ribosomal RNA (rRNA), molecule in cells that forms part of the protein-synthesizing organelle known as a ribosome and that is exported to the cytoplasm to help translate the information in messenger RNA (mRNA) into protein. The three major types of RNA that occur in cells are rRNA, mRNA, and transfer RNA (tRNA).
Molecules of rRNA are synthesized in a specialized region of the cell nucleus called the nucleolus, which appears as a dense area within the nucleus and contains the genes that encode rRNA. The encoded rRNAs differ in size, being distinguished as either large or small. Each ribosome contains at least one large rRNA and at least one small rRNA. In the nucleolus, the large and small rRNAs combine with ribosomal proteins to form the large and small subunits of the ribosome (e.g., 50S and 30S, respectively, in bacteria). (These subunits generally are named according to their rate of sedimentation, measured in Svedberg units [S], in a centrifugal field.) Ribosomal proteins are synthesized in the cytoplasm and transported to the nucleus for subassembly in the nucleolus. The subunits are then returned to the cytoplasm for final assembly.
