Final answer:
In translation, the large ribosomal subunit joins the small subunit after the AUG start codon is recognized, securing the initiator tRNA in the P site and enabling the ribosome to proceed to elongation.
Step-by-step explanation:
The large ribosomal subunit (50S in prokaryotes or 60S in eukaryotes), during step 3 of translation, plays a critical role in the initiation of protein synthesis. After the small subunit identifies the AUG start codon and the initiator tRNA has bound to it, the large ribosomal subunit joins the initiation complex. This important event is facilitated by initiation factors that help align the large subunit correctly, establishing the functional sites required for tRNA entry and exit during the subsequent stages of elongation. Completing this association of the large subunit ensures that the initiator tRNA is positioned within the P site of the ribosome, setting the stage for the addition of new aminoacyl-tRNAs at the A site as translation progresses into the elongation phase.