Proteins are assembled on molecular machines that can be seen with an electron microscope. The ribosome is a large complex of RNA and protein that reads messenger RNA and links amino acids in the correct order. Every living cell depends on ribosomes for protein synthesis.

Bacterial ribosomes differ slightly from eukaryotic ones, a difference exploited by many antibiotics. The ribosome has three sites—A, P, and E—where transfer RNAs successively bind, add their amino acid, and exit. Peptide-bond formation is catalyzed by ribosomal RNA, making the ribosome a ribozyme.

Cells adjust ribosome number according to growth conditions. Cryo-electron microscopy has revealed the ribosome’s structure in atomic detail and shown how antibiotics and regulatory factors interact with it. The machine is both ancient and remarkably conserved across the tree of life.

• Cellular machine that synthesizes proteins
• Composed of RNA and protein subunits
• Reads mRNA and assembles amino acids
• Target of many antibiotics