Answer:
25nm
Step-by-step explanation:
Microtubules are polymers of tubulin and they are part of the cytoskeleton. The roles of the microtubules are to maintain the shape and structure of the cell, intracellular transport, they are structural part of the cilia an flagella, they are involved in cell division etc.
Tubulin that is the basic unit of microtubules is composed of two globular proteins, alpha and beta tubulin. Alpha and beta subunits spontaneously form dimers which then assemble into oligomers, then in chains (protofilaments); that can form a variety of intermediate structures such as single and double rings or spirals. Protofilaments that associate side-by-side form sheets of protofilaments which then curle to form a tube or cylinder (closing of microtubule). Continued growth of microtubules occurs by the direct addition of more dimers on + side.