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Genetics: A Conceptual Approach

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Chapter 15 The Genetic Code and Translation Dr. Ed Otto George Mason University
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Key objectives Examine the relationship between genes and proteins Understand the genetic code– how to read it, its characteristics, and how it was deciphered Understand the mechanism of translation in bacteria Compare translation process in bacterial and eukaryotic cells
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Relationship Between Genes & Proteins
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Genes and Enzymes Garrod proposed in 1902 that genes encode enzymes However it was not until the 1940s, when George Beadle and Edward Tatum examined the genetic basis of biochemical pathways, that the relationship between genes and enzymes became widely accepted
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Beadle & Tatum Beadle and Tatum used Neurospora (bread mold) to study the biochemical consequences of mutations - Wild-type Neurospora grows on minimal medium because it is able to synthesize all the biological molecules it needs from the basic compounds in the medium - Beadle and Tatum irradiated the fungus to induce mutations that destroy the fungus’ ability to synthesize one or more essential biological compounds - These nutritionally-deficient mutants, ____________ , no longer grew on minimal medium, but grew on medium supplemented with the substance that they could no longer synthesize
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Isolation of Auxotrophic Mutants
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Isolation of Auxotrophic Mutants
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Analysis of Auxotrophic Mutants A series of auxotrophic mutants whose growth required arginine were tested for their ability to grow on three compounds: ornithine, citrulline, and arginine Mutants fell into three groups From these results, they were able to construct a biochemical pathway & identify genes that controlled each step
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One Gene, One Enzyme Hypothesis Genetic crosses and mapping studies demonstrated that the mutations affecting any one step in the pathway always mapped to the same chromosomal location Thus, Beadle & Tatum reasoned that a single gene must control each step and that each gene encodes one enzyme This idea became know as the one gene, one enzyme hypothesis
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One Gene, One Polypeptide Today we know that not all proteins are enzymes and that many proteins are composed of more than one polypeptide… …so Beadle and Tatum’s model was modified to become one gene, one polypeptide
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Structure and Function of Proteins
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Protein Functions Proteins are central to all living organisms and have a variety of functions in the cell Functions Examples - catalyst (enzyme) - structural/support - transport - regulatory - communication - defense RNA polymerase nuclear scaffold proteins ion channels transcription factors peptide hormones antibodies
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Protein Structure All proteins are composed of amino acids that are linked end to end Each amino acid consists of a central carbon atom attached to: (1) an amino group (NH 3 + ), a carboxyl group (COO - ), (3) hydrogen atom (H), and (4) a radical group (R)
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Protein Structure There are 20 common amino acids that have different R groups Amino acids can be grouped based on the chemical properties of their R groups: - nonpolar, aliphatic - polar, uncharged - aromatic - positively charged - negatively charged
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Common Amino Acids
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Common Amino Acids
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Common Amino Acids
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Common Amino Acids
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