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LECTURE CONNECTIONS 15 | The Genetic Code and Translation © 2009 W. H. Freeman and Company 15.3 Amino Acid Are Assembled into a Protein Through the Mechanism of Translation • Translation takes place on ribosomes. • A ribosome attaches near the 5’ end of an mRNA strand and moves towards the 3’ end, translating the codons as it goes. • Synthesis begin at the amino end of the protein and the protein is elongated by the addition of new aa to the carboxyl end. • Interactions between mRNA and the rRNA that hold the mRNA in the ribosome. The Binding of Amino Acids to Transfer RNAs • Aminoacyl-tRNA synthetases and tRNA charging • The specificity between an amino acid and its tRNA is determined by each individual aminoacyl-tRNA synthesis. • There are exactly 20 different aminoacyl-tRNA syntheses in a cell. Each synthetase recognizes a particular aa. • Recognition of tRNAs by a synthetase depends on the differing nucleotide sequences of the tRNAs tRNA charging requires energy, which is supplied by adenosine triphosphate (ATP) The Initiation of Translation • mRNA, small and large subunits of ribosome • A set of 3 proteins called the initiation factors: IF-1, IF-2 and IF-3 • initiator tRNA with N-formylmethionine attached to form fMet-tRNAfMet • Energy molecule: GTP The Initiation of Translation • The Shine–Dalgarno consensus sequence in bacterial cells is recognized by the small unit of ribosome. • The Kozak sequence in eukaryotic cells facilitates the identification of the start codon. Elongation • Exit site E • Peptidyl site P • Aminoacyl site A • Elongation factors: Tu, Ts, and G Concept Check 3 In elongation, the creation of peptide bonds between amino acids is catalyzed by . a. b. c. d. rRNA protein in the small subunit protein in the large subunit tRNA Termination • Termination codons: UAA, UAG, and UGA • Release factors 15.4 Additional Properties of RNA and Ribosomes Affect Protein Synthesis • The three-dimensional structure of the ribosome • Polyribosomes: – An mRNA with several ribosomes attached Concept Check 4 In a polyribosome, the polypeptides associated with which ribosomes will be the longest? a. b. c. d. Those at the 5′ end of mRNA Those at the 3′ end of mRNA Those in the middle of mRNA All polypeptides will be the same length. 15.4 Additional Properties of RNA and Ribosomes Affect Protein Synthesis • Messenger RNA surveillance: • Detect and deal with errors in mRNA • Nonsense – mediated mRNA decay: eliminating mRNA containing premature termination codons • The posttranslational modifications of proteins