Molecular rulers?

What is a ‘molecular ruler’? The lengths of many linear polymers in biology are specified with remarkably accuracy, indicating they have very similar or the same numbers of subunits. Examples include bacteriophage tails, bacterial flagellum hooks, bacterial ‘injectisome’ needles, and the thin and thick filaments of striated muscles. Such exact assembly requires explanation, because the same subunits in the test-tube generally polymerize to highly variable lengths. One attractively simple explanation for precise length is that there is a ‘ruler’ molecule whose size measures off the final size of the growing polymer. An interaction between the ruler and the polymer might involve only the ends of both — a simple ruler — or interactions could occur along their entire lengths through multiple binding sites, with the ruler acting as a template. In either case, a change in polymerization conditions will occur when the polymer reaches the end of the ruler, stopping further elongation and possibly also initiating binding of a terminator or capping molecule.