Our 9th grade biology text describes the procedure usedmto create a
karyotype of human chromosomes. It involves getting a blood sample,
treating it with a chemical that stops cell division, adding water
which causes the blood cells to expland and break open, spinning down
the broken cells, and then taking a sample of that and looking at it
under the microscope. A student has asked me if the chromosomes come
from many blood cells, how does a person know if there are too many
or too few chromosomes in one cell if the chromosomes from all of the
cells are mixed together. I can't seem to find an answer.
I'm sorry it took so long to get back but I had to ask someone about the
answer. What a great question for a kid to ask. In all the years I have
been teaching about karyotyping no one has EVER asked that question. You can
tell your student it took some thinking to come up with it. Anyway-when you
make the cell mixture the cells become diluted. A chemical is added to make
the nuclear membrane unstable. The pipet is held above the slide at a
distance and then a drop, containing many cells, is dropped onto a slide.
The cells break as they hit the slide. Sometimes the cells fall very close
to each other and the chromosomes mix together. These would not be used.
The technologist keeps scanning the slide until an isolated cell "plop" is
found with all of the chromosomes isolated from another cell's chromosomes.
I learned something new myself. Thanks for asking the question!
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Update: June 2012