How does translation on ribosomes synthesize polypeptides?

Sourav PanNovember 8, 2024

Answered

Translation is the process by which ribosomes synthesize polypeptides (proteins) from messenger RNA (mRNA) sequences. This intricate process involves several key components and occurs in three main stages: initiation, elongation, and termination.

Overview of Translation

1. Initiation:
   • Ribosome Assembly: Translation begins with the assembly of the ribosome around the mRNA molecule. The small ribosomal subunit binds to the mRNA at the start codon (AUG), which specifies the first amino acid, methionine .
   • tRNA Binding: An initiator tRNA carrying methionine binds to the start codon in the ribosome’s P-site (peptidyl site). The large ribosomal subunit then joins this complex, forming a functional ribosome ready for elongation .
2. Elongation:
   • Amino Acid Addition: During elongation, tRNAs bring specific amino acids to the ribosome. Each tRNA has an anticodon that pairs with the corresponding codon on the mRNA. The ribosome has three binding sites: A-site (aminoacyl site), P-site, and E-site (exit site) .
   • Peptide Bond Formation: The next tRNA enters the A-site, bringing an amino acid that is complementary to the mRNA codon. The ribosome catalyzes a reaction where the carboxyl end of the growing polypeptide chain (attached to the tRNA in the P-site) forms a peptide bond with the amino group of the amino acid on the tRNA in the A-site. This reaction is facilitated by the peptidyl transferase activity of the ribosome’s large subunit, linking amino acids together into a polypeptide chain .
   • Translocation: After peptide bond formation, the ribosome shifts one codon down the mRNA. The tRNA that was in the P-site moves to the E-site and exits, while the tRNA in the A-site moves to the P-site. This cycle repeats as new amino acids are added sequentially based on the mRNA codons .
3. Termination:
   • Stop Codon Recognition: Translation continues until a stop codon (UAA, UAG, or UGA) is encountered on the mRNA. These codons do not have corresponding tRNAs and signal that protein synthesis should end .
   • Release of Polypeptide: When a stop codon is reached, release factors bind to the ribosome, prompting it to release the completed polypeptide chain from the tRNA in the P-site. Subsequently, the ribosomal subunits dissociate from each other and from the mRNA, allowing them to be reused for another round of translation .