## Translation Elongation Phase: Prokaryotic Mechanism The elongation phase of translation proceeds through a highly ordered, cyclical sequence of three major steps. ### Step-by-Step Sequence 1. **Aminoacyl-tRNA Binding (A-site entry)** - Aminoacyl-tRNA (aa-tRNA) enters the A site of the ribosome - Facilitated by EF-Tu (elongation factor Tu) in prokaryotes - GTP hydrolysis provides energy for correct codon–anticodon pairing 2. **Peptide Bond Formation** - Catalyzed by peptidyl transferase activity of the 23S rRNA (large subunit) - The carboxyl group of the peptidyl-tRNA (in P site) attacks the amino group of the aa-tRNA (in A site) - Forms a new peptide bond; deacylated tRNA moves to E site 3. **Translocation** - Ribosome moves exactly 3 nucleotides (one codon) along the mRNA - Facilitated by EF-G (elongation factor G) and GTP hydrolysis - P-site tRNA moves to E site; A-site tRNA moves to P site; E-site tRNA exits - A site becomes vacant and ready for the next aa-tRNA **Key Point:** This cycle repeats until a stop codon is reached. The order is invariant: binding → catalysis → movement. **High-Yield:** Remember the mnemonic **"A-P-E"** for the three ribosomal sites and **"BAT"** for the three elongation steps: **B**inding, **A**ctylation (peptide bond), **T**ranslocation. **Mnemonic:** **"Bring Amino-acids, Then bond, Then Travel"** — captures the sequence and logic of elongation. ### Why This Order Is Essential - **Binding first** ensures the correct tRNA is selected (proofreading by EF-Tu) - **Peptide bond formation second** commits the amino acid to the growing chain - **Translocation last** clears the A site for the next cycle and prevents premature dissociation [cite:Molecular Biology of the Gene, Watson et al., Ch 17]
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