mRNA is transported to the cytosol from the nucleus and is used as informative matrix for the synthesis of protein polypeptide chains by ribosomes (translation). Large and small ribosomal subunits with rRNA and 75 different proteins are also assembled in the nucleus and thus transported into the cytoplasm through a nuclear pore complex. The polypeptide chain synthesis only starts when a ribosome is completely assembled from subunits attached to a mRNA strand. It means that in a cell there are no free ribosomes that are not synthesizing peptide chains. More than that, a single mRNA strand is generally used to produce a protein by several ribosomes at the same time. These ribosomes joined to the same mRNA (12-15 of them usually) are organized in round-shaped clusters or polyribosomes (polysomes) #1, #2. If a protein is intended for cytosol-associated or other similar function (e.g., regulatory proteins, proteins of cytoskeletal fibrillar structures, the majority of mitochondrial proteins), it is produced by free polysomes of the cytosol. In contrast, cell membrane proteins, lysosomal enzymes, and exported proteins are produced by ribosomes that are attached to the rough endoplasmic reticulum (for more details, see further). The proteins are always enclosed by membranes or are incorporated into them.

The image depicts a fragment of a cardiac muscle cell including its perinuclear space with no contractile structures that are found further: myofibrils #1, #2. The polyribosomes of this perinuclear cytosol are probably producing proteins to construct myofibrils (actin and actin-associated proteins, myosin, etc.). Nuclear pores #1, #2, #3 are concentrated on the nuclear envelope surface that is near the protein-producing area. The synthesis requires energy provided by mitochondria #1, #2. The mitochondria of cardiac muscle cells are marked by a large number of inner membrane folds or cristae.