Translation part 2 Chain Initiation,Chain elongation,Chain Termination

Translation (Protein Synthesis) – Part 2 1. Chain Initiation Chain initiation is the first stage of translation in which the ribosome assembles on the mRNA and begins protein synthesis. Steps: 1. The small ribosomal subunit binds to the mRNA. 2. The initiator tRNA carrying methionine (Met) binds to the start codon (AUG). In prokaryotes: N-formyl methionine (fMet) is the first amino acid. In eukaryotes: Methionine (Met) is the first amino acid. 3. Initiation factors (IFs in prokaryotes, eIFs in eukaryotes) assist the process. 4. The large ribosomal subunit joins the complex, forming a complete initiation complex. 2. Chain Elongation During elongation, amino acids are added one by one to the growing polypeptide chain. Steps: 1. A charged tRNA enters the A (aminoacyl) site of the ribosome. 2. Codon–anticodon pairing occurs between mRNA and tRNA. 3. Peptidyl transferase enzyme forms a peptide bond between adjacent amino acids. 4. The ribosome moves one codon forward on the mRNA (translocation). 5. The empty tRNA leaves through the E (exit) site. 6. The cycle repeats until a stop codon is reached. 3. Chain Termination Termination occurs when the ribosome reaches a stop codon. Stop Codons: UAA UAG UGA Steps: 1. No tRNA recognizes stop codons. 2. Release factors bind to the stop codon. 3. The completed polypeptide chain is released. 4. Ribosomal subunits dissociate from the mRNA. 4. Post-Translational Modification After translation, newly synthesized proteins undergo modifications to become fully functional. Types of Post-Translational Modifications: Folding: Protein attains its three-dimensional structure. Cleavage: Removal of extra amino acid sequences. Phosphorylation: Addition of phosphate groups. Glycosylation: Addition of carbohydrate groups. Methylation: Addition of methyl groups. Acetylation: Addition of acetyl groups. Disulfide Bond Formation: Stabilizes protein structure. Importance: Activates proteins. Increases protein stability. Determines protein localization and function. Enables proper biological activity. Summary Translation = Initiation → Elongation → Termination → Post-Translational Modification → Functional Protein.