title: Chromosomes attain a metaphase position on half-spindles in the absence of an opposing spindle pole.
abstract: To examine the relative roles of chromosomes, spindle poles and microtubules in the formation of the metaphase spindle and metakinesis, I have experimentally placed an extra centrosome-free pronucleus close to a forming bipolar spindle in a living cell. The chromosomes from the extra nucleus induce the formation of an extra half-spindle from one pole of the otherwise normal bipolar spindle with chromosomes positioned at the putative metaphase plate. I conclude that chromosomes determine the location of half-spindles by sustaining a higher than normal density of microtubules. These results are surprising for two reasons: first, because previous in vivo experiments in tissue culture cells show that mono-oriented chromosomes with functional attachments to spindle microtubules do not support half-spindle formation but oscillate unstably or move to one spindle pole. Additionally, the generally accepted view is that chromosomes attain a metastable condition at the metaphase plate as a result of a balance between forces directed to opposite spindle poles. However, our observation that chromosomes on extra half-spindles attain a metastable position in the absence of an opposing spindle pole, suggests that Ostergren's model does not account for metakinesis in sea urchin embryos.
authors.affiliation: Zoology Department, University of California, Davis 95616.
authors.forename: R J
authors.initials: RJ
authors.lastname: Leslie
mesh_terms: D000818:Animals; D002875:Chromosomes / Q000502:physiology* / Q000648:ultrastructure; D004625:Embryo, Nonmammalian / Q000648:ultrastructure; D008677:Metaphase / Q000502:physiology*; D008870:Microtubules / Q000502:physiology / Q000648:ultrastructure; D012617:Sea Urchins; D008941:Spindle Apparatus / Q000502:physiology* / Q000648:ultrastructure