13 - Translation of mRNA

13 - Translation of mRNA - Chapter 13 Translation of mRNA...

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Chapter 13 Translation of mRNA I. The Genetic Basis for Protein Synthesis a. Structural genes: encode an amino acid sequence b. Messenger RNA (mRNA): RNA that is transcribed from structural genes c. Archibald Garrod Proposed that Some genes code for the production of a single enzyme i. Prior to Garrod, biochemists had studied many metabolic pathways within living cells that consist of a series of metabolic conversions of one molecule to the next each step catalyzed by an enzyme 1. each enzyme is a different protein ii. Garrod studied patients with defects in metabolizing certain compounds, especially the inherited disease alkaptonuria 1. patient’s body accumulates abnormal levels of homogentisic acid (alkapton) which is excreted in urine causing it to appear black, also get bluish black coloration of cartilage and skin (ochronosis) 2. Garrod proposed the accumulation of homogentisic acid is due to a missing enzyme, homotentisic acid oxidase iii. Garrod knew alkaptonuria is inherited in a recessive pattern iv. Alkaptonuria is an inborn error of metabolism 1. this is first connection of function of genes and their production of enzymes d. Beadle and Tatum’s experiments with Neurospora led them to propose the one gene- one enzyme theory i. Used Neurospora crassa, a bread mold ii. Has few nutritional requirements, a carbon source (sugar), inorganic salts, and one vitamin (biotin) iii. Irradiated to induce mutations, found three strains unable to grow on the minimal medium and in each strain it required only one vitamin to restore its growth on minimal medium iv. Concluded a single gene controlled the synthesis of a single enzyme v. Theory is modified later 1. enzymes are only one category of cellular proteins, all proteins are encoded by genes and many of them don’t function as enzymes 2. some proteins are composed of two or more different polypeptides so a structural gene encodes a polypeptide (a linear sequence of amino acids) 3. protein denotes function, some proteins are one polypeptide but sometimes its more than one e. during translation, the genetic code within mRNA is used to make a polypeptide with a specific amino acid sequence i. 20 amino acids, so minimum of 20 codons needed to specify each type ii. Genetic code is degenerate because the number of possible codons exceeds 20 (and the number of amino acids) more than one codon can specify the same amino acid 1. the third base in the codon is the degenerate base or the wobble base iii. genetic code is nearly universal iv. Francis Crick isolated mutants in T4 bactereophages where one + mutation can be counteracted by a second – mutation in order to restore the reading frame and produce a protein with nearly normal amino acid sequence 1. suggested genetic code is read in triplets f. synthetic RNA helped to decipher the genetic code i. Nirenberg, Ochoa, Khorana ii. Nirenberg 1. added RNA back to DNase-treated extracts and therby regained polypeptide synthesis, radiolabeled amino acids were added to the extracts and the synthesized polypeptides
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13 - Translation of mRNA - Chapter 13 Translation of mRNA...

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