Final answer:
The site within the ribosome that holds the growing polypeptide chain during translation is the P (peptidyl) site. This site is where peptide bonds form, elongating the polypeptide as tRNAs move from the A (aminoacyl) site to the P site and then to the E (exit) site for expulsion.
Step-by-step explanation:
During translation, the site within the ribosome that holds the growing polypeptide chain is the P (peptidyl) site. This site is crucial in the protein synthesis process as it binds charged transfer RNAs (tRNAs) that carry amino acids already attached to the growing polypeptide chain. As the ribosome translocates along the mRNA, new amino acids are added to the chain by forming peptide bonds. This occurs when a charged tRNA carrying an amino acid enters the A (aminoacyl) site, and a peptide bond is formed between the new amino acid and the growing chain in the P site. Upon the formation of each peptide bond, the tRNA previously in the P site, now uncharged, moves to the E (exit) site and is subsequently expelled, making room for the tRNA that was in the A site, now carrying the growing polypeptide chain, to shift to the P site.