As just mentioned, elongation requires the help of elongation factor proteins. The tRNAs with attached amino acids (called aminoacyl tRNAs) are brought onto the ribosome by one such elongation factor . This factor is called EF-Tu in prokaryotes and EF1 in eukaryotes. Its job is to bring aminoacyl tRNAs onto the ribosome and then to help the ribosome make sure that this tRNA has the correct amino acid attached to it. The ribosome has three aminoacyl tRNA binding sites: the acceptor site (A), the peptidyl site (P), and the exit site (E). The tRNA that has the growing protein attached to it binds in the P site (hence the name peptidyl, for peptide). The incoming aminoacyl tRNA, containing the next amino acid to be added, binds in the A site. The A site is where decoding of the genetic code takes place; the correct aminoacyl tRNA is selected to match the next codon of the mRNA. Spent tRNAs that no longer have an amino acid or the growing peptide chain attached to them end up in the E site, from Figure 2. Peptide bond formation by the ribosome. The three lines between the mRNAs and the tRNA indicate base pairing between the codon of the mRNA and the anticodon of the tRNA.