the polysome
/ POL-ee-some /
If one ribosome reading one mRNA makes one protein at a time, the cell faces an obvious bottleneck when it needs many copies of a protein fast. Its elegant solution is not to wait: as soon as the first ribosome has moved a little way along the mRNA, a second ribosome hops on at the start and follows behind, then a third, and so on. A single mRNA threaded with many ribosomes at once is a polysome.
A polysome looks, under the electron microscope, like beads on a string: a row of ribosomes spaced out along one mRNA, each at a different stage of reading the same message, each trailing a protein chain of a different length. They all move in the same direction (5' to 3') like a line of readers following one another through the same sentence. The closer the ribosomes are packed, the more protein per second that mRNA yields. In bacteria, where there is no nucleus to separate the steps, polysomes can even form on an mRNA that is still being transcribed — ribosomes start translating one end while RNA polymerase is still writing the other (coupled transcription and translation).
The polysome is why a cell can make a lot of a given protein without needing a lot of copies of its gene or its mRNA — many ribosomes squeeze maximum output from each message. The number of ribosomes loaded on an mRNA is also a readable signal: researchers measure it (in 'polysome profiling') to see which messages a cell is actively translating right now, which can differ sharply from which mRNAs are merely present. A heavily loaded mRNA is being read hard; a bare one is sitting idle.
A single mRNA can carry many ribosomes at once — densely translated messages may hold a ribosome roughly every eighty nucleotides — so one mRNA simultaneously produces many copies of the same protein, each ribosome a little further along than the one behind it.
Many ribosomes read one mRNA at once, like beads on a string, multiplying output.
Every ribosome on a polysome is reading the same mRNA in the same frame — they do not make different proteins. They are simply staggered copies of one job, multiplying how much of one protein is produced.