Figure 1: Chromosome.
(1)
Chromatid. One of
the two identical parts of
the chromosome after
S phase. (2)
Centromere. The point
where the two chromatids
touch, and where the
microtubules attach. (3)
Short arm (4) Long arm.
A chromosome, from the
ancient Greekχρωμα
(color) and σωμα (body) is,
minimally, a very long, continuous
piece of DNA, which contains many
genes,
regulatory elements and other
intervening
nucleotide sequences. In the
chromosomes of
eukaryotes, the uncondensed
DNA exists in a quasi-ordered
structure inside the
nucleus, where it wraps around
histones (structural
proteins, Fig. 1), and where
this composite material is called
chromatin. During
mitosis (cell division), the
chromosomes are condensed and a
spindle composed of microtubules
is formed. Microtubules are
self-assembled from
dimers of alpha and beta
tubulin. Microtubules attach
to chromosomes at specialized
structures, the kinetochores, one
of which is present on each sister
chromatid. Sister chromatids are
attached at an area called the
centromere. This term is
sometimes misleading, however,
because they are not necessarily
joined at the center of the
chromosome. A special DNA base
sequence in the region of the
kinetochores provides, along with
special proteins, longer-lasting
attachment in this region. This is
the only natural context in which
individual chromosomes are visible
with an optical
microscope. Each chromosome
has two arms, the shorter one
called p arm (from the
Frenchpetit, small)
and the longer one q arm (q
following p in the Latin
alphabet).
Prokaryotes do not possess
histones or nuclei. In its relaxed
state, the DNA can be accessed for
transcription, regulation, and
replication.
Eukaryotes (cells
with nuclei such as plants, yeast,
and animals) possess multiple
linear chromosomes contained in
the cell's nucleus. Each
chromosome has one
centromere, with one or two
arms projecting from the
centromere. The ends of the
chromosomes are special structures
called
telomeres. DNA replication
begins at many different locations
on the chromosome.
Bacterial chromosomes are often
circular but sometimes linear.
Some bacteria have one chromosome,
while others have a few. Bacterial
DNA also exists as
plasmids. The distinction
between plasmids and chromosomes
is poorly defined, though size and
necessity are generally taken into
account. Bacterial chromosomes
initiate replication and one
origin of replication.
When linear, bacterial
chromosomes tend to be tethered to
the
plasma membrane of the
bacteria. In molecular biology
application, this allows for its
isolation from plasmid DNA by
centrifugation of lysed bacteria
and pelleting of membranes (and
the attached DNA).
Euchromatin, which consists
of DNA that is active, e.g.,
expressed as protein.
Heterochromatin, which
consists of mostly inactive DNA.
It seems to serve structural
purposes during the chromosomal
stages. Heterochromatin can be
further distinguished into two
types:
Constitutive
heterochromatin, which is
never expressed. It is located
around the centromere and
usually contains
repetitive sequences.
Facultative
heterochromatin, which is
sometimes expressed.
Figure 2: Different
levels of DNA condensation.
(1) Single DNA strand. (2)
Chromatin strand (DNA
with
histones). (3)
Chromatin during
interphase with
centromere. (4)
Condensed chromatin during
prophase. (Two copies of
the DNA molecule are now
present) (5) Chromosome
during
metaphase.
In the very early stages of
mitosis, the chromatin strands
become more and more condensed.
They cease to function as
accessible genetic material and
become a compact transport form.
Eventually, the two matching
chromatids (condensed
chromatin strands) become visible
as a chromosome, linked at the
centromere. Long
microtubules are attached at
the centromere and two opposite
ends of the cell. During mitosis,
the microtubules pull the
chromatids apart, so that each
daughter cell inherits one set of
chromatids. Once the cells have
divided, the chromatids are
uncoiled and can function again as
chromatin. In spite of their
appearance, chromosomes are highly
structured (Fig. 2). For example,
genes with similar functions are
often kept close together in the
nucleus, even if they are far
apart on the chromosome. The short
arm of a chromosome can be
extended by a
satellite chromosome that
contains codes for
ribosomal
RNA.
Normal members of a particular
species all have the same
number of chromosomes (see the
table).
Asexually reproducing species
have one set of chromosomes, which
is the same in all body cells.
Gametes, reproductive cells,
are
haploid [n] and have one set
of chromosomes.
Sexually reproducing species
have
somatic cells, body cells,
which are
diploid [2n] having two sets
of chromosomes, one from the
mother and one from the father.
Gametes are produced by
meiosis of a diploid
germ line cell. During
meiosis, the matching chromosomes
of father and mother can exchange
small parts of themselves (crossover),
and thus create new chromosomes
that are not inherited solely from
either parent. When a male and a
female gamete merge (fertilization),
a new diploid organism is formed.
Some animal and plant species
are
polyploid [Xn] and have more
than two sets of chromosomes.
Agriculturally important plants
such as tobacco or wheat are often
polyploid compared to their
ancestral species. Wheat has a
haploid number of seven
chromosomes, still seen in some
cultivars as well as the wild
progenitors. The more common pasta
and bread wheats are polyploid
having 28 (tetraploid) and 42 (hexaploid)
chromosomes compared to the 14
(diploid) chromosomes in the wild
wheat[1].
Historical note: In 1921,
Theophilus Painter claimed,
based on his observations, that
human sex cells had 24
chromosomes, giving humans 48
chromosomes total. It wasn't until
1955 that the number was clearly
shown to be 23.
To determine the (diploid)
number of chromosomes of an
organism, cells can be locked in
metaphase
in vitro (in a reaction vial)
with
colchicine. These cells are
then stained (the name chromosome
was given because of their ability
to be stained), photographed and
arranged into a
karyotype (an ordered set of
chromosomes, Fig. 3), also called
karyogram. Like many
sexually reproducing species,
humans have special
gonosomes (sex chromosomes, in
contrast to
autosomes for body functions).
These are XX in females and XY in
males. In females, one of the two
X chromosomes is inactive and can
be seen under a microscope as
Barr bodies.
Some chromosome abnormalities
do not cause disease in carriers,
such as
translocations, or
chromosomal inversions,
although they may lead to a higher
chance of having a child with a
chromosome disorder. Abnormal
numbers of chromosomes or
chromosome sets,
aneuploidy, may be lethal or
give rise to genetic disorders.
Genetic counseling is offered
for families that may carry a
chromosome rearrangement.
The gain or loss of chromosome
material can lead to a variety of
genetic disorders. Examples
include:
Cri du chat, which is caused
by the
deletion of part of the
short arm of chromosome 5. "Cri
du chat" means "cry of the cat"
in French, and the condition was
so-named because affected babies
make high-pitched cries that
sound like a cat. Affected
individuals have wide-set eyes,
a small head and jaw and are
moderately to severely mentally
retarded and very short.
Wolf-Hirschhorn syndrome,
which is caused by partial
deletion of the short arm of
chromosome 4. It is
characterized by severe growth
retardation and severe to
profound mental retardation.
Down syndrome, usually is
caused by an extra copy of
chromosome 21 (trisomy
21). Characteristics include
decreased muscle tone,
asymmetrical skull, slanting
eyes and mild to moderate mental
retardation.
Edward's syndrome, which is
the second most common trisomy
after Down syndrome. It is a
trisomy of chromosome 18.
Symptoms include mental and
motor retardation as well as
numerous congenital anomalies
causing serious health problems.
Ninety percent die in infancy;
however, those who live past
their first birthday usually are
quite healthy thereafter. They
have a characteristic hand
appearance with clenched hands
and overlapping fingers.
Patau Syndrome, also called
D-Syndrome or trisomy-13.
Symptoms are somewhat similar to
those of trisomy-18, but they do
not have the characteristic hand
shape.
Jacobsen syndrome, also
called the terminal 11q deletion
disorder.[3]
This is a very rare disorder.
Those affected have normal
intelligence or mild mental
retardation, with poor
expressive language skills. Most
have a bleeding disorder called
Paris-Trousseau syndrome.
Klinefelter's syndrome (XXY).
Men with Klinefelter syndrome
are usually sterile, and tend to
have longer arms and legs and to
be taller than their peers. Boys
with the syndroms are often shy
and quiet, and have a higher
incidence of speech delay and
dyslexia. During puberty,
some of them grow breasts and
get a curvy figure.
Turner syndrome (X instead
of XX or XY). In Turner
syndrome, female sexual
characteristics are present but
underdeveloped. People with
Turner syndrome often have a
short stature, low hairline,
abnormal eye features and bone
development and a "caved-in"
appearance to the chest.
XYY syndrome. XYY boys are
usually taller than their
brothers. They are more likely
to be hyperactive, enjoying
active games. Despite what was
previously believed, XYY boys
are no more likely than other
boys to be violent.
Triple-X syndrome (XXX). XXX
girls tend to be tall and thin
and are often shy. They have a
higher incidence of dyslexia.
