The Ribosome 423

The Ribosome Is Composed of a Large and a Small Subunit 425 Tile Large and Small Subunits Undergo Association and Dissociation during each Cycle of Translation 425

New Amino Acids Are Attached to the C-Terminus of the Growing Polypeptide Chain 427 Peptide Bonds Are Formed by Transfer of the Glowing Polypeptide Chain from One tRNA to Another 428 Ribosomal RNAs Are Both Structural and Catalytic

Determinants of the Ribosome 428 The Ribosome l ias Three Binding Sites for tRNA 429 Channels through the Ribosome Allow the mRNA and Growing Polypeptide to Enter and/or Exif the Ribosome 430 INITIATION OF TRANSLATION 432 Prokaryotic mRN As Are Initially Recruited to the

Small Subunit by Base-Pairing to rRNA 433 A Specialised tRNA Charged with a Modified Methionine Binds Directly to the Prokaryotic Small Subunit 433 Three Initiation Factors Direct the Assembly of an Initiation Complex that Contains mRN/\ and the Initiator tRNA 433

Eukaryotic Ribosomes Are Recruited to the mRNA

by the 5' Cap 435 The Stan Codon Is Found by Scanning Downstream from the 5' End of the mRNA 437 Translation Initiation Factors i fold Eukaryotic mR-NAs in Circles 438

Box ¡4-2 uORFs and JRESs. Exceptions that Prove the Ride 439 TRANSLATION ELONGATION 440 Aminoacyl-tRNAs Are Delivered to the A Site by Elongation Factor EF-Tu 441 The Ribosome Uses Multiple Mechanisms to Select Against Incorrect Aminoacyl-tRNAs 441

The Ribosome Is a Rihozyirie 442 Peptide Bond Formation and the Elongation Factor EF-G Drive Translocation of the tRNAs and the mRNA 444

EF-G Drives Translocation by Displacing the cRNA Bound to the A Site 445 EF-Tu-GDP and EF-G-GDP Must Exchange GDP forGTP Prior to Participating m a New Round of Elongation 446 A Cycle of Peptide Bond Formation Consumes Two Molecules of GTP and One Molecule of ATP 446 Box ¡4-3 GTP-BindingProteins,Conformational Switching, and the fidelity and Ordering of the Li'ents of Translation 447

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