Cooperation of kinetochores and pole in the establishment of monopolar mitotic apparatus.

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Abstract
Monopolar mitotic apparatus can be produced in sea urchin [Strongylocentrotus purpuratus] eggs by a maneuver that distributes the 4 poles of the 2nd mitosis into 4 separate blastomeres. The pole of the monopolar mitotic apparatus generates a 1/2-spindle that is similar in structural details to the 1/2-spindle of a normal bipolar mitotic apparatus, although the chromosomes are not as well aligned as in a normal metaphase plate. The chromosomes are oriented; 1 kinetochore faces the pole; its sister kinetochore faces away from the pole. The poleward kinetochore is connected to the pole by bundles of microtubules. No microtubules are seen on the sister kinetochore that faces away from the pole. A single pole can direct most of the events in the establishment of a mitotic apparatus. The cooperation of kinetochores and poles are examined in the formation of microtubules between them, stressing the 1/2-spindle as the medium of cooperation and leaving open the question whether the kinetochores are origins or terminations of microtubules.