FIGURE 1:

(A) Microtubules are polymers of α/β-tubulin dimers typically composed of 13 protofilaments. (B) Adjacent protofilaments have a longitudinal offset of 9.2 Å between tubulins such that 13-protofilament microtubules have straight protofilaments (left). The lattice of non–13-protofilament microtubules (14 protofilaments shown) must accommodate by imposing a protofilament supertwist (right; not to scale). (C) The functional unit of cilia and flagella (left) is the 9+2 axoneme, with 2 central pair singlets and 9 outer microtubule doublets (middle). Doublets (right) are composed of an incomplete microtubule (10-protofilament B-tubule) clutched to the side of a complete 13-protofilament microtubule (A-tubule). pf, protofilament.