Final answer:
A polyribosome, or polysome, is a structure where an mRNA molecule is translated by many ribosomes at once, increasing the efficiency of protein synthesis.
Step-by-step explanation:
A polyribosome, or polysome, is an mRNA transcript that has many bound ribosomes in the act of translation. This structure consists of a single mRNA molecule being simultaneously translated by multiple ribosomes, generally all moving in the 5' to 3' direction. This enhances the efficiency of protein synthesis, as each ribosome works to assemble a polypeptide chain, effectively producing multiple copies of a protein very swiftly compared to what would be possible with a single ribosome.
In prokaryotic cells, transcription and translation can occur concurrently, leading to the formation of polysomes before the transcription of mRNA is even complete, which allows for a rapid response to environmental changes. In contrast, in eukaryotic cells, polysomes form only after RNA synthesis is complete and the RNA molecule has been modified and exported out of the nucleus.