CHRONIC
C
CHROMOSOMES: EGG AND SPERM CELLS
These differ from other body cells in that they contain only 23 chromosomes -
one from each of the 22 autosome pairs plus an X chromosome (in the case of an
egg) and either an X or a Y (in the case of a sperm). Because they have only half
the normal complement, they are called haploid, while other cells are diploid.
Female
Normal diploid cell
XX
Female
Chromosomes
under
microscope
Haploid cells
Egg and sperm
cells have half the
normal number of
chromosomes
and are called
haploid cells
Male
Normal diploid cell
XY
Male
W hen egg or sperm cells are formed,
by a process called
meiosis
,
there are
two
important departures
from
the
norm al process of chromosome divi-
sion. First, after the DNA has been
copied (before division takes place),
some sections of chromosomal materi-
al
are
exchanged between the
two
members of each chromosome pair.
This helps to ensure that each of a per-
son’s
eggs
or
sperm
contains
a
different combination of chromosomal
material, and this is the reason why
siblings (with the exception of identi-
cal twins) have a unique appearance.
Next, the cells undergo two consecu-
tive divisions. Therefore, the original
parent cell gives rise to four separate
egg or sperm cells, each of w hich has
only
23
chromosomes.
DISORDERS
Defective chromosome division during
the formation of eggs and sperm (or,
more rarely, during the first few divi-
sions of a fertilized egg) can lead to
various
chromosomal abnormalities
.
The
precise nature of the abnormality can
be investigated by detailed
chromosome
analysis
.
(See also
genetic disorders
. )
chronic
A term that is used to describe a disor-
der or a set of symptoms that has
persisted over a long time. The term
“ chronic illness” implies a continuing
disease process with progressive deteri-
oration (sometimes despite treatment).
Chronic disorders are usually contrast-
ed w ith
acute
disorders, w hich are of
sudden onset and are short in duration.
A
chronic
illness
produces
little
change in symptoms from day to day,
and an affected person may be able, to
carry out normal activities. The term
“acute” ,
in
contrast,
suggests
rapid
onset of severe symptoms such as high
fever, intense pain, or breathlessness,
with a rapid change in the person’s
condition from one day to the next.
A person w ith a chronic disease may
suffer an acute exacerbation (flare-up)
of symptoms. On the other hand, peo-
ple who have had a
stroke
or some
other acute illness may be left with
permanent disabilities, but their condi-
tion is not chronic.
chronic bronchitis
See
pulmonary disease, chronic obstructive
.
chronic fatigue syndrome
A condition that causes extreme fatigue
over a prolonged period, often for sev-
eral
years.
It
is
most
common
in
women aged between
25
and
4 5
.
CAUSES
The cause is unclear. In some cases, the
condition develops after a viral infec-
tion or after a stressful event such as
bereavement. In other cases, there is no
such preceding illness or event.
SYMPTOMS
The main symptom of chronic fatigue
syndrome is persistent tiredness. Other
symptoms vary, but commonly include
impairment of short-term memory or
concentration, sore throat, tender
lymph
nodes
,
muscle and joint pain, muscle
fatigue, unrefreshing sleep, and head-
aches. The disorder is often associated
with
depression
or
anxiety
.
DIAGNOSIS
There is no diagnostic test for chronic
fatigue syndrome, and investigations of
the
condition
are
usually
aimed
at
excluding other possible causes of the
symptoms, such as
anaemia
.
A physical
examination, blood tests, and a psycho-
logical assessment may be carried out.
If no cause can be found, a diagnosis of
chronic fatigue syndrome can be made
from the symptoms.
TREATMENT
Analgesic drugs
(painkillers) or
antide-
pressant drugs
may help to relieve the
symptoms.
Physiotherapy
or
psychothera-
py
may also be helpful. Chronic fatigue
syndrome is a long-term disorder, but
the symptoms clear up after several
years in some people.
chronic glaucoma
See
glaucoma
.
chronic heart failure
See
heart failure
.
chronic obstructive lung
disease
See
pulmonary disease, chronic obstructive.
chronic obstructive pulmonary
disease
See
pulmonary disease, chronic obstructive.
174
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