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Department of Molecular, Cellular, and Developmental Biology,
University of Colorado, Boulder, Colorado 80309
Mitotic movements of chromosomes are usually coupled to the
elongation and shortening of the microtubules to which they are bound.
The lengths of kinetochore-associated microtubules change by incorporation or loss of tubulin subunits, principally at their chromosome-bound ends. We have reproduced aspects of this phenomenon in
vitro, using a real-time assay that displays directly the movements of
individual chromosome-associated microtubules as they elongate and
shorten. Chromosomes isolated from cultured Chinese hamster ovary cells
were adhered to coverslips and then allowed to bind labeled
microtubules. In the presence of tubulin and GTP, these microtubules
could grow at their chromosome-bound ends, causing the labeled segments
to move away from the chromosomes, even in the absence of ATP.
Sometimes a microtubule would switch to shortening, causing the
direction of movement to change abruptly. The link between a
microtubule and a chromosome was mechanically strong; 15 pN of tension
was generally insufficient to detach a microtubule, even though it
could add subunits at the kinetochore-microtubule junction. The behavior of the microtubules in vitro was regulated by
the chromosomes to which they were bound; the frequency of transitions
from polymerization to depolymerization was decreased, and the speed of
depolymerization-coupled movement toward chromosomes was only one-fifth
the rate of shortening for microtubules free in solution. Our results
are consistent with a model in which each microtubule interacts with an
increasing number of chromosome-associated binding sites as it
approaches the kinetochore.
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