The ribosome is one of the most abundant molecular complexes in the cell: a single bacterial cell may contain as many as 20,000 ribosomes, and eukaryotic cells possess even more. Ribosomes typically contain about 80% of the total cellular RNA. They are complex structures, each consisting of more than 50 different proteins and RNA molecules. A functional ribosome consists of two subunits, a large ribosomal subunit, and a small ribosomal subunit, each of which consists of one or more RNA molecules and a number of proteins. The sizes of the ribosomes and their RNA components are given in Svedberg (S) units (a measure of how rapidly an object sediments in a centrifugal field). (It is important to note that S units are not additive: combining a 10S structure and a 20S structure does not necessarily produce a 30S structure because the sedimentation rate is affected by the three-dimensional structure of an object as well as by its mass). The three-dimensional structure of the bacterial ribosome has been elucidated in great detail through X-ray crystallography.
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