In prokaryotes, ribosomes attach to the $5^{\prime}$ end of mRNA as soon as transcription begins. A bunch of ribosome moves along a single mRNA molecule adding 15 amino acids/second to the polypeptide chain, almost the same speed at which RNA polymerase transcribes the mRNA. The group of ribosomes together with the single mRNA molecules, they are translating is called polysome. The use of polysomes is advantageous to a cell since, the overall rate of protein synthesis is increased compared to the rate that would occur if there were no polysomes.
In eukaryotic cells the ribosomes are attached to rough endoplasmic reticulum by ribophorin protein. Electron microscopy reveals that membranes of homogenised endoplasmic reticulum disrupt to form closed vesicles called microsomes. Microsomes derived from rough endoplasmic reticulum are studied with ribosomes and are called rough ribosomes.