rRNA: Ribosomal RNA, a molecular component of a ribosome, the cell's essential protein factory. Strictly speaking, ribosomal RNA (rRNA) does not make proteins. It makes polypeptides (assemblies of amino acids) that go to make up proteins.
A tiny particulate structure located in the cytoplasm of the cell (outside the nucleus), the ribosome is composed of two subunits, one larger than the other. Both subunits were believed to contain both rRNA and protein but this is not the full story. Molecular maps of the ribosome have revealed startling details about its structure that boost support for an "RNA world" as the model for the origin of life on Earth. Although the ribosome consists of both ribosomal RNA (rRNA) and protein, the "active site" on the large unit of the ribosome -- the site of the chemical reaction that changes genetic information into the beginnings of a protein -- contains only rRNA. This suggests that the ribosome is actually a ribozyme, an RNA molecule that can catalyze its chemical reactions. RNA's starring role in the ribosome may therefore support the idea that life on Earth began with RNA.