CH 17 Termination of translation o No tRNAs for any of the three stop codons o Release factor enters the A site and the peptide chain is transferred to water not another amino acid o Releases protein from ribosome o Hydrolysis reaction Polyribosome polysome o Number of ribosomes can translate a single mRNA simultaneously o They enable a cell to make many copies of a polypeptide very quickly Targeting proteins to specific locations o In euk proteins destined for the endomembrane sys ER Golgi lysosomes cell membrane etc or the outside world are made on ribosomes attached to the ER o Signal peptide 1st 20 aa sequence recognized by SRP signal recognition particle takes protein to ER Mutations o Without signal peptide signaling for ER ribosome will form protein in the cytoplasm o Changes in the nucleotide sequence can affect protein structure and function o Start codon defines reading frame o Addition of a U shifts the reading frame and changes the codons and amino acids specified o Point mutation chemical changes in one base pair of a gene o Base pair substitution silent missense nonsense mutation Silent no effect on amino acid Missense when the change makes a different amino acid Nonsense creates a premature stop codon o Insertion or deletion Causes frame shift mutations far more disastrous Frame shift causing immediate nonsense
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