Space-fill drawing of the 70S ribosome in three stages of elongation during the mRNA-to-protein translation process. At left, the EF-Tu protein (purple) delivers a new tRNA to the A site. At center, the new peptide bond is being formed between the amino acid from the A-site tRNA and the growing protein chain held by the P-site tRNA. At right, the EF-G protein (purple) helps move everything forward one step, ejecting the tRNA that had been in the E (exit) site. Drawn by David Goodsell from PDB files 2wrn, 2wro, 2wdk, 2wdl, 2wri, and 2wrj.