Lecture18 Biol302 Spring 2011

Lecture18 Biol302 Spring 2011 - Polypeptide Chain Elongation

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Polypeptide Chain Elongation An aminoacyl-tRNA binds to the A site of the ribosome. The growing polypeptide chain is transferred from the tRNA in the P site to the tRNA in the A site by the formation of a new peptide bond. The ribosome translocates along the mRNA to position the next codon in the A site. At the same time, The nascent polypeptide-tRNA is translocated from the A site to the P site. The uncharged tRNA is translocated from the P site to the E site. http://www.molecularmovies.com/showcase/
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Elongation of Fibroin Polypeptides (A mRNA can have multiple Ribosomes
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Polypeptide Chain Termination Polypeptide chain termination occurs when a chain-termination codon (stop codon) enters the A site of the ribosome. The stop codons are UAA, UAG, and UGA. When a stop codon is encountered, a release factor binds to the A site. A water molecule is added to the carboxyl terminus of the nascent polypeptide, causing termination.
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No tRNA exists for stop codons!
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Dissociation upon finish of protein synthesis
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Fig1
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The Genetic Code The genetic code is a nonoverlapping code, with each amino acid plus polypeptide initiation and termination specified by RNA codons composed of three nucleotides.
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Properties of the Genetic Code The genetic code is composed of nucleotide triplets. The genetic code is nonoverlapping. (?) The genetic code is comma-free. (?) The genetic code is degenerate. (yes) The genetic code is ordered. (5’ to 3’) The genetic code contains start and stop codons. (yes) The genetic code is nearly universal. YES :)
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A Triplet Code*
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A Single-Base Pair Insertion Alters the Reading Frame*
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A suppressor mutation restores the original reading frame.*
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Insertion of 3 base pairs does not change the reading frame.*
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Evidence of a Triplet Code: In Vitro Translation Studies Trinucleotides were sufficient to stimulate specific binding of aminoacyl-tRNAs to ribosomes. Chemically synthesized mRNAs containing repeated
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Lecture18 Biol302 Spring 2011 - Polypeptide Chain Elongation

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