Final answer:
At the end of each elongation cycle, the amino acid in the P site joins the polypeptide chain and the E site becomes available for a new tRNA.
Step-by-step explanation:
At the end of each cycle of elongation, the amino acid that was in the P site, is added to the polypeptide chain and the E site is free to accept another tRNA. During this process, peptide bonds form between the amino group of the amino acid attached to the A-site tRNA and the carboxyl group of the amino acid attached to the P-site tRNA. This crucial event is catalyzed by peptidyl transferase, an enzyme integrated into the ribosomal subunit.
The tRNA that was in the P site, now attached to the growing polypeptide chain, moves into the A site, allowing a new aminoacyl tRNA to enter the P site. As the ribosome continues its journey along the mRNA strand, the now uncharged tRNA in the P site is translocated to the E site, where it is eventually expelled, making room for the cycle to repeat. This dynamic process is essential for the synthesis of proteins and is highly efficient, with bacteria like E. coli capable of adding an amino acid to a polypeptide chain in mere fractions of a second.