bio- chap 17

bio- chap 17 - From Gene to Protein The Connection Between...

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From Gene to Protein The Connection Between Genes and Proteins - 1909 Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical processes in the cell---he postulated that symptoms of an inherited disease reflect person’s inability to make a particular enzyme; “inborn errors of metabolism.” - Garrod gave the example of the hereditary condition called alkaptonuria (urine appears black b/c it contains the chemical alkapton which darkens upon exposure to air), Garrod reasoned that normal individuals have an enzyme which breaks down alkapton whereas alkaptonuric ppl have inherited the inability to make enzyme that metabolizes alkapton ~ How Genes Control Metabolism - function of a gene is to dictate the production of a specific enzyme - cells synthesize and degrade most organic molecules via metabolic pathways in which each chemical reaction in a sequence is catalyzed by a specific enzyme - 1930’s George Beadle and Boris Ephrussi speculated that each of the various mutations affecting eye color in Drosophila blocks pigment synthesis at a specific step by preventing production of the enzyme that catalyzes that step - breakthrough in demonstrating relationship between genes and enzymes came after Beadle & Edward Tatum began to search for mutants of bread mold, Neurospora crassa Discovered mutants differed from wild type mold in nutritional needs: Wild- type Neurospora has modest food requirements--survives on agar mixed w/ inorganic salts, sucrose and the vitamin biotin (minimal medium), from mm mold uses metabolic pathways to produce all the other molecules it needs They identified mutants which could not survive on minimal medium, apparently b/c they were unable to synthesize certain essential molecules from the minimal ingredients. *auxotrophs, can survive on complete growth medium--minimal medium supplemented w/ 20 amino acids & other nutrients To pinpoint an auxotrophs metabolic defect, Beadle and Tatum took samples from the mutant growing on complete medium and distributed them to several vials---each vial contained minimal medium plus a single additional nutrient, therefore the particular supplement that allowed growth indicated the metabolic defect They concluded that each mutant lacked a different enzyme. Assuming that each mutant was defective in a single gene the formulated the *one gene-one enzyme hypothesis: the function of a gene is to dictate production of a specific enzyme ~ One Gene-One Polypeptide - not all proteins are enzymes --many proteins are constructed from 2 or more different polypeptide chains and each polypeptide is specified by its own gene - we can restate Beadle and Tatum’s idea as the *one gene-one polypeptide hypothesis Transcription and translation are the two main processes linking gene to protein - Genes provide instructions for making specific proteins but does not build protein directly--bridge between DNA and protein synthesis is RNA - RNA: always single stranded, contains ribose as its sugar, has nitrogenous base uracil
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bio- chap 17 - From Gene to Protein The Connection Between...

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