Batch 2 phages were grown with radioactive phosphorus

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Batch 2: Phages were grown with radioactive phosphorus ( 32 P ), which was incorporated into phage DNA (blue). 1 2 3 4 Agitated in a blender to separate phages outside the bacteria from the bacterial cells. Mixed radioactively labeled phages with bacteria. The phages infected the bacterial cells. Centrifuged the mixture so that bacteria formed a pellet at the bottom of the test tube. Measured the radioactivity in the pellet and the liquid Phage proteins remained outside the bacterial cells during infection, while phage DNA entered the cells. When cultured, bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus. Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material. RESULTS CONCLUSION EXPERIMENT Radioactivity (phage DNA) in pellet Figure 16.4
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Additional Evidence That DNA Is the Genetic Materia Prior to 1950s, already known that DNA polymer of nucleotides, each consisting of 3 components: nitrogenous base, sugar, & phosphate group Sugar-phosphate backbone Nitrogenous bases 5 end O O P O CH 2 5 4 O H H O H H H 3 1 H O CH 3 N O N H Thymine (T) O O P O O CH 2 H H O H H H H N N N H N H H Adenine (A) O O P O O CH 2 H H O H H H H H H H N N N O Cytosine (C) O O P O CH 2 5 4 O H O H H 3 1 OH 2 H N N N H O N N H H H H Sugar (deoxyribose) 3 end Phosphate Guanine (G) DNA nucleotide 2 N Figure 16.5
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Erwin Chargaff analyzed base composition of DNA, from different organisms 1947, Chargaff reported DNA composition varies from one species to next Evidence of molecular diversity among species Made DNA a more credible candidate for genetic material
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Building a Structural Model of DNA: Scientific Inquiry Once most biologists were convinced that DNA was genetic material Challenge was to determine how structure of DNA could account for its role in inheritance
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Maurice Wilkins and Rosalind Franklin Using X-ray crystallography to study molecular structure Rosalind Franklin Produced a picture of DNA molecule (a) Rosalind Franklin Franklin’s X-ray diffraction Photograph of DNA (b) Figure 16.6 a, b
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Watson & Crick deduced that DNA was a double helix Through observations of X-ray crystallographic images of DNA Figure 16.7a, c C T A A T C G G C A C G A T A T A T T A C T A 0.34 nm 3.4 nm (a) Key features of DNA structure G 1 nm G (c) Space-filling model T
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Franklin had concluded that DNA Was composed of 2 antiparallel sugar- phosphate backbones, with nitrogenous bases paired in molecule’s interior Nitrogenous bases Paired in specific combinations: adenine with thymine, cytosine with guanine
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O O O OH O O O O H 2 C O O O O H 2 C O O O O OH O O O T A C G C A T O O O CH 2 O O O O CH 2 CH 2 CH 2 5 end Hydrogen bond 3 end 3 end G P P P P O OH O O O O P P O O O O P O O O O P (b) Partial chemical structure H 2 C 5 end Figure 16.7b O
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Watson & Crick reasoned there must be
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