How is the process of cell division regulated so that all of the chromosomes attach correctly? The answer lies in
controlling the activity of the "scissors" that divide the cell. In the event that the chromosomes are not working correctly, the
spindle checkpoint de-activates the scissors.
Murray describes the spindle checkpoint in detail, asking "what the spindle checkpoint has to do, whether it can
succeed in doing it, and how that might be relevant to the most obvious and mysterious fact about Down Syndrome, which is that its
incidence rises very sharply as the mothers of children get older and older."
Chromosomes are linked to each other by recombination, a process of cutting and pasting maternal and paternal DNA,
creating a linkage that holds the chromosomes together, and allows them to be instructed that they have been attached to either ends of
this separational machinery. Murray describes an important experiment that suggests that cells know the difference between having their
centromeres correctly attached to opposite ends, as compared to the same ends, of this separational machinery. This experiment demonstrates that
cells have devices that monitor and then attempt to reorient errant chromosomes into their proper positions.
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