17_Lecture_Post - From Gene to Protein Overview The Flow of...

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Unformatted text preview: From Gene to Protein. Overview: The Flow of Genetic Information The information content of DNA is in the form of specific sequences of nucleotides The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins Proteins are the links between genotype and phenotype Gene expression , the process by which DNA directs protein synthesis, includes two stages: transcription and translation Evidence from the Study of Metabolic Defects In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway Nutritional Mutants in Neurospora: Scientific Inquiry George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine They developed a one gene – one enzyme hypothesis, which states that each gene dictates production of a specific enzyme Figure 17.2 Minimal medium No growth: Mutant cells cannot grow and divide Growth: Wild-type cells growing and dividing EXPERIMENT RESULTS CONCLUSION Classes of Neurospora crassa Wild type Class I mutants Class II mutants Class III mutants Minimal medium (MM) (control) MM ornithine MM citrulline Condition MM arginine (control) Summary of results Can grow with or without any supplements Can grow on ornithine, citrulline, or arginine Can grow only on citrulline or arginine Require arginine to grow Wild type Class I mutants (mutation in gene A ) Class II mutants (mutation in gene B ) Class III mutants (mutation in gene C ) Gene (codes for enzyme) Gene A Gene B Gene C Precursor Precursor Precursor Precursor Enzyme A Enzyme A Enzyme A Enzyme A Enzyme B Enzyme B Enzyme B Enzyme B Enzyme C Enzyme C Enzyme C Enzyme C Ornithine Ornithine Ornithine Ornithine Citrulline Citrulline Citrulline Citrulline Arginine Arginine Arginine Arginine The Products of Gene Expression: A Developing Story Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene – one protein Many proteins are composed of several polypeptides, each of which has its own gene Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene – one polypeptide hypothesis Note that it is common to refer to gene products as proteins rather than polypeptides Basic Principles of Transcription and Translation RNA is the bridge between genes and the proteins for which they code Transcription is the synthesis of RNA under the direction of DNA Transcription produces messenger RNA (mRNA) Translation is the synthesis of a polypeptide, using information in the mRNA Ribosomes are the sites of translation...
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This note was uploaded on 04/30/2011 for the course BIOL 1362 taught by Professor Loeblich during the Spring '08 term at University of Houston.

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17_Lecture_Post - From Gene to Protein Overview The Flow of...

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