Which site of the ribosome is primarily responsible for holding the growing polypeptide chain?

The primary site of the ribosome responsible for holding the growing polypeptide chain is the P site. During the process of translation, the P site (peptidyl site) is where the tRNA carrying the growing polypeptide chain is located. As new amino acids are added to the chain, this site facilitates peptide bond formation between the amino acids, effectively elongating the polypeptide.

The P site plays a crucial role in the coordination of protein synthesis. As the ribosome moves along the messenger RNA (mRNA), the tRNA in the P site provides the growing chain with the amino acids that are sequentially added, maintaining the integrity of the polypeptide as it is formed. The activity of the P site is essential for ensuring that the ribosome can correctly assemble the amino acid sequence as dictated by the mRNA template. This explains why this site is fundamental to the translation process in protein synthesis.