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, or rRNA) and numerous proteins. In eukaryotes, large and small subunits are assembled at the nucleolus and then exit the nucleus, via the nuclear pores, to become fully functional ribosomes in the cytoplasm. During translation, the ribosome subunits assemble around the messenger RNA; the ribosome also has three binding sites for transfer RNA molecules. 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. The eukaryote large ribosomal subunit comprises three different rRNA molecules plus 49 different proteins; the small subunit contains a single rRNA and 33 proteins. Usually there are many ribosomes in a cell, either attached to the endoplasmic reticulum or nuclear envelope or free in the cytoplasm. During protein synthesis they are associated with messenger RNA as polyribosomes in the process of translation.