On the ribosome, there are 4 binding sites – 3 for tRNA and 1 for mRNA. A, P and E are the three t-RNA sites. The P site or peptidyl site binds to the tRNA that is holding the developing polypeptide chain of amino acids. The A or acceptor site binds to the aminoacyl tRNA, which holds the newly added amino acid to the polypeptide chain.