Stages of protein synthesis. Illustration of the stages in protein synthesis in cells: initiation (top), elongation (middle), and termination (bottom). Messenger ribonucleic acid (mRNA, red) is read by ribosomes (blue), moving from the 5' to the 3' end of mRNA. The first transfer RNA (tRNA, key-shaped) and amino acid (orange dot) binds to the ribosome's P site (upper centre). Incoming tRNA fragments bring more amino acids to the A site, with the polypeptide chain growing (centre) from the P site until the termination codon is reached (bottom). A release factor (purple) frees the ribosome subunits until needed again. For this artwork with labels, see C023/8874.