###### Ribosomes, polyribosomes  and protein synthesis
Ribosomes are macromolecular machines that catalyse the synthesis of
proteins from amino acids; synthesis and assembly into subunits takes
place in the nucleolus and includes the association of ribosomal RNA
(rRNA) with ribosomal proteins translocated from their site of synthesis
in the cytoplasm. The individual subunits are then transported into the
cytoplasm, where they remain separate from each other when not
actively synthesizing proteins. Ribosomes are granules approximately
25 nm in diameter, composed of rRNA molecules and proteins assembled into two unequal subunits. The subunits can be separated by their
sedimentation coefficients (S) in an ultracentrifuge into larger 60S and
smaller 40S components. These are associated with 73 different proteins (40 in the large subunit and 33 in the small), which have structural
and enzymatic functions. Three small, highly convoluted rRNA strands
(28S, 5.8S and 5S) make up the large subunit, and one strand (18S) is
in the small subunit.