The ribosome binds to one end of a messenger RNA and initiates protein biosynthesis, moving along the mRNA. mRNAs that are being translated actively are associated with multiple ribosomes, forming polysomes. Because the length of mRNA covered by one ribosome is 30 to 35 nucleotides, the size of the polyribosome varies, depending on the length of the mRNA as well as on its translatability.
In prokaryotic cells, where translation and transcription are coupled in the same cellular compartment, mRNA is bound with ribosomes to start translation even before the transcript is complete, forming polysomes. The eukaryotic translation machinery has been found to be localized near the nuclear pore complex and to bind to the Cap signal of mRNA when translocated from the nucleus to the cytoplasm. During translation of eukaryotic secretary or membrane-associated proteins, ribosomes are bound to the endoplasmic reticulum (ER) through the signal peptide of the nascent polypeptide. The polysomes are visible there, giving this part of the endoplasmic reticulum the name rough endoplasmic reticulum.
