geneExpression

geneExpression - Gene Expression Chapter 8 PROTEIN...

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Gene Expression Chapter 8
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PROTEIN STRUCTURE: Read pp. 234-239 Note the following terms: - Primary structure - Secondary structure - Tertiary structure - Quarternary structure
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The genetic code How triplets of the four nucleotides unambiguously specify 20 amino acids, making it possible to translate information from a nucleotide chain to a sequence of amino acids?
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The Genetic Code: 61 triplet codons represent 20 amino acids; 3 triplet codons signify stop Fig. 8.3
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HOW HAS CODE BEEN DECIPHERED?
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A gene’s nucleotide sequence is colinear the amino acid sequence of the encoded polypeptide Charles Yanofsky Used E. coli gene for a subunit of tryptophan synthetase; compared mutations within a gene to particular amino acid substitutions Trp - mutants in trpA Fine structure recombination map Determined amino acid sequences of mutants
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COLINEARITY OF GENES AND PROTEINS Fig. 8.4
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CONCLUSION Different point mutations may affect same amino acid Codon contains more than one nucleotide Each point mutation altered only one amino acid Each nucleotide is part of only a single codon
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Logically, the minimal size of codon is three nucleotides 20 amino acids are encoded for by 4 nucleotides By deduction: 1 nucleotide/amino acid = 4 1 = 4 combinations 2 nucleotides/amino acid = 4 2 = 16 combinations 3 nucleotides/amino acid = 4 3 = 64 combinations Must be at least triplet combinations that code for amino acids
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MODELS OF GENETIC CODE Yanofsky’s results supported non-overlapping code
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Studies of frameshift mutations in bacteriophage T4 rIIB gene A (Crick, Brenner et al.) Fig. 8.5
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CRICK’S LOGIC Let’s assume that FCO (+) is an addition, and suppressor (-) is a deletion WILD TYPE CAT ATE TEA CAT ATE TEA Normal function (+) MUTANT CAT T AT ETE ACA TAT ETE A Loss of function (+) (-) DOUBLE MUTANT CAT T AE TEA CAT ATE TEA Normal function (-T)
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WILD TYPE CAT ATE TEA CAT ATE TEA Normal function (+) MUTANT CAT T AT ETE ACA TAT ETE A Loss of function (+) (+) DOUBLE MUTANT CAT T AT T ET EAC ATA TET EA No function (+) (+) (+) TRIPLE MUTANT CAT T AT T E T TEA CAT ATE TEA Restoration of function THREE IS A MAGIC NUMBER OF GENETIC CODE CRICK’S LOGIC Let’s assume that FCO (+) is an addition, and suppressor (-) is a deletion
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geneExpression - Gene Expression Chapter 8 PROTEIN...

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