DNA & Genes - The Griffith Experiment • What is a...

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The Griffith Experiment What is a gene? The work of Sutton and Morgan established that genes reside on chromosomes But chromosomes contain proteins and DNA So which one is the hereditary material Several experiments ultimately revealed the nature of the genetic material
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The Griffith Experiment In 1928, Frederick Griffith discovered transformation while working on Streptococcus pneumoniae The bacterium exists in two strains S Forms smooth colonies in a culture dish Cells produce a polysaccharide coat and can cause disease R Forms rough colonies in a culture dish Cells do not produce a polysaccharide coat and are therefore harmless
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How Griffith discovered transformation Thus, the dead S bacteria somehow “transformed” the live R bacteria into live S bacteria
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The Avery and Hershey-Chase Experiments Two key experiments that demonstrated conclusively that DNA, and not protein, is the hereditary material Oswald Avery and his coworkers Colin MacLeod and Maclyn McCarty published their results in 1944 Alfred Hershey and Martha Chase published their results in 1952
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Avery and his colleagues prepared the same mixture of dead S and live R bacteria as Griffith did They then subjected it to various experiments All of the experiments revealed that the properties of the transforming principle resembled those of DNA 1. Same chemistry and physical properties as DNA 2. Not affected by lipid and protein extraction 3. Not destroyed by protein- or RNA-digesting enzymes 4. Destroyed by DNA-digesting enzymes The Avery Experiments
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Viruses that infect bacteria have a simple structure DNA core surrounded by a protein coat Hershey and Chase used two different radioactive isotopes to label the protein and DNA Incubation of the labeled viruses with host bacteria revealed that only the DNA entered the cell Therefore, DNA is the genetic material The Hershey-Chase Experiment
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The Hershey- Chase Experiment Thus, viral DNA directs the production of new viruses
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Discovering the Structure of DNA DNA is made up of nucleotides Each nucleotide has a central sugar, a phosphate group and an organic base The bases are of two main types Purines – Large bases Adenine (A) and Guanine (G) Pyrimidines – Small bases Cytosine (C) and Thymine (T)
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The four nucleotide subunits that make up DNA 5-C sugar Nitrogenous base
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Erwin Chargaff made key DNA observations that became known as Chargaff’s rule Rosalind Franklin’s X-ray diffraction experiments revealed that DNA had the shape of a coiled spring or helix Purines = Pyrimidines A = T and C = G Rosalind Franklin (1920-1958)
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In 1953, James Watson and Francis Crick deduced that DNA was a double helix James Watson (1928- ) Francis Crick (1916-2004) They came to their conclusion using Tinkertoy models and the research of Chargaff and Franklin
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The DNA double helix Dimensions suggested by X- ray diffraction The two possible basepairs
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