Question

when the ribosome reaches a stop codon on the mrna, no corresponding trna enters the a site. if the translation reaction were to be experimentally stopped at this point, which of the following sets of structures would you be able to isolate from the cell?

○ an assembled ribosome with a polypeptide attached to the trna in the p site

○ separated ribosomal subunits, a polypeptide, and free trna

○ an assembled ribosome with a separated polypeptide

○ separated ribosomal subunits with a polypeptide attached to the trna

Explanation:

During translation, when a stop codon is reached, the ribosome (still assembled) has the polypeptide attached to the tRNA in the P site (since no new tRNA enters A site, and the polypeptide isn't released yet, ribosome subunits don't separate immediately). The first option matches this: assembled ribosome, polypeptide on tRNA in P site. The second option is wrong (subunits not separated yet), third (polypeptide isn't separated from tRNA yet), fourth (subunits aren't separated).

Answer:

A. an assembled ribosome with a polypeptide attached to the tRNA in the P site