FIGURE 18-11 Dynamic instability depends on the presence or absence of a GTP-β-tubulin cap. Images taken in the electron microscope of frozen samples of a growing microtubule (upper) and a shrinking microtubule (lower). Notice that the end of the growing microtubule has a blunter end, whereas the end of the shrinking one has curls like a ram’s horns. As the diagram shows, a microtubule with GTP-β-tubulin on the end of each protofilament is strongly favored to grow. However, a microtubule with GDP-β-tubulin at the ends of the protofilaments forms a curved structure and will undergo rapid disassembly. Switches between growing and shrinking phases, called rescues and catastrophes, can occur, and the rate of switching is regulated by associated proteins. See A. Desai and T. J. Mitchison, 1997, Annu. Rev. Cell Dev. Bi. 13:83–117.