ribosome
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 they 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’. There are two main types of ribosomes, distinguished by their size: 70S ribosomes occur in prokaryotes and in the matrix of chloroplasts and mitochondria; the slightly larger 80S ribosomes occur in the cytoplasm of eukaryotes. (The ‘S’ refers to the Svedberg unit.)
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ribosome
