Elongation: Building the Polypeptide Chain
Definition
Elongation
Elongation is the stage where the polypeptide chain grows, one amino acid at a time.
- It involves three key steps:
- Codon Recognition
- Peptide Bond Formation
- Translocation
Remember: The ribosome moves along the mRNA in the 5′ to 3′direction.
Step 1: Codon Recognition
- A tRNA molecule carrying an amino acid enters the A site of the ribosome.
- The anticodon of the tRNA pairs with the complementary codon on the mRNA.
- This ensures the correct amino acid is added to the chain.
If the mRNA codon is AUG, the tRNA with the anticodon UAC will bind, bringing methionine.
Step 2: Peptide Bond Formation
- The ribosome catalyzes the formation of a peptide bond between the amino acid in the A site and the growing polypeptide chain in the P site.
- This reaction is facilitated by the ribosomal RNA (rRNA) in the large subunit, acting as a ribozyme.
The growing polypeptide chain is temporarily transferred to the tRNA in the A site.
Step 3: Translocation
- The ribosome shifts one codon forward along the mRNA.
- The tRNA in the P site moves to the E site and exits the ribosome.
- The tRNA in the A site (now holding the polypeptide chain) moves to the P site, leaving the A site ready for the next tRNA.
- Don’t confuse the direction of ribosome movement.
- It always moves from the 5′ to 3′end of the mRNA.
- How does the precision of protein synthesis reflect the interconnectedness of biological systems?
- What might happen if even a single step in elongation fails?
