# Chapter 18 - Protein Synthesis - There is a “gear ratio” that occurs during the transition from nucleotides to amino acids (3 → 1) - The process of protein synthesis can be considered a “lossy” compression method ## Initiation - The only time a tRNA comes into the P site is during initiation. Anytime after it will be coming in through the A site. This is because at this stage the - Shine Dalgarno (SD) ←> Kozak Sequence - Both are ribosome binding sites (RBSs) **Prokaryote Initiation** - fMet (N-formylmethionine) will only work as the first methionine. Regular methionines will be used for intermediate - This is because fMet can only be charged into tRNAi (initiating tRNA) tRNA$^\text{fMet}$ (Initiation) tRNA$^\text{Met}$ (Elongation) The aaRS for methionine is loading both - “There’s no 21st aaRS for fMet” - There is a secondary --- **Eukarotic Initiation** - The same mRNA molecule can be used to transcribe a protein multiple times. - A cyclical structure is able to be formed, allowing a ribosome ## 18.4 - Elongation and Termination of the Polypeptide Chain - [?] How does the correct aa-tRNA get identified by the ribosome - tRNA in the A site performs a flipping motion - Occurs after the GDP --- > [!question] How do ribosomes handle defective mRNA? >Types of errors: > - Truncated mRNA > - Premature stop codons > - Non-existent stop codons ## 18.5 - Translation-Coupled Removal of Defective mRNA --- **Types of Assisting Molecules** - Initiation Factors (IF) - Elongation Factors (EF) - Ternary Complex (TC) - 43S preinitiation complex ## 18.6 Protein Folding, Covalent Modification, and Targeting