Final answer:
The A site on a ribosome during translation's elongation cycle functions in receiving a new tRNA with the correct amino acid, facilitating the addition of this amino acid to the growing polypeptide chain.
Step-by-step explanation:
During the elongation cycle of translation, the A site on a ribosome is responsible for receiving a new tRNA with the correct amino acid. This is a crucial step in protein synthesis where the correct aminoacyl tRNA pairs with the mRNA codon, ensuring the appropriate amino acid is added to the growing polypeptide chain.
Each time a new aminoacyl tRNA enters the A site, a peptide bond forms between the new amino acid and the existing chain. This process is catalyzed by peptidyl transferase, an RNA-based enzyme integrated into the larger ribosomal subunit. After the peptide bond is made, the tRNA carrying the growing polypeptide chain translocates from the A site to the P site. This action makes room for the next charged tRNA to enter the A site, continuing the process of elongation.
The efficiency of this process is remarkable in organisms like E. coli, where it takes only about 0.05 seconds to add each amino acid to the chain. So, for a 200-amino acid polypeptide, the entire elongation process can occur in as little as 10 seconds.