Release factor causes addition of water molecule instead of an aa to polypeptide chain 2. This breaks (hydrolyzes) bond between completed polypeptide and tRNA in P site, releasing polypeptide thru exit tunnel of large sub 3. Remainder of translation assembly comes apart in multistep process, aided by other protein factors, requires hydrolysis of 2 GTP molecules h. Polyribosomes i. Single ribosome can make avg. polypeptide quickly 1. However, multiple ribosomes translate an mRNA at the same time; a single mRNA used to make many copies of polypeptide simultaneously 2. Once ribosome is far enough past start codon, 2 nd ribosome can attach to mRNA, resulting in number of ribosomes trailing mRNA 3. Such strings of ribosomes are polyribosomes , found in both bacterial and eukaryotic cells ii. Enable cell to make many copies of polypeptide quickly i. Completing and targeting the functional protein i. Process of translation often not sufficient to make functional protein ii. Polypeptide chains undergo modifications after translation process iii. Mechanisms are used to target completed proteins to specific sites in cell j. Protein folding and post-translational modifications i. During its synthesis, polypeptide chain coils, folds spontaneously as result of its aa sequence (primary structure), forming protein with specific shape ii. 3D molecule with secondary and tertiary structure iii. Thus genes determine primary structure, which determines shape iv. Many cases, chaperone protein (chaperonin) helps polypeptide fold correctly
v. Additional steps (post-translational modifications) may be required before protein can being its job in cell 1. Certain amino acids may be chemically modified by attachment of sugars, lips, phosphate groups, etc. 2. Enzymes may remove 1+ amino acids from leading (amino) end of polypeptide chain 3. Polypeptide chain may be enzymatically cleaved 4. 2+ polypeptides synthesized separately may come together, becoming subunits of protein with quaternary structure k. Targeting polypeptides to specific locations i. 2 populations of ribosomes and polyribosomes are: free and bound ii. Free ribosomes suspended in cytosol, synthesize proteins that stay in cytosol and function there iii. Bound ribosomes attached to cytosolic side of ER or to nuclear envelope 1. Make proteins of endomembrane system (nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, plasma membrane) and proteins secreted out of the cell 2. Ribosomes are identical, can switch from free to bound, vice versa iv. Polypeptide synthesis always begins in cytosol as free ribosome translates mRNA molecule 1. Process continues unless growing polypeptide itself cues ribosome to attach to ER 2. Polypeptides of proteins destined for ER/secretion are marked by signal peptide , targets protein for ER a. leading end (N-terminus) of polypeptide is recognized as it emerges from ribosomes by protein-RNA complex called a signal-recognition particle (SRP) b. This functions as an escort that brings ribosome to receptor protein built into ER membrane c.
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