Even before an mRNA is translated, a cell must invest energy to build each of its ribosomes. In E. coli, there are between 10,000 and 70,000 ribosomes present in each cell at any given time.

A ribosome is a complex macromolecule composed of structural and catalytic rRNAs and many distinct polypeptides.

In eukaryotes, the nucleolus is completely specialized for the synthesis and assembly of rRNAs. Ribosomes exist in the cytoplasm of prokaryotes and in the cytoplasm and rough endoplasmic reticulum of eukaryotes.

Mitochondria and chloroplasts also have their ribosomes in the matrix and stroma, which look more similar to prokaryotic ribosomes (and have similar drug sensitivities) than the ribosomes just outside their outer membranes in the cytoplasm.

Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassociate during the initiation of translation.