Murray explains the basic principles of cell division, where each chromosome is copied, paired with a sister chromosome,
and then separated to produce two cells. However, when sister chromosomes are segregated incorrectly, a cell can be created that is
missing a chromosome. As an example of what happens when a cell is missing a chromosome, Murray describes retinoblastoma, a cancer of
the eye developed in children. In Down Syndrome, the mechanism that segregates chromosomes into two different cells fails, leaving an
extra copy of chromosome 21 inside a cell destined to become an egg.
"What are the things that we need to make sure that the chromosomes segregate properly?" asks Murray. "First of all, we
need...some dynamic mechanical structure that will move the chromosome through space, organize the cell, and instruct where the purse
string is to form that will pinch the cell in half.... Finally, we'd like a checking mechanism... to make sure that everything's been
lined up correctly." Murray describes the "locomotive tracks," or microtubules, that search through space looking for sister chromosomes
to drag apart. Once chromosomes have been located and attached to microtubules, a molecular protein cuts the hoops that bonds the sister
chromosomes together; the chromosomal separation machinery moves apart; and the cell pinches itself in two, creating two cells.
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