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Question set 6.docx

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University of Ottawa
Doug Johnson

 Transformed bacteria were altered Question Set #6 permanently; smooth, infective trait was stably inherited by descendants of transformed bacteria 12.1 Establishing DNA as the Hereditary   Conversion of R to S  Molecule th transformation • First half of the 20 century, many scientists believed that proteins were the most likely  Agent responsible  transforming candidate for the hereditary molecules since principle they appeared to offer greater opportunities for information than nucleic acids • Proteins and nucleic acids are built of various combinations of different amino • Proteins contain 20 types of amino acids acids and nucleotides (gives then a complexity of structure that makes them • Frederick Griffith (1928) used 2 strains of likely candidates for carrying information bacterium needed for transformation)  Smooth strain, S, has • Oswald Avery (1940s) performed polysaccharide capsule surrounding experiment designed to identify chemical each cell and forms colonies that nature of transforming principle that can appear smooth and glossy when change R to S grown on a culture plate  Attempted to reproduce  The S was injected into mice transformation using bacteria causing pneumonia and it killed the growing in culture tubes mice in a day or 2  Used heat to kill virulent S and then  Rough strain, R, does not have the treated macromolecules extracted polysaccharide capsule and forms from cells with enzymes that break colonies with a nonshiny, rough down each of 3 main candidate appearance molecules (protein, DNA and other nucleic acids, RNA)  R was injected and the mice lived  When DNA was destroyed  no  Capsule was responsible for the transformation occurred virulence (risk of infection) of the S strain  1944, transforming principle was DNA; believed that the genetic  If S bacteria was killed by heating before injecting them into the mice material was protein (remained healthy)  Unlikely to be DNA, since it has 4 different components  But if he injected living R bacteria along with the heat-killed S, many of  Protein with 20 different components the mice died seemed a far superior medium for coding information  In some way, living R bacteria had acquired the ability to make the • Alfred D. Hershey (1952) removed any polysaccharide capsule from the remaining doubts that DNA is the hereditary dead S bacteria  changed to S cells molecule  Studied infection of E. coli by  Labelled proteins no radioactivity in bacteriophage T2 (viruses that infect bacterial cells but could not easily bacteria) measure it in material removed by mixing (also found no radioactivity in  Phage replication cycle begins when progeny phages) a phage attaches to surface of the bacteria; T2: infected cell quickly  Labelled DNA  radioactivity inside producing its own molecules and infected bacteria but none in phage instead starts making progeny coats removed by mixing phages (whole cycle takes 90 mins) (radioactivity was seen in progeny phages)  T2 studied consists of only a core of DNA surrounded by proteins; hence
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