
A chromosome is a
package of
DNA containing part or all of the
genetic material
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic aci ...
of an
organism
An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
. In most chromosomes, the very long thin DNA fibers are coated with
nucleosome
A nucleosome is the basic structural unit of DNA packaging in eukaryotes. The structure of a nucleosome consists of a segment of DNA wound around eight histone, histone proteins and resembles thread wrapped around a bobbin, spool. The nucleosome ...
-forming packaging
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s; in
eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
cells, the most important of these proteins are the
histones. Aided by
chaperone proteins, the histones bind to and
condense the DNA molecule to maintain its integrity.
These eukaryotic chromosomes display a complex
three-dimensional structure that has a significant role in
transcriptional regulation.
Normally, chromosomes are visible under a
light microscope only during the
metaphase of
cell division, where all chromosomes are aligned in the center of the cell in their condensed form. Before this stage occurs, each chromosome is duplicated (
S phase), and the two copies are joined by a
centromere—resulting in either an X-shaped structure if the centromere is located equatorially, or a two-armed structure if the centromere is located distally; the joined copies are called '
sister chromatids'. During
metaphase, the duplicated structure (called a 'metaphase chromosome') is highly condensed and thus easiest to distinguish and study.
In animal cells, chromosomes reach their highest compaction level in
anaphase during
chromosome segregation.
Chromosomal
recombination during
meiosis and subsequent
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
plays a crucial role in
genetic diversity
Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species. It ranges widely, from the number of species to differences within species, and can be correlated to the span of survival for a species. It is d ...
. If these structures are manipulated incorrectly, through processes known as
chromosomal instability and
translocation, the cell may undergo
mitotic catastrophe. This will usually cause the cell to initiate
apoptosis
Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
, leading to its own
death
Death is the end of life; the irreversible cessation of all biological functions that sustain a living organism. Death eventually and inevitably occurs in all organisms. The remains of a former organism normally begin to decompose sh ...
, but the process is occasionally hampered by cell mutations that result in the progression of
cancer
Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...
.
The term 'chromosome' is sometimes used in a wider sense to refer to the individualized portions of
chromatin in cells, which may or may not be visible under light microscopy. In a narrower sense, 'chromosome' can be used to refer to the individualized portions of chromatin during cell division, which are visible under light microscopy due to high condensation.
Etymology
The word ''chromosome'' () comes from the
Ancient Greek
Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
words (', "colour") and (', "body"), describing the strong
staining produced by particular
dyes. The term was coined by the German anatomist
Heinrich Wilhelm Waldeyer, referring to the term '
chromatin', which was introduced by
Walther Flemming.
Some of the early
karyological terms have become outdated. For example, 'chromatin' (Flemming 1880) and 'chromosom' (Waldeyer 1888) both ascribe color to a non-colored state.
History of discovery
Otto Bütschli was the first scientist to recognize the structures now known as chromosomes.
In a series of experiments beginning in the mid-1880s,
Theodor Boveri gave definitive contributions to elucidating that chromosomes are the
vectors of
heredity
Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic infor ...
, with two notions that became known as 'chromosome continuity' and 'chromosome individuality'.
Wilhelm Roux suggested that every chromosome carries a different
genetic configuration, and Boveri was able to test and confirm this hypothesis. Aided by the rediscovery at the start of the 1900s of
Gregor Mendel's earlier experimental work, Boveri identified the connection between the rules of inheritance and the behaviour of the chromosomes. Two generations of American
cytologists were influenced by Boveri:
Edmund Beecher Wilson,
Nettie Stevens,
Walter Sutton and
Theophilus Painter (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' (sometimes known as the 'Sutton–Boveri chromosome theory').
Ernst Mayr remarks that the theory was hotly contested by some famous geneticists, including
William Bateson,
Wilhelm Johannsen,
Richard Goldschmidt and
T.H. Morgan, all of a rather dogmatic mindset. Eventually, absolute proof came from chromosome maps in Morgan's own laboratory.
The number of human chromosomes was published by Painter in 1923. By inspection through a microscope, he counted 24 pairs of chromosomes, giving 48 in total. His error was copied by others, and it was not until 1956 that the true number (46) was determined by Indonesian-born
cytogeneticist Joe Hin Tjio.
Prokaryotes
The
prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s –
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
and
archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
– typically have a single
circular chromosome. The chromosomes of most bacteria (also called
genophores), can range in size from only 130,000
base pair
A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
s in the
endosymbiotic bacteria ''
Candidatus Hodgkinia cicadicola''
and ''
Candidatus Tremblaya princeps'', to more than 14,000,000 base pairs in the soil-dwelling bacterium ''
Sorangium cellulosum''.
Some bacteria have more than one chromosome. For instance,
Spirochaetes such as ''
Borrelia burgdorferi'' (causing
Lyme disease), contain a single ''linear'' chromosome. ''
Vibrios'' typically carry two chromosomes of very different size. Genomes of the genus ''
Burkholderia'' carry one, two, or three chromosomes.
Structure in sequences
Prokaryotic chromosomes have less sequence-based structure than eukaryotes. 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, unlike eukaryotes.
DNA packaging
Prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
s 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
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.
Certain bacteria also contain
plasmid
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
s or other
extrachromosomal DNA. These are circular structures in the
cytoplasm
The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
that contain cellular DNA and play a role in
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...
.
In prokaryotes and viruses,
the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by
histone-like proteins.
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
replication.
Eukaryotes

Each eukaryotic chromosome consists of a long linear
DNA molecule associated with
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s, forming a compact complex of proteins and DNA called ''
chromatin.'' Chromatin contains the vast majority of the DNA in an organism, but a
small amount inherited maternally can be found in the
mitochondria. It is present in most
cells, with a few exceptions, for example,
red blood cells.
Histones are responsible for the first and most basic unit of chromosome organization, the
nucleosome
A nucleosome is the basic structural unit of DNA packaging in eukaryotes. The structure of a nucleosome consists of a segment of DNA wound around eight histone, histone proteins and resembles thread wrapped around a bobbin, spool. The nucleosome ...
.
Eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s (
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
cytoplasm
The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
ic chromosomes.

In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around
histones (structural proteins), forming a composite material called chromatin.
Interphase chromatin
The packaging of DNA into nucleosomes causes a 10 nanometer fibre which may further condense up to 30 nm fibres.
Most of the euchromatin in interphase nuclei appears to be in the form of 30-nm fibers.
Chromatin structure is the more decondensed state, i.e. the 10-nm conformation allows transcription.

During
interphase (the period of the
cell cycle
The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...
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 sequences.
** ''Facultative heterochromatin'', which is sometimes expressed.
Metaphase chromatin and division

In the early stages of
mitosis
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
or
meiosis (cell division), the chromatin double helix becomes more and more condensed. They cease to function as accessible genetic material (
transcription stops) and become a compact transportable form. The loops of thirty-nanometer chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells. The DNA is thus condensed about ten-thousand-fold.
The
chromosome scaffold, which is made of proteins such as
condensin,
TOP2A and
KIF4, plays an important role in holding the chromatin into compact chromosomes. Loops of thirty-nanometer structure further condense with scaffold into higher order structures.
This highly 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
A microscope () is a laboratory equipment, laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic ...
.
Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops.
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 nucleus.
Human chromosomes
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. Human cells 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
The human genome is a complete set of nucleic acid sequences for humans, encoded as the DNA within each of the 23 distinct chromosomes in the cell nucleus. A small DNA molecule is found within individual Mitochondrial DNA, mitochondria. These ar ...
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.
Based on the micrographic characteristics of size, position of the
centromere and sometimes the presence of a
chromosomal satellite, the human chromosomes are classified into the following groups:
Karyotype

In general, the
karyotype
A karyotype is the general appearance of the complete set of chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is discerned by de ...
is the characteristic chromosome complement of a
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
. 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
germline and
soma (between
gamete
A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
s 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 fertilized egg.
The technique of determining the karyotype is usually called ''karyotyping''. Cells can be locked part-way through division (in metaphase)
in vitro
''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...
(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.
History and analysis techniques
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. In 1922,
Painter 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
Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...
in
metaphase by a solution of
colchicine
# Pretreating cells in a
hypotonic solution , 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 indisputable karyogram.
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.
Aberrations

Chromosomal aberrations are disruptions in the normal chromosomal content of a cell. They can cause 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 disorders.
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 disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosome abnormality. Although polygenic disorders ...
s. Human examples include:
*
Cri du chat, 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.
*
DiGeorge syndrome, also known as 22q11.2 deletion syndrome. Symptoms are mild learning disabilities in children, with adults having an increased risk of
schizophrenia
Schizophrenia () is a mental disorder characterized variously by hallucinations (typically, Auditory hallucination#Schizophrenia, hearing voices), delusions, thought disorder, disorganized thinking and behavior, and Reduced affect display, f ...
. Infections are also common in children because of problems with the immune system's T cell-mediated response due to an absence of hypoplastic thymus.
*
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 11q terminal deletion disorder. Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called
Paris-Trousseau syndrome.
*
Klinefelter syndrome (XXY). Men with Klinefelter syndrome are usually sterile, and tend to be taller than their peers, with longer arms and legs. 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 genetic material.
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 with 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, 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 difficulties.
Sperm aneuploidy
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,
insecticide
Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. The major use of insecticides is in agriculture, but they are also used in home and garden settings, i ...
s,
and perfluorinated compounds.
Increased aneuploidy is often associated with increased DNA damage in spermatozoa.
Number in various organisms
In eukaryotes
The number of chromosomes in
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s is highly variable. It is possible for chromosomes to fuse or break and thus evolve into novel karyotypes. Chromosomes can also be fused artificially. For example, when the 16 chromosomes of
yeast were fused into one giant chromosome, it was found that the cells were still viable with only somewhat reduced growth rates.
The tables below give the total number of chromosomes (including sex chromosomes) in a cell nucleus for various eukaryotes. Most are
diploid, such as
humans who have 22 different types of
autosomes—each present as two homologous pairs—and two
sex chromosomes, giving 46 chromosomes in total. Some other organisms have more than two copies of their chromosome types, for example
bread wheat which is ''hexaploid'', having six copies of seven different chromosome types for a total of 42 chromosomes.
Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes. 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.
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) that are
diploid n having two sets of chromosomes (23 pairs in humans), one set from the mother and one from the father.
Gamete
A gamete ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
s (reproductive cells) are
haploid having one set of chromosomes. Gametes are produced by
meiosis of a diploid
germline cell, during which 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 during
fertilization
Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...
, a new diploid organism is formed.
Some animal and plant species are
polyploid n having more than two sets of
homologous chromosomes. Important crops 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
cultivar
A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...
s as well as the wild progenitors. The more common types of pasta and bread wheat are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in wild wheat.
In prokaryotes
Prokaryote
A prokaryote (; less commonly spelled procaryote) is a unicellular organism, single-celled organism whose cell (biology), cell lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Gree ...
species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. For example, ''
Buchnera'', a
symbiont
Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can fo ...
of
aphid
Aphids are small sap-sucking insects in the Taxonomic rank, family Aphididae. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Eriosomatinae, woolly ...
s, has multiple copies of its chromosome, ranging from 10 to 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.
See also
*
Chromomere
*
Cohesin
*
Epigenetics
In biology, epigenetics is the study of changes in gene expression that happen without changes to the DNA sequence. The Greek prefix ''epi-'' (ἐπι- "over, outside of, around") in ''epigenetics'' implies features that are "on top of" or "in ...
*
Genetic genealogy
*
Lampbrush chromosome
*
Locus (genetics)
In genetics, a locus (: loci) is a specific, fixed position on a chromosome where a particular gene or genetic marker is located. Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total numb ...
– explains gene location nomenclature
*
Minichromosome
*
Neochromosome
*
Nondisjunction
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (mitosis/meiosis). There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I ...
*
Parasitic chromosome
*
Polytene chromosome
*
Secondary chromosome
*
Sex-determination system
**
Maternal influence on sex determination
**
Temperature-dependent sex determination
Temperature-dependent sex determination (TSD) is a type of environmental sex determination in which the temperatures experienced during embryonic/larval development determine the sex of the offspring. It is observed in reptiles and teleost fish, ...
Notes and references
External links
An Introduction to DNA and Chromosomesfrom
HOPES: Huntington's Outreach Project for Education at Stanford
Chromosome Abnormalities at AtlasGeneticsOncologyOn-line exhibition on chromosomes and genome (SIB)What Can Our Chromosomes Tell Us? from the University of Utah's Genetic Science Learning Center
Try making a karyotype yourself from the University of Utah's Genetic Science Learning Center
Chromosome News from Genome News Network European network for Rare Chromosome Disorders on the Internet
Ensembl.org Ensembl project, presenting chromosomes, their
gene
In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
s and
syntenic loci graphically via the web
Genographic Project
Home reference on Chromosomesfrom the U.S. National Library of Medicine
Visualisation of human chromosomesand comparison to other species
Unique – The Rare Chromosome Disorder Support GroupSupport for people with rare chromosome disorders
{{Authority control
Nuclear substructures
Cytogenetics