darush at pacbell.net
Mon Apr 7 11:52:12 EST 1997
Hopefully this will answer your question:
Each of the two joined chromatids of a chromosome has a protein
structure called a kinetochore located at the centromere region. Some
microtubules called kinetochore microtubules attach to the kinetochore.
During prometaphase, kinetochore microtubules from one pole of the cell
may attach to a kinetochore first, and the chromosome begins to move
toward that pole. However, this movement is checked as soon as
micrtubules from the opposite pole attach to the chromosome's other
kinetochore. What happens next is like a tug-of-war that ends in a draw.
The chromosome moves first in one direction,then the other, back and
forth, finally settling at the cell's midpoint. Apparently, microtubules
can only remain attached to a kinetochore when there is a force exerted
on the chromosome from the opposite pole of the cell. If microtubules
from just one pole attach to a chromosome, they lose their grip.
Attachment to one kinetochore is stabilized only when microtubules from
the opposite pole hook to the other kinetochore. This check-and-balance
system equalizes the number of microtubules attached to the two
kinetochores of a duplicated chromosome and moves the chromosome to the
midline of the cell. At metaphase, all duplicated chromosomes are
aligned on the cell's midline, or metaphase plate.
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