EXPERIMENTAL FIGURE 18-8 Microtubules grow preferentially at the (+) end. A fragment of a microtubule bundle from a flagellum was used as a nucleus for the in vitro addition of αβ-tubulin. The nucleating flagellar fragment is the thick bundle seen in this electron micrograph, with the newly formed microtubules (MT) radiating from its ends. The greater length of the microtubules at one end, the (+) end, indicates that tubulin subunits are added preferentially to this end.

[Courtesy of Gary G. Borisy.]