Definition of Ribosomal RNA Ribosomal RNA: A molecular component
of a ribosome, the cell's essential protein factory. Ribosomal
RNA (rRNA) fabricates polypeptides (assemblies of amino acids
that 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