polysomes

Polysomes, or polyribosomes, are clusters of ribosomes bound to a single messenger RNA (mRNA), working in concert to translate the genetic message into protein. In eukaryotes they can be free in the cytosol or attached to the rough endoplasmic reticulum, forming membrane-bound polysomes; in prokaryotes they are typically free in the cytoplasm. A polysome contains multiple ribosomes, each at a different stage of elongation, spaced along the mRNA so that several copies of the polypeptide can be synthesized in parallel.

Translation occurs as the ribosome moves along the mRNA in the 5' to 3' direction, each ribosome synthesizing a polypeptide that emerges from the ribosomal exit tunnel. The presence of multiple ribosomes on a single transcript increases protein output and coordinates production of proteins required in large amounts. The ribosome size differs: in bacteria, ribosomes are 70S (30S and 50S subunits); in eukaryotes they are 80S (40S and 60S).

Polysome formation can be assessed by sucrose gradient centrifugation, which separates free ribosomal subunits, monosomes (single ribosome bound to mRNA), and polysomes. The proportion of mRNA in polysome fractions reflects translation activity. Polysome profiling combined with RNA analysis provides a snapshot of actively translated mRNAs and is widely used to study translational regulation under different conditions.