Complex sentence: These α/β-tubulin dimers polymerize end-to-end into linear protofilaments that associate laterally to form a single microtubule, which can then be extended by the addition of more α/β-tubulin dimers.
Split into simpler sentences: These α/β-tubulin dimers polymerize end-to-end into linear protofilaments that serve as building-blocks for the microtubule structure. These protofilaments associate laterally to form a single microtubule, which can then be extended by the addition of more α/β-tubulin dimers.