ribosome

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ribosome A small spherical body within a living cell that is the site of protein synthesis. Ribosomes consist of two subunits, one large and one small, each of which comprises a type of RNA (called ribosomal RNA) and protein. Ribosomes are described in terms of their sedimentation coefficients (i.e. their rates of sedimentation in an ultracentrifuge), which are measured in Svedberg units (symbol S). The prokaryote (70S) ribosome comprises a 50S (large) subunit and a 30S (small) subunit; the eukaryote (80S) ribosome has large 60S and small 40S subunits. Usually there are many ribosomes in a cell, either attached to the endoplasmic reticulum or free in the cytoplasm. During protein synthesis they are associated with messenger RNA as polyribosomes in the process of translation.

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