A CHROMOSOME (from ancient Greek : χρωμόσωμα, _chromosoma,
chroma_ means color, _soma_ means body) is a
DNA molecule with part or
all of the genetic material (genome ) of an organism.
Chromosomes are normally visible under a light microscope only when
the cell is undergoing the metaphase of cell division . Before this
happens, every chromosome is copied once (
S phase ), and the copy is
joined to the original by a centromere , resulting in an X-shaped
structure. The original chromosome and the copy are now called sister
chromatids . During metaphase, when a chromosome is in its most
condensed state, the X-shape structure is called a metaphase
chromosome. In this highly condensed form chromosomes are easiest to
distinguish and study.
Chromosomes vary widely between different organisms . Some species
such as certain bacteria , which lack histones , also contain plasmids
or other extrachromosomal
DNA . These are circular structures in the
cytoplasm that contain cellular
DNA and play a role in horizontal gene
transfer . In prokaryotes (see nucleoids ) and viruses , the
often densely packed and organized; in the case of archaea , by
homology to eukaryotic histones, and in the case of bacteria, by
Compaction of the duplicated chromosomes during cell division
(mitosis or meiosis ) results either in a four-arm structure (pictured
to the right) if the centromere is located in the middle of the
chromosome or a two-arm structure if the centromere is located near
one of the ends. Chromosomal recombination during meiosis and
subsequent sexual reproduction play a significant role in genetic
diversity. If these structures are manipulated incorrectly, through
processes known as chromosomal instability and translocation, the cell
may undergo mitotic catastrophe and die, or it may unexpectedly evade
apoptosis , leading to the progression of cancer .
Some use the term chromosome in a wider sense, to refer to the
individualized portions of chromatin in cells, either visible or not
under light microscopy. However, others use the concept in a narrower
sense, to refer to the individualized portions of chromatin during
cell division, visible under light microscopy due to high
* 1 History of discovery
* 2 Prokaryotes
* 2.1 Structure in sequences
Metaphase chromatin and division
* 4 Number in various organisms
* 4.1 In eukaryotes
* 4.2 In prokaryotes
* 5.1 Historical note
* 6 Aberrations
* 6.1 Sperm aneuploidy
* 7 See also
* 8 Notes and references
* 9 External links
HISTORY OF DISCOVERY
Walter Sutton (left) and
Theodor Boveri (right)
independently developed the chromosome theory of inheritance in 1902.
The word _chromosome_ (/ˈkroʊməˌsoʊm, -ˌzoʊm/ ) comes from
the Greek χρῶμα (_chroma_, "colour") and σῶμα (_soma_,
"body"), describing their strong staining by particular dyes .
Schleiden , Virchow and Bütschli were among the first scientists
who recognized the structures now familiar as chromosomes. The term
was coined by von Waldeyer-Hartz , referring to the term chromatin ,
which was introduced by
Walther Flemming .
In a series of experiments beginning in the mid-1880s, Theodor Boveri
gave the definitive demonstration that chromosomes are the vectors of
heredity . His two principles were the _continuity_ of chromosomes and
the _individuality_ of chromosomes. It is the second of these
principles that was so original.
Wilhelm Roux suggested that each
chromosome carries a different genetic load . Boveri was able to test
and confirm this hypothesis. Aided by the rediscovery at the start of
the 1900s of
Gregor Mendel 's earlier work, Boveri was able to point
out the connection between the rules of inheritance and the behaviour
of the chromosomes. Boveri influenced two generations of American
Edmund Beecher Wilson ,
Nettie Stevens , Walter Sutton
Theophilus Painter were all influenced by Boveri (Wilson, Stevens,
and Painter actually worked with him).
In his famous textbook _The Cell in Development and Heredity_, Wilson
linked together the independent work of Boveri and Sutton (both around
1902) by naming the chromosome theory of inheritance the
Boveri–Sutton chromosome theory (the names are sometimes reversed).
Ernst Mayr remarks that the theory was hotly contested by some famous
William Bateson ,
Wilhelm Johannsen , Richard Goldschmidt
T.H. Morgan , all of a rather dogmatic turn of mind. Eventually,
complete proof came from chromosome maps in Morgan's own lab.
The number of human chromosomes was published in 1923 by Theophilus
Painter . By inspection through the microscope, he counted 24 pairs,
which would mean 48 chromosomes. His error was copied by others and it
was not until 1956 that the true number, 46, was determined by
Joe Hin Tjio .
The prokaryotes – bacteria and archaea – typically have a single
circular chromosome , but many variations exist. The chromosomes of
most bacteria, which some authors prefer to call genophores , can
range in size from only 130,000 base pairs in the endosymbiotic
bacteria _Candidatus Hodgkinia cicadicola _ and _Candidatus Tremblaya
princeps _, to more than 14,000,000 base pairs in the soil-dwelling
Sorangium cellulosum _. Spirochaetes of the genus
_Borrelia_ are a notable exception to this arrangement, with bacteria
such as _
Borrelia burgdorferi _, the cause of
Lyme disease ,
containing a single _linear_ chromosome.
STRUCTURE IN SEQUENCES
Prokaryotic chromosomes have less sequence-based structure than
Bacteria typically have a one-point (the origin of
replication ) from which replication starts, whereas some archaea
contain multiple replication origins. The genes in prokaryotes are
often organized in operons , and do not usually contain introns ,
STRUCTURE OF CHROMOSOME:
* Each chromosome is made up of TWO CHROMATIDS(chromosomal arms)
which are joined to each other at a small constricted region called
the centromere.(PRIMARY CONSTRICTION).
* The centromere helps the chromatids to get attached to the spindle
fibres during cell division, it is also concerned with the ANAPHASE
MOVEMENT of the chromosomes, by which the spindle fibers pull the
chromatids to the two opposite poles by their contraction during
* Besides the primary constriction, in certain chromosomes there is
a secondary constriction as well. Because a the small portion is
pinched off from the chromosomal body; this portion is called a
'satellite' and the chromosome is called an SAT chromosome.
* The two chromatids are made up of very thin chromatin fibres which
are made up of 40%
DNA and 60% histone proteins
* Each chromatin fibre consists of one
DNA strand coiled around
eight histone molecules like a loop; such a complex is called
* The primary constriction is a non-stainable region; during cell
division this region appears like a gap.
* Within the primary constriction there is a clear zone called
* The centromere with the
DNA and histone proteins bound to them
form a disc shaped structure called _KINETOCHORE_.
* the chromonemata represents a chromatid in the early stage of
Prokaryotes do not possess nuclei. Instead, their
DNA is organized
into a structure called the nucleoid . The nucleoid is a distinct
structure and occupies a defined region of the bacterial cell. This
structure is, however, dynamic and is maintained and remodeled by the
actions of a range of histone-like proteins, which associate with the
bacterial chromosome. In archaea , the
DNA in chromosomes is even
more organized, with the
DNA packaged within structures similar to
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 the membranes (and the attached DNA).
Prokaryotic chromosomes and plasmids are, like eukaryotic DNA,
generally supercoiled . The
DNA must first be released into its
relaxed state for access for transcription , regulation, and
DNA in a eukaryotic cell. See also: Eukaryotic
chromosome fine structure
In eukaryotes, nuclear chromosomes are packaged by proteins into a
condensed structure called chromatin . This allows the very long DNA
molecules to fit into the cell nucleus . The structure of chromosomes
and chromatin varies through the cell cycle . Chromosomes are even
more condensed than chromatin and are an essential unit for cellular
division. Chromosomes must be replicated, divided, and passed
successfully to their daughter cells so as to ensure the genetic
diversity and survival of their progeny . Chromosomes may exist as
either duplicated or unduplicated. Unduplicated chromosomes are single
double helixes, whereas duplicated chromosomes contain two identical
copies (called chromatids or sister chromatids ) joined by a
centromere . The major structures in
DNA , the
nucleosome , the 10 nm "beads-on-a-string" fibre, the 30 nm fibre and
the metaphase chromosome.
Eukaryotes (cells with nuclei such as those found in plants, fungi,
and animals) possess multiple large linear chromosomes contained in
the cell's nucleus. Each chromosome has one centromere , with one or
two arms projecting from the centromere, although, under most
circumstances, these arms are not visible as such. In addition, most
eukaryotes have a small circular mitochondrial genome , and some
eukaryotes may have additional small circular or linear cytoplasmic
In the nuclear chromosomes of eukaryotes , the uncondensed
in a semi-ordered structure, where it is wrapped around histones
(structural proteins ), forming a composite material called chromatin
Chromatin is the complex of
DNA and protein found in the eukaryotic
nucleus, which packages chromosomes. The structure of chromatin varies
significantly between different stages of the cell cycle , according
to the requirements of the DNA.
During interphase (the period of the cell cycle where the cell is not
dividing), two types of chromatin can be distinguished:
Euchromatin , which consists of
DNA that is active, e.g., being
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
* _Facultative heterochromatin_, which is sometimes expressed.
Chromatin And Division
See also: mitosis and meiosis
Human chromosomes during
In the early stages of mitosis or meiosis (cell division), the
chromatin double helix become more and more condensed. They cease to
function as accessible genetic material (transcription stops) and
become a compact transportable form. This compact form makes the
individual chromosomes visible, and they form the classic four arm
structure, a pair of sister chromatids attached to each other at the
centromere . The shorter arms are called _p arms_ (from the French
_petit_, small) and the longer arms are called _q arms_ (_q_ follows
_p_ in the Latin alphabet; q-g "grande"; alternatively it is sometimes
said q is short for _queue_ meaning tail in French ). This is the only
natural context in which individual chromosomes are visible with an
optical microscope .
Mitotic metaphase chromosomes are best described by a linearly
organized longitudinally compressed array of consecutive chromatin
During mitosis, microtubules grow from centrosomes located at
opposite ends of the cell and also attach to the centromere at
specialized structures called kinetochores , one of which is present
on each sister chromatid . A special
DNA base sequence in the region
of the kinetochores provides, along with special proteins,
longer-lasting attachment in this region. The microtubules then pull
the chromatids apart toward the centrosomes, so that each daughter
cell inherits one set of chromatids. Once the cells have divided, the
chromatids are uncoiled and
DNA can again be transcribed. In spite of
their appearance, chromosomes are structurally highly condensed, which
enables these giant
DNA structures to be contained within a cell
Chromosomes in humans can be divided into two types: autosomes (body
chromosome(s)) and allosome (sex chromosome (s)). Certain genetic
traits are linked to a person's sex and are passed on through the sex
chromosomes. The autosomes contain the rest of the genetic hereditary
information. All act in the same way during cell division.
have 23 pairs of chromosomes (22 pairs of autosomes and one pair of
sex chromosomes), giving a total of 46 per cell. In addition to these,
human cells have many hundreds of copies of the mitochondrial genome .
Sequencing of the human genome has provided a great deal of
information about each of the chromosomes. Below is a table compiling
statistics for the chromosomes, based on the
Sanger Institute 's human
genome information in the Vertebrate
Genome Annotation (VEGA) database
. Number of genes is an estimate, as it is in part based on gene
predictions . Total chromosome length is an estimate as well, based on
the estimated size of unsequenced heterochromatin regions. Estimated
number of genes and base pairs (in mega base pairs) on each human
TOTAL BASE PAIRS
% OF BASES
SEQUENCED BASE PAIRS
X (sex chromosome)
Y (sex chromosome)
NUMBER IN VARIOUS ORGANISMS
List of organisms by chromosome count
These tables give the total number of chromosomes (including sex
chromosomes) in a cell nucleus. For example, human cells are diploid
and have 22 different types of autosome , each present as two copies,
and two sex chromosomes . This gives 46 chromosomes in total. Other
organisms have more than two copies of their chromosome types, such as
bread wheat , which is _hexaploid_ and has six copies of seven
different chromosome types – 42 chromosomes in total.
Chromosome numbers in some plants
Arabidopsis thaliana _ (diploid)
Einkorn wheat (diploid)
Maize (diploid or palaeotetraploid)
Durum wheat (tetraploid)
Bread wheat (hexaploid)
Adder\'s tongue fern (diploid)
Chromosome numbers (2n) in some animals
Common fruit fly
Pill millipede (_Arthrosphaera fumosa_)
Earthworm (_Octodrilus complanatus_)
Rabbit (_Oryctolagus cuniculus_)
Guppy (_poecilia reticulata_)
Gorillas , chimpanzees
Chromosome numbers in other organisms
Trypanosoma brucei _
(_Columba livia domestics_) 18
2 sex chromosomes
Normal members of a particular eukaryotic species all have the same
number of nuclear chromosomes (see the table). Other eukaryotic
chromosomes, i.e., mitochondrial and plasmid-like small chromosomes,
are much more variable in number, and there may be thousands of copies
per cell. The 23 human chromosome territories during prometaphase
in fibroblast cells.
Asexually reproducing species have one set of chromosomes that are
the same in all body cells. However, asexual species can be either
haploid or diploid.
Sexually reproducing species have somatic cells (body cells), which
are diploid having two sets of chromosomes (23 pairs in humans with
one set of 23 chromosomes from each parent), one set from the mother
and one from the father. Gametes , reproductive cells, are haploid :
They have one set of chromosomes. 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 : They have more than two
sets of homologous chromosomes . Plants important in agriculture such
as tobacco or wheat are often polyploid, compared to their ancestral
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 wheat types are polyploid, having 28 (tetraploid) and
42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes
in the wild wheat.
Prokaryote species generally have one copy of each major chromosome,
but most cells can easily survive with multiple copies. For example,
_Buchnera _, a symbiont of aphids has multiple copies of its
chromosome, ranging from 10–400 copies per cell. However, in some
large bacteria, such as _
Epulopiscium fishelsoni _ up to 100,000
copies of the chromosome can be present. Plasmids and plasmid-like
small chromosomes are, as in eukaryotes, highly variable in copy
number. The number of plasmids in the cell is almost entirely
determined by the rate of division of the plasmid – fast division
causes high copy number.
Karyotype Karyogram of a human male
In general, the KARYOTYPE is the characteristic chromosome complement
of a eukaryote species . The preparation and study of karyotypes is
part of cytogenetics .
Although the replication and transcription of
DNA is highly
standardized in eukaryotes , _the same cannot be said for their
karyotypes_, which are often highly variable. There may be variation
between species in chromosome number and in detailed organization. In
some cases, there is significant variation within species. Often there
is: 1. variation between the two sexes 2. variation between the
germ-line and soma (between gametes and the rest of the body) 3.
variation between members of a population, due to balanced genetic
polymorphism 4. geographical variation between races 5. mosaics or
otherwise abnormal individuals.
Also, variation in karyotype may occur during development from the
The technique of determining the karyotype is usually called
_karyotyping_. Cells can be locked part-way through division (in
metaphase) in vitro (in a reaction vial) with colchicine . These cells
are then stained, photographed, and arranged into a _karyogram_, with
the set of chromosomes arranged, autosomes in order of length, and sex
chromosomes (here X/Y) at the end.
Like many sexually reproducing species, humans have special gonosomes
(sex chromosomes, in contrast to autosomes ). These are XX in females
and XY in males.
Investigation into the human karyotype took many years to settle the
most basic question: _How many chromosomes does a normal diploid human
cell contain?_ In 1912, Hans von Winiwarter reported 47 chromosomes in
spermatogonia and 48 in oogonia , concluding an XX/XO sex
determination mechanism . Painter in 1922 was not certain whether the
diploid number of man is 46 or 48, at first favouring 46. He revised
his opinion later from 46 to 48, and he correctly insisted on humans
having an XX/XY system.
New techniques were needed to definitively solve the problem:
* Using cells in culture
* Arresting mitosis in metaphase by a solution of colchicine
* Pretreating cells in a hypotonic solution 0.075 M KCl, which
swells them and spreads the chromosomes
* Squashing the preparation on the slide forcing the chromosomes
into a single plane
* Cutting up a photomicrograph and arranging the result into an
It took until 1954 before the human diploid number was confirmed as
46. Considering the techniques of Winiwarter and Painter, their
results were quite remarkable. Chimpanzees , the closest living
relatives to modern humans, have 48 chromosomes as do the other great
apes : in humans two chromosomes fused to form chromosome 2 .
Argument from authority#Inaccurate chromosome number )
In Down syndrome, there are three copies of chromosome 21
Chromosomal aberrations are disruptions in the normal chromosomal
content of a cell and are a major cause of genetic conditions in
humans, such as
Down syndrome , although most aberrations have little
to no effect. Some chromosome abnormalities do not cause disease in
carriers, such as translocations , or chromosomal inversions ,
although they may lead to a higher chance of bearing a child with a
chromosome disorder. Abnormal numbers of chromosomes or chromosome
sets, called aneuploidy , may be lethal or may give rise to genetic
Genetic counseling is offered for families that may carry
a chromosome rearrangement.
The gain or loss of
DNA from chromosomes can lead to a variety of
genetic disorders .
Human 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;
the condition was so-named because affected babies make high-pitched
cries that sound like those of a cat. Affected individuals have
wide-set eyes, a small head and jaw, moderate to severe mental health
problems, and are very short.
Down syndrome , the most common trisomy, usually caused by an
extra copy of chromosome 21 (trisomy 21 ). Characteristics include
decreased muscle tone, stockier build, asymmetrical skull, slanting
eyes and mild to moderate developmental disability.
Edwards syndrome , or trisomy-18, the second most common trisomy.
Symptoms include motor retardation, developmental disability and
numerous congenital anomalies causing serious health problems. Ninety
percent of those affected die in infancy. They have characteristic
clenched hands and overlapping fingers.
Isodicentric 15 , also called idic(15), partial tetrasomy 15q, or
inverted duplication 15 (inv dup 15).
Jacobsen syndrome , which is very rare. It is also called the
terminal 11q deletion disorder. Those affected have normal
intelligence or mild developmental disability, with poor expressive
language skills. Most have a bleeding disorder called Paris-Trousseau
Klinefelter syndrome (XXY). Men with
Klinefelter syndrome are
usually sterile and tend to be taller and have longer arms and legs
than their peers. Boys with the syndrome are often shy and quiet and
have a higher incidence of speech delay and dyslexia . Without
testosterone treatment, some may develop gynecomastia during puberty.
Patau Syndrome , also called D-Syndrome or trisomy-13. Symptoms
are somewhat similar to those of trisomy-18, without the
characteristic folded hand.
Small supernumerary marker chromosome . This means there is an
extra, abnormal chromosome. Features depend on the origin of the extra
Cat-eye syndrome and isodicentric chromosome 15
syndrome (or Idic15) are both caused by a supernumerary marker
chromosome, as is
Pallister–Killian syndrome .
Triple-X syndrome (XXX). XXX girls tend to be tall and thin and
have a higher incidence of dyslexia.
Turner syndrome (X instead of XX or XY). In Turner syndrome,
female sexual characteristics are present but underdeveloped. Females
Turner syndrome often have a short stature, low hairline,
abnormal eye features and bone development and a "caved-in" appearance
to the chest.
Wolf–Hirschhorn syndrome , which is caused by partial deletion
of the short arm of chromosome 4. It is characterized by growth
retardation, delayed motor skills development, "Greek Helmet" facial
features, and mild to profound mental health problems.
XYY syndrome . XYY boys are usually taller than their siblings.
Like XXY boys and XXX girls, they are more likely to have learning
Exposure of males to certain lifestyle, environmental and/or
occupational hazards may increase the risk of aneuploid spermatozoa.
In particular, risk of aneuploidy is increased by tobacco smoking,
and occupational exposure to benzene, insecticides, and
perfluorinated compounds. Increased aneuploidy is often associated
DNA damage in spermatozoa.
* For information about chromosomes in genetic algorithms , see
chromosome (genetic algorithm)
List of number of chromosomes of various organisms
* Locus (explains gene location nomenclature)
Maternal influence on sex determination
XY sex-determination system
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