Polysomes. (a) Polysomes consist of multiple ribosomes associated with a single mRNA. (b) Polysomes can be separated on a sucrose density gradient. The polysome profile indicates total RNA content at increasing sucrose density. The results show that 30S and 50S ribosomal subunits and 70S ribosomes migrate near the top of the gradient, whereas polysomes, comprising two or more ribosomes associated with mRNA transcripts, migrate in the heavier (more dense) fractions of the gradient.