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1. A submicroscopic cluster of proteins and RNA, occurring in great numbers in the cytoplasm of living cells, which takes part in the manufacture of proteins.
2. A tiny, somewhat mitten-shaped organelle occurring in great numbers in the cell cytoplasm either freely, in small clusters, or attached to the outer surfaces of endoplasmic reticula, and functioning as the site of protein development.

This is the cell's essential protein factory; that is, 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 RNA 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 Earth.

Although the ribosome consists of both ribosomal RNA (rRNA) and protein, the "active site" on the large unit of the ribosome; that is, the site of the chemical reaction that changes genetic information into the beginnings of a protein which contains only rRNA.

This suggests that the ribosome is actually a ribozyme, an RNA molecule that can catalyze its chemical reactions.

—Compiled from excerpts found in
MedicineNet, Inc. and Webster's New World Medical Dictionary,
Third Edition; Wiley Publishing, Inc.; Hoboken, New Jersey; 2008.