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# Long description

In (a) the microfilament comprises two chains of actin monomers in a loose spiral structure the diameter of which is about 6 nanometres. Part (b) includes a schematic cross-section through a microtubule which is a ring of 13 tubulin monomers each of diameter about 5 nanometres; there is also a side-on view of part of a microtubule, showing the dumb-bells-shaped tubulin monomers arranged in a spiral structure to form the tubule. The external diameter of the tubule is about 25 nanometres. The intermediate filament shown in part (c) comprises about eight narrow protofilaments, which do not have a subunit structure. The protofilaments are wound around each other in a rope-like structure 8-10 nanometres in diameter.