BS 161 1st Edition Lecture 32 Outline of Last Lecture I Transcription translation Outline of Current Lecture II Transcription translation continued Current 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 ER
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