Chapter 17 (2)

Chapter 17 (2) - Chapter 17 From Gene to Protein Overview:...

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Unformatted text preview: Chapter 17 From Gene to Protein Overview: The Flow of Genetic Information The information content of DNA Is in form of specific sequences of nucleotides along DNA strands DNA inherited by an organism Leads to specific traits by dictating synthesis of proteins Process by which DNA directs protein synthesis, gene expression Includes 2 stages, transcription & translation Concept 17.1: Genes specify proteins via transcription and translation In 1909, British physician Archibald Garrod Was first to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell Nutritional Mutants in Neurospora: Scientific Inquiry Beadle and Tatum causes bread mold to mutate with X-rays Creating mutants that could not survive on minimal medium Beadle and Tatum developed one gene one enzyme hypothesis States that function of a gene is to dictate production of a specific enzyme Using genetic crosses They determined that their mutants fell into three classes, each mutated in a different gene Figure 17.2 Working with the mold Neurospora crassa , George Beadle and Edward Tatum had isolated mutants requiring arginine in their growth medium and had shown genetically that these mutants fell into three classes, each defective in a different gene. From other considerations, they suspected that the metabolic pathway of arginine biosynthesis included the precursors ornithine and citrulline. Their most famous experiment, shown here, tested both their one geneone enzyme hypothesis and their postulated arginine pathway. In this experiment, they grew their three classes of mutants under the four different conditions shown in the Results section below. The wild-type strain required only the minimal medium for growth. The three classes of mutants had different growth requirements EXPERIMENT RESULTS Class I Mutants Class II Mutants Class III Mutants Wild type Minimal medium (MM) (control) MM + Ornithine MM + Citrulline MM + Arginine (control) CONCLUSION From the growth patterns of the mutants, Beadle and Tatum deduced that each mutant was unable to carry out one step in the pathway for synthesizing arginine, presumably because it lacked the necessary enzyme. Because each of their mutants was mutated in a single gene, they concluded that each mutated gene must normally dictate the production of one enzyme. Their results supported the one geneone enzyme hypothesis and also confirmed the arginine pathway. (Notice that a mutant can grow only if supplied with a compound made after the defective step.) Class I Mutants (mutation in gene A ) Class II Mutants (mutation in gene B ) Class III Mutants (mutation in gene C ) Wild type Gene A Gene B Gene C Precursor Precursor Precursor Precursor Ornithine Ornithine Ornithine Ornithine Citrulline Citrulline Citrulline Citrulline Arginine Arginine Arginine Arginine Enzyme A Enzyme B Enzyme C A A A B B B C C C The Products of Gene Expression: A Developing...
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Chapter 17 (2) - Chapter 17 From Gene to Protein Overview:...

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