Question

Give a brief account of the different steps involved in the translation of mRNA into a polypeptide in prokaryotes.

Answer 1

In prokaryotes, translation is the process by which the genetic message in mRNA is decoded into a polypeptide chain. It occurs in the cytoplasm and involves three main stages:

1. Initiation:
   • Ribosome Binding: The small ribosomal subunit (30S) binds to the mRNA at the Shine-Dalgarno sequence, located just upstream of the start codon (AUG).
   • Start Codon Recognition: A specialised initiator tRNA, carrying N-formylmethionine (fMet), binds to the AUG start codon through its anticodon.
   • Assembly: The large ribosomal subunit (50S) then joins the complex, positioning the initiator tRNA in the P-site (Peptidyl site) and leaving the A-site (Aminoacyl site) vacant.
2. Elongation:
   • tRNA Entry: A new aminoacyl-tRNA, carrying the next amino acid, enters the A-site by base-pairing with the next mRNA codon.
   • Peptide Bond Formation: The enzyme peptidyl transferase (part of the large subunit) forms a peptide bond between the amino acid in the P-site and the one in the A-site. The growing chain is transferred to the tRNA in the A-site.
   • Translocation: The ribosome moves (translocates) three bases forward along the mRNA. The empty tRNA moves to the E-site (Exit) and leaves, while the tRNA holding the polypeptide chain shifts from the A-site to the P-site.
3. Termination:
   • Stop Codon: Elongation continues until the ribosome reaches a stop codon (UAA, UAG, or UGA) at the A-site.
   • Release Factors: Since no tRNA matches a stop codon, proteins called release factors bind to the A-site.
   • Dissociation: These factors trigger the hydrolysis and release of the completed polypeptide chain. The ribosomal subunits, mRNA, and tRNAs then dissociate and are recycled.