BS 161 1st Edition Lecture 32 Outline of Last Lecture I. Transcription translationOutline of Current Lecture II. Transcription translation continuedCurrent Lecture-ribosomes have 3 binding sites-P site holds the tRNA that carries the growing polypeptide chain-A site holds the tRNA that carries the next amino acid to be added to the chain-E site is the exit site, where discharged tRNAs leave the ribosome-translation occurs in three stages-initiation: brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits-elongation: amino acids are added one by one to the preceding amino acid-termination: occurs when a stop codon in the mRNA reaches the A site of the ribosome-polyribosome (polysome): enable a cell to make many copies of a polypeptide very quickly-RNA polymerase transcribes exons and introns-transcription and translation can occur on the same mRNA in prokaryotes..major difference between prokaryotes and eukaryotes-Some polypeptides are activated by enzymes that cleave them-Other polypeptides come together to form the subunits of a protein-Others are modified by additions of sugars, methyl groups, phosphate groups, etc.-All directed to various cellular locales-Polypeptide synthesis always begins in the cytosol-Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER-Polypeptides destined for the ER or for secretion are marked by a signal peptide-Signal Recognition Particle (SRP): brings the signal peptide and its ribosome to the
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