Chromosomes and human GenetiCs 387D Put cells on
a microscope
slide. Observe.E Photograph one cell through microscope.
Enlarge image of its chromosomes. Cut the
image apart. Arrange chromosomes as a set.B Centrifugation forces cells to
bottom of tube. Draw off culture
medium. Add a dilute saline
solution to tube. Add a fixative.C Prepare and
stain cells for
microscopy.A Add cells from a small blood
sample to a medium that has
a chemical stimulator for mitosis.
Add colchicine to arrest mitosis
at metaphase. Transfer culture to
a centrifuge. (This motor-driven
rotary device spins test tubes
at high speed. Tube contents
respond to the centrifugal force
according to their mass, density,
and shape.) Charles D. Winters/Science Source123456713 14 15 16 17 18 19 20 21 22 XX (or XY)8910 11 12F
a diagram called a karyotype can help answer questions
about a person’s chromosomes. Chromosomes are the
most condensed and easiest to identify at the phase of
mitosis called metaphase (section 18.3).
a technician who wants to make a karyotype doesn’t
assume that it will be possible to find a body cell that
is dividing. instead, cells are cultured in the laboratory
along with chemicals that stimulate the cells to grow
and to divide by mitosis. Blood cells are often used for
this purpose.
once the cell culture is established, a chemical called
colchicine is added to stop mitosis at metaphase. after
the colchicine treatment, the
“soup” of cultured cells is
placed into glass tubes that
are whirled in a centrifuge.
the spinning force moves
the cells to the bottom of
the test tubes.
next the cells are transferred to a saline (salt–water)
solution. they swell (by osmosis) and separate, as do
the metaphase chromosomes. at this point the cells are
placed on a microscope slide, “fixed” (stabilized by
air-drying or some other method), and stained so that
they are easy to see.
the chromosomes are photographed through the
microscope, and the image is enlarged. then the
photograph is cut apart, one chromosome at a time. the
cutouts are arranged in order, essentially from largest to
smallest (Figure 20.3). the sex chromosomes are placed
last. all pairs of homologous chromosomes are aligned
horizontally, by their centromeres. Figure 20.3F shows a
karyotype diagram prepared this way.picturing Chromosomes with Karyotypes
Figure 20.3 Animated! A karyotype gives a portrait of
the chromosomes in a cell. A–E The diagrams show how to
prepare a karyotype. F A human karyotype. Human somatic
cells have twenty-two pairs of autosomes and one pair of sex
chromosomes (XX or XY). These are metaphase chromosomes
from a female; each has been duplicated. In the orange box
at the far right are the two sex chromosomes (XY) of a male.
(© Cengage Learning)20.2
karyotype A preparation of
an individual’s metaphase
chromosomes arranged by
length, shape, and the loca-
tion of the centromere.SCIENCE COMES TO LIFE
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