- The cell's essential protein factory. 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 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 thus support the idea that life on Earth began with RNA.
* * *A granule of ribonucleoprotein, 120–150 Å in diameter, that is the site of protein synthesis from aminoacyl-tRNAs as directed by mRNAs. SYN: Palade granule.
* * *ri·bo·some 'rī-bə-.sōm n any of the RNA- and protein-rich cytoplasmic granules that are sites of protein synthesisri·bo·som·al .rī-bə-'sō-məl adj
* * *n.a particle, consisting of RNA and protein, that occurs in cells and is the site of protein synthesis in the cell (see translation). Ribosomes are either attached to the endoplasmic reticulum or free in the cytoplasm as polysome.• ribosomal adj.
* * *ri·bo·some (riґbo-sōm) [ribonucleic acid + -some] a large molecular structure, approximately 12 nm wide and 25 nm long, having two dissociable subunits, that is the site of protein synthesis (see translation). Ribosomes found in the cytosol of eukaryotes have a sedimentation coefficient of 80S; the subunits have coefficients of 60S and 40S and together contain four different ribosomal RNA (rRNA) chains and about 80 different proteins. Ribosomes found in prokaryotes and mitochondria are smaller (70S, with 50S and 30S subunits containing a total of three rRNAs and 55 proteins) and also differ from eukaryotic ribosomes in their sensitivity to certain antibiotics. ribosomal adj
Medical dictionary. 2011.