Cilia and flagella contain a structure called an axoneme at their centers (see INTERNET LINK here). An axoneme is composed of microtubules. The internal structure of the axoneme is truly remarkable. The most obvious feature is the arrangement of microtubules known as a 9 + 2 array: two central microtubules ringed by nine microtubule doublets. The single microtubules in the center are complete, each having 13 of tubulin dimers. By contrast, each of the nine surrounding doublets is composed of one complete microtubule to which is fused an incomplete microtubule, carrying only 10 or 11 protofilaments. Closer inspection of electron micrographs reveals even greater complexity, as diagrammed in Figure 8.23. The outer doublets are periodically interconnected by a protein called nexin and carry at regular intervals sidearms composed of the protein dynein. In addition, radial spokes, each consisting of a head and an arm, project from the outer doublets to connect with the central pair.

Another form of dynein, called cytoplasmic dynein, is a molecular motor that transports materials from the plus end of the microtubule toward the minus end. The protein kinesin transports them in the opposite direction.

See also: Motions of Cilia and Flagella, Intracellular Transport of Materials, Microtubule Systems