Translation Process Overview

Steps of Translation

1. Initiation

• The small ribosomal subunit binds to initiator tRNA carrying methionine.
• This complex attaches to the 5’ cap structure of mRNA and scans for the start codon (AUG).
• Mediated by several initiation factors.
• At the start codon:
  • The large ribosomal subunit joins the complex.
  • All initiation factors are released.
• Ribosome Structure:
  • A-site: Entry site for new tRNA charged with amino acids (aminoacyl-tRNA).
  • P-site: Occupied by peptidyl-tRNA (carries the growing polypeptide chain).
  • E-site: Exit site for tRNA after delivering the amino acid.
• The initiator tRNA is positioned in the P-site.

2. Elongation

• A new tRNA carrying an amino acid enters the A-site of the ribosome.
• Anticodon of the incoming tRNA matches against the mRNA codon in the A-site.
• Proof-reading occurs:
  • tRNA with incorrect anticodons are rejected.
  • New tRNA are checked again.
• When the correct aminoacyl-tRNA enters the A-site:
  • A peptide bond is formed between adjacent amino acids.
  • tRNA in the P-site releases the amino acids onto the tRNA in the A-site and becomes empty.
• Ribosome moves one triplet forward on the mRNA:
  • The empty tRNA moves to the E-site.
  • Peptidyl-tRNA moves to the P-site.
  • The A-site is now unoccupied and ready for a new tRNA.
• This cycle is repeated for each codon on the mRNA.

3. Termination

• Occurs when a stop codon (one of three) is positioned in the A-site.
• No tRNA can fit in the A-site (no matching tRNA exists for stop codons).
• Stop codons are recognized by a release factor (a protein).
• Binding of the release factor:
  • Catalyzes cleavage of the bond between the polypeptide and tRNA.
  • The polypeptide is released from the ribosome.
• The ribosome disassembles into subunits, ready for a new round of translation.