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Ribosomes are responsible for assembling the proteins of the cell.
Ribosomal subunits are synthesized by the nucleolus . Depending on the protein production level of a particular cell, ribosomes may number in the millions. Ribosome attached to endoplasmic reticulum, Image courtesy of The Virtual Cell . Small and large ribosome subunits, Image courtesy of The Virtual Cell . These ribosomes are called free ribosomes and bound ribosomes respectively.