Practice question

Co-translational modifications occur simultaneously with nascent chain elongation inside ribosome exit tunnel, including methionine aminopeptidase removal, N-terminal acetylation, and selenocysteine insertion. Selenocysteine insertion qualifies as cotranslational because SECIS-mediated recoding delivers Sec-tRNASec to A-site before termination, integrating Sec directly into primary sequence during elongation. In contrast, phosphorylation, acetylation, methylation typically occur post-translationally after folding via separate kinases, acetyltransferases, methyltransferases adding functional groups to side chains. This distinction matters for topology and enzyme targeting. Selenocysteine integration requires active ribosome, GTP hydrolysis, mRNA structure, illustrating direct encoding rather than side-chain decoration after polypeptide release from large subunit.