I
CHROMOSOMES
Chromosome analysis is also carried
out when a baby is stillborn without an
obvious cause, or when a baby is born
with abnormal physical characteristics
that suggest
a
chromosomal
defect,
such as
Turner’s syndrome
.
The analysis of the sex chromosomes
may be carried out in order to establish
the chromosomal sex of a child whose
genitals have an ambiguous appearance
(see
genitalia, ambiguous
) ;
to confirm or
to exclude the diagnosis of
chromosomal
abnormalities
;
or to investigate
infertility
.
HOW IT IS DONE
Fetal cells for analysis can be obtained
by
amniocentesis
or
chorionic villus sam-
pling
.
Chromosome analysis in children
and adults uses white blood cells taken
from a blood sample.
chromosomes
Threadlike structures that exist inside
the nuclei (see
nucleus
)
of cells and
carry inherited inform ation governing
all
cellular
activities
and
functions.
Each
chromosome
contains
around
3
0 ,0 0 0
genes
(units of inheritance),
w hich are arranged in single file along
a long double filament of
DNA
.
The
sequence of chemical units, or bases,
in the DNA provides the coded instruc-
tions for cellular activities. Each cell
Appearance of a dividing chromosome
Just after a chromosome has copied itself, the
copies are joined at a constriction (centromere)
that divides them into long and short “arms”.
contains the chemical machinery for
decoding these instructions (see
genet-
ic code; nucleic acids).
All
of an
individual’s
body
cells
(with the exception of egg or sperm
cells)
carry
the
same
chromosomal
material, w hich has been copied by a
process of
cell division
from the original
material
in
the
fertilized
egg.
Each
human cell norm ally contains
46
chro-
mosomes made up of
23
pairs. H alf of
each pair of chromosomes comes from
the mother and half from the father.
TYPES OF CHROMOSOMES
In each set of chromosomes,
22
pairs
are autosomal, w hich are the same in
both sexes, and the remaining pair are
the sex chromosomes.
There are two types of sex chromo-
some, called X and Y. In females, the sex
chromosomes are a matched pair of X
chromosomes. In males, one is an X
and the other is a Y. The mother’s egg
contributes an X chromosome to every
offspring. The father’s sperm contri-
butes the other chromosome in the
pair: an X in girls or a Y in boys.
The Y chromosome is thought to
provide all of the information required
for the development of male sexual
characteristics. In the absence of the Y
chromosome,
the
female
pattern
of
development takes place.
CHROMOSOME DIVISION
W hen
a
cell
divides,
all
of
its
components,
including
the
chromo-
somes,
are
duplicated into the two
offspring cells.
The process by w hich most body
cells divide is called
mitosis
.
Shortly
before cell division, the DNA in each
chromosome is copied; if viewed under
a microscope,
chromosomes
at
this
stage appear as double rods joined at an
area called the centromere. As cell divi-
sion proceeds, the duplicated chromo-
somes are pulled apart, divided at the
centromeres, so that each daughter cell
w ill receive a single copy of each of the
usual
46
chromosomes.
C
PROCEDURE FOR CHROMOSOME ANALYSIS
I
In antenatal testing,
a collection of fetal
cells is obtained by
amniocentesis or by
chorionic villus sampling.
In a test on a baby, child,
or adult, white blood cells
are obtained from a
sample ofblood.
2
The cells are
suspended in a
medium containing
substances that
encourage them to
divide. Chemicals are
then added that stop
the cells from dividing
at a stage where their
chromosome content
is most easily visible.
3
The cells are spread on a
microscope slide and stained.
A selected few (in which the
chromosomes are clearly visible
and well separated) have their
nuclei photographed or are
closely examined through a high-
powered microscope.
Ü n
U
M
n
иànnnnn
и
и и
13
14
15
XX
XX
XX
16
17
18
XX
19
XX
20
XX
XX
21
22
и
X Y
4
The chromosomes
are matched up and
arranged into the 22 pairs
of autosomes and the sex
chromosomes. A study of
the complete setwill
reveal any abnormalities.
173
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