ribosome

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ribosome A subcellular granule, 10–20 nm in diameter, composed of RNA and protein, which is found in large numbers in all types of cells and in some subcellular organelles. Ribosomes are the site of protein synthesis: m-RNA attaches to them and there receives t-RNA molecules bearing amino acids. In eukaryotic cells ribosomes are synthesized in the nucleolus but are found predominantly in the cytoplasm, singly or in chains (polysomes or polyribosomes, probably linked by the m-RNA), or attached to the endoplasmic reticulum, which is then termed ‘rough ER’. Ribosomes have a sedimentation factor: 70S in bacteria; 80S in eukaryotic cytoplasm; and 70S in mitochondrial ribosomes. See MITOCHONDRION.

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