Translating an mRNA molecule. The three-step cycle shown is repeated over and over during the synthesis
of a protein chain. An aminoacyl-tRNA molecule binds to the A-site on the ribosome in step 1, a new peptide bond is formed in step 2, and the small subunit moves a distance of three nucleotides along the mRNA chain in step 3, ejecting the spent tRNA molecule and "resetting" the ribosome so that the next aminoacyl-tRNA molecule can bind. As indicated, the mRNA is translated in the 5?to-3?direction, and the amino-terminal end of a protein is made first.