3 Hydrogen bonds AP Biology Base Pairing – Chargaff’s Rule Amount of T = Amount of A Amount of G = Amount of C Ex: Approximately how much thymine would be found in the wheat DNA? The amount of G’s and C’s is approx. 45% so, the amount of A’s and T’s should be close to 55%. Thus, 55-28 = 27% AP Biology Semi-Conservative Replication Replication of DNA Base pairing allows each side of original strand serves as a template for a new strand New strand is 1/2 parent template & 1/2 new DNA AP Biology Prokaryotic DNA Replication Replication moves in two directions. Always occurs 5’ to 3’ only. AP Biology Eukaryotic DNA Replication Multiple origin sites AP Biology DNA Replication: The Process DNA is unwound by Helicase enzyme Breaks the hydrogen bonds between the nitrogen bases of each strand Creates the replication forks SSB’s stabilize replication fork single-stranded binding proteins AP Biology replication fork Making New Strands Free, unbound nucleotides are in the nucleus RNA Primase puts on RNA Primer to start new strand Designates start region of replication DNA Polymerase III – enzyme that bonds new complementary nucleotides to old strand. makes new 5’ to 3’strands can only add nucleotides to 3’ end of parent strand Leading Strand: Follows replication fork (as helicase unwinds) One continuous strand Lagging Strand: Opposite direction of fork Lays down multiple strands (fragments) AP Biology Making Strands…. AP Biology Replication in a Bubble.. 3 5 5 3 DNA polymerase III leading strand 5 3 3 5 3 5 5 5 3 lagging strand 3 5 3 5 lagging strand 5 5 leading strand 3 growing replication fork leading strand 3 lagging strand 5 5 AP Biology growing replication fork 5 5 5 3 Lagging Strands = Okazaki fragments! DNA polymerase I replaces RNA primers with DNA nucleotides DNA ligase attaches the DNA pieces together AP Biology Replication fork Animation DNA polymerase 5’ 3’ DNA ligase lagging strand RNA primase Okazaki fragments 5’ 3’ 5’ SSB 3’ helicase DNA polymerase 5’ 3’ leading strand direction of replication AP Biology SSB = single-stranded binding proteins Editing & Proofreading DNA Specific Enzymes that can: Cuts and removes abnormal bases proofreads repairs mismatched Mutations are a permanent change in DNA AP Biology Telomeres Repeating, non-coding sequences at the end of chromosomes Shorten after each cell division shorter telomeres = aging cells Telomerase – doesn’t let telomeres shorten AP Biology works in stem cells and cancer cells