Final answer:
Microtubules consist of 13 protofilaments made up of alpha-tubulin and beta-tubulin, forming a circular pattern that creates the microtubule's hollow structure with a diameter of about 25 nm.
Step-by-step explanation:
Microtubules are essential components of the cell's cytoskeleton. The wall of a microtubule is composed of 13 protofilaments, which are longitudinal rows of polymerized alpha-tubulin and beta-tubulin dimers. These protofilaments align side by side in a circular pattern to form the hollow structure of the microtubule. With a diameter of approximately 25 nm, microtubules assist in maintaining cell shape, facilitate intracellular transport, and are critical during cell division as they pull replicated chromosomes apart. They can also quickly disassemble and reform, which is necessary for their various functions within the cell.