A metaphase spindle formed in vitro by adding Xenopus sperm to an extract of Xenopus eggs (see Figure 17-9) has incorporated three fluorescent markers: rhodamine-labeled tubulin (red) to mark all of the microtubules, a blue DNA-binding dye that labels the chromosomes, and caged-fluorescein-labeled tubulin, which is also incorporated into all of the microtubules but is invisible because it is nonfluorescent until activated by ultraviolet light. (A) The distribution of the chromosomes and microtubules in the spindle. (B) A beam of ultraviolet light was used to uncage the caged-fluorescein-labeled tubulin locally, mainly just to the left side of the metaphase plate. Over the next few minutes (after 1.5 minutes in C, after 2.5 minutes in D), the uncaged fluorescein-tubulin signal is seen to move toward the left spindle pole, indicating that tubulin is continuously moving poleward, even though the spindle (visualized by the red rhodamine-tubulin fluorescence) remains largely unchanged. The caged fluorescein signal also diminishes in intensity, indicating that the individual microtubules are continually depolymerizing and being replaced. (From K.E. Sawin and T.J. Mitchison, J. Cell Biol. 112:941–954, 1991. © The Rockefeller University Press.)