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Lecture 13

Lecture 13th - BIOA01H3.docx

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Biological Sciences
Mark Fitzpatrick

thBIOA01H3Lecture 13 stWeek of October 1DNA the stuff of heredity Chapter 12 121 Establishing DNA as the Hereditary Moleculeth In first half of 20 century many scientists believes that proteins were hereditary molecules bc offered greater opportunities for info coding than nucleic acids20 types of amino acids nucleic acids4 different nitrogen basesOther scientists believed DNA was hereditary molecule121a Experiments begin when Griffith found substance that could genetically transform pneumonia bacteriaUsed bacterium streptococcus penumoniaecauses severe pneumonia in mammals Used 2 strains smooth strain S and rough strain R S strain has polysaccharide capsule whereas R strain doesntS strain virulent highly infective or pathogenic and R strain is avirulent bc s strain has polysaccharide capsule it inhibits hosts immune systemcan therefore multiplycause fatal pneumoniacapsule responsible for S strains virulence 1 Experiment 1 Mice injected w live S cells result mice die Live S cells in blood 2 Experiment 2 Mice injected w live R cells result mice live No live R cells in blood 3 Experiment 3 Mice injected w heatkilled S cells result mice live No live S cells in blood req live S cells to be virulent 4 Experiment 4 Mice injected w heatkilled S cells along w R cells result Mice die Live S cells in blood living R cells can convert to virulent S cellsConclusion some molecules released when S cells killed could change living nonvirulent R cells genetically to virulent S form Called molecule transforming principle and process of genetic change transformation 121b Avery and his coworkers identified DNA as the molecules that transforms avirulent rough streptococcus to the virulent smooth formAttempted to reproduce transformation using bacteria growing in culture tubesUsed heat to kill virulent S bacteria then treated macromolecules extracted from cells w enzymes to break down 3 main candidates for hereditary moleculesprotein DNA and RNAWhen RNA destroyed no effect extra S bacteria still transformed R bacteria into virulent S bacteriaWhen DNA destroyed no transformation occurred Scientists still wary that DNA could ever be complex enough to hold genetic info121c Hershey and Chase found the final evidence establishing DNA as the hereditary molecule Studied infection of bacterium Escherichia coli by bacteriophage T2E coli normally found in intestines of mammals Bacteriophages are viruses that infect bacteria Virus is infectious agent that contains either DNA or RNA surrounded by a protein coat Cannot reproduce without host cellPhage replication cycle begins when phage attaches to surface of bacterium T2 phage causes infected cell to quickly stop producing own moleculesinstead progeny phages Viral enzyme breaks down cell wall killing cell and releasing new phagesT2 phage consists only of core DNA surrounded by proteins therefore one of these molecules must be genetic material that enters bacterial cell and directs infective cycle within 1 Infected E coli growing in presence of radioactive 32P or 35S w phage T2 Progeny phages labeled in protein w 35S or DNA with 32P 2 Separate cultures of E coli infected w radioactively labeled phages 3 Bacteria mixed in blender shearing phage coats from cell surface that did not enter bacteria Components then analyzed for radioactivity4 Results No radioactivity within cell 35S in phage coat32P within cell not in phage coat 32P in progeny phages1
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