A typical cell contains millions of ribosomes. They may form groups

(polyribosomes or polysomes) attached to messenger RNA (mRNA),
which they translate during protein synthesis for use outside the system
of membrane compartments, e.g. enzymes of the cytosol and cytoskeletal proteins. Some of the cytosolic products include proteins that can
be inserted directly into (or through) membranes of selected organelles,
such as mitochondria and peroxisomes. Ribosomes may be attached to
the membranes of the rough endoplasmic reticulum (see Fig. 1.5E).

In a mature polyribosome, all the attachment sites of the mRNA are