Final answer:
The addition of amino acids to a polypeptide chain during protein synthesis takes place at the A, P, and E sites on the ribosome, involving the formation of peptide bonds and the transfer of tRNAs through these sites.
Step-by-step explanation:
The addition of amino acids to a polypeptide chain occurs at specific sites within the ribosome. These sites are known as the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. The process of elongation during protein synthesis involves charged transfer RNAs (tRNAs) entering the A site, where the amino acid that they carry is added to the growing polypeptide chain through the formation of a peptide bond. This bond is formed 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. Peptidyl transferase, an enzyme that is part of the ribosome, catalyzes the formation of this bond. Once the peptide bond is established, the tRNA in the A site then shifts to the P site, allowing the next charged tRNA to enter the A site. The tRNA previously in the P site moves to the E site and is eventually expelled from the ribosome, completing the cycle and elongating the polypeptide chain by one amino acid.