The Golgi apparatus, however, is a transit place only for a fraction of the proteins which are actually produced by eukaryotic cells. The synthesis of all such proteins invariably begins in the soluble part of the cytoplasm (the cytosol), and during this first step they also receive a signal that specifies their geographic destination. The piece of amino acid chain that is emerging first from the ribosome machine – the so-called peptide leader – can contain a sequence that the cell interprets as an exportation signal to the endoplasmic reticulum. If such signal is present, the ribosome binds to the reticulum and delivers the protein in its lumen. If the peptide leader does not carry such a signal, the synthesis continues on free ribosomes, and the resulting proteins are shed in the cytosol. Of these, however, only a fraction is destined to remain there, because the amino acid chain can carry, in its interior, one or more signals which specify other destinations. More precisely, there are signals for protein exportation to the nucleus, to mitochondria, and to other cell compartments. Proteins, in conclusion, are carrying with them the signals of their geographic destination, and even the absence of such signals has a meaning, because it implies that the protein is destined to remain in the cytosol.
The crucial point is that there is no necessary correspondence between protein signals and geographic destinations. The exportation-to-the-nucleus signals, for example, could have been used for other compartments, or could have been totally different, just as the names which are given to cities, to airports and to holiday resorts. The existence of eukaryotic compartments, in conclusion, is based on natural conventions, and to these rules of correspondence we can legitimately give the name of compartments codes.

Chromosomes

Bacteria have a single chromosome which has a circular form and no stable association with structural proteins, while eukaryotes contain various chromosomes which are open-ended (or linear) molecules, and bind large amounts of structural proteins which fold the DNA thread many times over.