ribosome

The ribosome is the cell's protein-building workshop — a molecular machine that takes the instructions in mRNA and physically assembles the corresponding protein, one amino acid at a time. Every living cell, from bacteria to humans, is packed with them, often thousands or more.

Structurally a ribosome is made of two subunits built from ribosomal RNA and proteins. It has slots where the mRNA threads through and where transfer RNAs dock; as it ratchets along the message, it matches each codon to its tRNA and forges the chemical bond that links one amino acid to the next.

Strikingly, the bond-forming chemistry is performed by the ribosome's RNA, not its proteins, making the ribosome a ribozyme — an RNA-based enzyme. Because bacterial and human ribosomes differ in detail, several antibiotics work by jamming the bacterial ribosome while leaving ours untouched.