SELF-ORGANIZATION OF MICROTUBULES INTO BIPOLAR SPINDLES AROUND ARTIFICIAL CHROMOSOMES IN XENOPUS EGG EXTRACTS

Functional nuclei and mitotic spindles are shown to assemble around DN A-coated beads incubated in Xenopus egg extracts. Bipolar spindles ass emble in the absence of centrosomes and kinetochores, indicating that bipolarity is an intrinsic property of microtubules assembling around chromatin in a mitotic cytoplasm. Microtubules nucleated at dispersed sites with random polarity rearrange into two arrays of uniform polari ty. Spindle-pole formation requires cytoplasmic dynein-dependent trans location of microtubules across one another. It is proposed that spind les form in the absence of centrosomes by motor-dependent sorting of m icrotubules according to their polarity.