Final answer:
Upon the appearance of a stop codon at the A site of a ribosome during protein synthesis, release factors bind to the site, leading to the hydrolysis of the bond between the polypeptide and the tRNA in the P site, the hydrolysis of GTP, and ultimately the disassembly of the ribosome into its subunits. The correct answer is option a 3.
Step-by-step explanation:
When one of the stop codons (UAA, UAG, or UGA) appears at the A site of the ribosome during the process of protein synthesis, it signals the termination of translation. There is no corresponding tRNA for these codons, which leads to the recruitment of release factors RF1 or RF2 that bind to the A site. This event triggers several actions: the hydrolysis of the ester bond linking the polypeptide to the tRNA in the P site, which is mediated by peptidyl transferase and leads to the release of the newly synthesized polypeptide chain.
Consequently, the GTP is hydrolyzed, which provides the necessary energy for the release factors to leave the A site. Finally, the ribosomal subunits, now separated from the mRNA strand, dissociate from each other; the 50S ribosomal subunit separates from the 30S subunit, effectively disassembling the 70S ribosome. These subunits can then be reused for another round of protein synthesis with new mRNA.
Regarding the options provided by the student, the correct sequence of events after a stop codon appears at the A site of the ribosome in prokaryotes is: (2) Release factor RF1 or RF2 binds to the A site, (5) the ester bond linking the polypeptide to the tRNA in the P site is hydrolyzed, (3) GTP is hydrolyzed, causing release of the RF from the A site, and (4) The 70S ribosome disassembles into 50S and 30S ribosomal subunits. Event (1) is not associated with the termination process but occurs at the initiation stage of protein synthesis.