show that functional mitotic spindles and nuclei form around DNA-coated beads incubated in Xenopus egg extracts and that bipolar arrays of microtubules are formed in the absence of centrosomes and kinetochores. Indeed, it has also been shown that laser ablation of centrosomes in vertebrate cells inhibits neither spindle assembly nor chromosome segregation. Under this scheme, the shape and size of the mitotic spindle are a function of the biophysical properties of the cross-linking motor proteins.

Chromatin-mediated microtubule nucleation by the Ran GTP gradient