How does the synthesis of a polypeptide proceed through a repeated cycle of events?

Sourav PanNovember 9, 2024

Answered

The synthesis of a polypeptide proceeds through a repeated cycle of events during the elongation phase of translation. This process involves the coordinated action of several components, including ribosomes, transfer RNA (tRNA), and amino acids. Here’s a detailed overview of how this cycle unfolds:

Overview of Translation Phases

Translation consists of three main phases: initiation, elongation, and termination. After the initiation phase, which sets up the ribosomal complex on the mRNA, the elongation phase involves the actual synthesis of the polypeptide chain.

Steps in the Elongation Cycle

1. Aminoacyl-tRNA Binding:
   • The cycle begins with a charged tRNA (aminoacyl-tRNA) entering the A site (aminoacyl site) of the ribosome. The anticodon of this tRNA pairs with the corresponding codon on the mRNA strand that is positioned in the ribosome. This ensures that the correct amino acid is added to the growing polypeptide chain based on the mRNA sequence.
2. Peptide Bond Formation:
   • Once the tRNA is correctly positioned in the A site, a peptide bond is formed between the amino acid attached to this tRNA and the growing polypeptide chain, which is held by the tRNA in the P site (peptidyl site). This reaction is catalyzed by peptidyl transferase activity, which is part of the large ribosomal subunit. The carboxyl end of the polypeptide chain is released from its tRNA in the P site and transferred to the free amino group of the amino acid on the tRNA in the A site, forming a new peptide bond.
3. Translocation:
   • After peptide bond formation, a series of conformational changes occur within the ribosome. The ribosome moves along the mRNA by one codon (three nucleotides), shifting the tRNAs:
     • The tRNA that was in the A site moves to the P site.
     • The empty tRNA that was in the P site moves to the E site (exit site), where it will be released from the ribosome .
   • This movement positions a new codon in the A site, ready for another charged tRNA to enter and repeat the cycle.

Repetition of Events

These steps—aminoacyl-tRNA binding, peptide bond formation, and translocation—are repeated for each codon along the mRNA until a stop codon is reached. Each cycle effectively elongates the polypeptide chain by one amino acid .

• The efficiency of this process allows for rapid protein synthesis; for example, in E. coli, it takes approximately 0.05 seconds to add each amino acid to a growing polypeptide chain