Lecture 14 - Molecular Genetics 10-24

Lecture 14 - Molecular Genetics 10-24 - Molecular basis of...

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Molecular basis of heredity Principles of Biology Lecture 14 Prof. David Fitch
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Information Complex alphabets convey more information in less space than do simple alphabets If genome size is constrained, then proteins (21 "letters") would be a better receptacle for genomic information than DNA (only 4 "letters") QuickTime™ and a TIF  (Uncompres ed) decompres or are ne ded to se  this picture. QuickTime™ and a TIF  (Uncompres ed) decompres or are ne ded to see this picture. QuickTime™ and a TIF  (Uncompres ed) decompres or are ne ded to se  this picture. David Fitch Linus Pauling, 1954
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Bacterial "transformation" Frederick Griffith (1928) Streptococcus pneumoniae R strain is "transformed" by killed S strain Living S (control) cells Living R (control) cells Heat-killed (control) S cells Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells are found in blood sample. Frederick Griffith in 1936
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Design an experiment! What is the transforming agent? Hypothesis 1: the polysaccharide coat of the S strain Hypothesis 2: some protein of the S strain Hypothesis 3: RNA Hypothesis 4: DNA Hypothesis 5: something else (none of the above) Design a series of experiments to discriminate between these hypotheses. What are the predictions that each hypothesis makes about the results expected from the experiment?
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DNA is the transforming agent Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944) (Rockefeller Institute, NYC) Polysaccharide coat did not transform R cells Only DNA-containing fraction transformed R cells Trypsin or RNase did not abolish transformation Only DNase destroyed this agent Thus, DNA was probably the transforming agent Streptococcus pneumoniae
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DNA responsible for phage heredity Hershey & Chase (1952) (Cold Spring Harbor Laboratory, NY) Which component of T2 bacteriophage is required for T2 reproduction? T2 is made of protein + DNA Label protein with 35 S (one batch of T2 grown in E. coli cells with 35 S) Label DNA with 32 P (another batch) Infect cells, remove phages attached to cells, pellet cells in centrifuge Which fraction has the label—pellet or supernatent? Phage head Tail Tail fiber DNA Bacterial cell 100 nm
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Hershey-Chase experiment Radioactivity (phage protein) in liquid Phage Bacterial cell Radioactive protein Empty protein shell Phage DNA DNA Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Centrifuge Pellet Radioactivity (phage DNA) in pellet Batch 1: Phages were grown with radioactive sulfur ( 35 S ), which was incorporated into phage protein (pink). Batch 2:
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Lecture 14 - Molecular Genetics 10-24 - Molecular basis of...

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