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A chromosome is a long
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
molecule with part or all of the
genetic material
genetic material
of an organism. Most
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
chromosomes include packaging proteins called
histone In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
s which, aided by
chaperone proteins Chaperone proteins participate in the folding of over half of all mammalian proteins. In molecular biology, molecular chaperones are proteins that assist the conformational folding or unfolding and the assembly or disassembly of other macromolecular ...
, bind to and
condense Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

condense
the DNA molecule to maintain its integrity. These chromosomes display a complex three-dimensional structure, which plays a significant role in
transcriptional regulation In and , transcriptional regulation is the means by which a cell regulates the conversion of to (), thereby . A single gene can be regulated in a range of ways, from altering the number of copies of RNA that are transcribed, to the temporal co ...
. Chromosomes are normally visible under a
light microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible spectrum, visible light and a system of lens (optics), lenses to generate magnified images of small objects. Optical microscopes ...
only during the
metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical s ...

metaphase
of
cell division Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

cell division
(where all chromosomes are aligned in the center of the cell in their condensed form). Before this happens, each chromosome is duplicated (
S phase S phase (Synthesis Phase) is the phase of the cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may ...
), and both copies are joined by a
centromere In this diagram of a duplicated chromosome, (2) identifies the centromere—the region that joins the two sister chromatids, or each half of the chromosome. In prophase of mitosis, specialized regions on centromeres called kinetochores attach chr ...
, resulting either in an X-shaped structure (pictured above), if the
centromere In this diagram of a duplicated chromosome, (2) identifies the centromere—the region that joins the two sister chromatids, or each half of the chromosome. In prophase of mitosis, specialized regions on centromeres called kinetochores attach chr ...
is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called
sister chromatids A sister chromatid refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the dup ...
. During metaphase the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study. In animal cells, chromosomes reach their highest compaction level in
anaphase Anaphase (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

anaphase
during
chromosome segregationChromosome segregation is the process in eukaryotes by which two sister chromatids formed as a consequence of DNA replication, or paired homologous chromosomes, separate from each other and migrate to opposite poles of the Cell nucleus, nucleus. This ...
. Chromosomal recombination during
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
and subsequent
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
play a significant 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 In biology, a species is the basic unit of biological classificati ...
. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo
mitotic catastropheMitotic catastrophe refers to a mechanism of delayed mitosis-linked cell death, a sequence of events resulting from premature or inappropriate entry of cells into mitosis that can be caused by chemical or physical stresses. Mitotic catastrophe is unr ...
. Usually, this will make the cell initiate
apoptosis Apoptosis (from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ) ...

apoptosis
leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer. Some use the term chromosome in a wider sense, to refer to the individualized portions of
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
in cells, either visible or not under light microscopy. 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 condensation.


Etymology

The word ''chromosome'' () comes from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
(''chroma'', "colour") and (''soma'', "body"), describing their strong staining by particular
dye A dye is a color Color (American English) or colour (Commonwealth English) is the visual perception, visual perceptual Physical property, property corresponding in humans to the categories called ''blue'', ''green'', ''red'', etc. Colo ...
s. The term was coined by the German anatomist Heinrich Wilhelm Waldeyer, referring to the term
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
, which was introduced by
Walther Flemming Walther Flemming (21 April 1843 – 4 August 1905) was a German biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has specialized knowledg ...

Walther Flemming
, the discoverer of
cell division Cell division is the process by which a parent cell (biology), cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative ...

cell division
. 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

The German scientists
Schleiden Schleiden is a town in North Rhine-Westphalia, Germany. It lies in the Eifel hills, in the district of Euskirchen (district), Euskirchen, and has 12,998 inhabitants as of 30 June 2017. Schleiden is connected by a tourist railway to Kall, Germany, K ...

Schleiden
, and Bütschli were among the first scientists who recognized the structures now familiar as chromosomes. In a series of experiments beginning in the mid-1880s,
Theodor Boveri Theodor Heinrich Boveri (12 October 1862 – 15 October 1915) was a German zoologist Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the animal k ...

Theodor Boveri
gave definitive contributions to elucidating that chromosomes are the
vectors Vector may refer to: Biology *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism; a disease vector *Vector (molecular biology), a DNA molecule used as a vehicle to artificially carr ...
of heredity, with two notions that became known as ‘chromosome continuity’ and ‘chromosome individuality’.
Wilhelm Roux Wilhelm Roux (9 June 1850 – 15 September 1924) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of Germany, see also ...
suggested that each 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 Gregor Johann Mendel (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a meteorologist, mathematician, biologist, AugustinianAugustinian may refer to: *Augustinians Augustinians are members of Christian religious orders tha ...

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 cytologists:
Edmund Beecher Wilson Edmund Beecher Wilson FRS (For) H FRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged to be "em ...
,
Nettie Stevens Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an American American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (U ...

Nettie Stevens
,
Walter Sutton Walter Stanborough Sutton (April 5, 1877 – November 10, 1916) was an American geneticist and physician A physician (American English), medical practitioner (English in the Commonwealth of Nations, Commonwealth English), medical doctor, o ...
and 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 Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists File:Francesco Redi.jpg, Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all ti ...
remarks that the theory was hotly contested by some famous geneticists:
William Bateson William Bateson (8 August 1861 – 8 February 1926) was an English biology, biologist who was the first person to use the term genetics to describe the study of heredity, and the chief populariser of the ideas of Gregor Mendel following their red ...

William Bateson
,
Wilhelm Johannsen Wilhelm Johannsen (3 February 1857 – 11 November 1927) was a Danish pharmacist, botanist Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist i ...
, Richard Goldschmidt and 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 Indonesia-born cytogeneticist .


Prokaryotes

The
prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contig ...
s – bacteria and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
 – typically have a single
circular chromosome A circular chromosome is a chromosome in bacteria, archaea, Mitochondrial DNA#Genome structure and diversity, mitochondria, and Chloroplast DNA#Molecular structure, chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromoso ...
, but many variations exist. The chromosomes of most bacteria, which some authors prefer to call
genophore The nucleoid (meaning ''nucleus-like'') is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. The chromosome A chromosome is a long DNA molecule with part or all of the genetic mate ...
s, can range in size from only 130,000
base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids Nucleic acids are biopolymer Biopolymers are natural polymers produced by the cells of Organism, living organisms. Biopolymers consist of monomeric units that are Covalent_ ...
s in the
endosymbiotic An endosymbiont or endobiont is any organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synony ...
bacteria '' Candidatus Hodgkinia cicadicola'' and '' Candidatus Tremblaya princeps'', to more than 14,000,000 base pairs in the soil-dwelling bacterium '' Sorangium cellulosum''.
Spirochaete A spirochaete () or spirochete is a member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical ...
s of the
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
''
Borrelia ''Borrelia'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a ...
'' are a notable exception to this arrangement, with bacteria such as ''
Borrelia burgdorferi ''Borrelia burgdorferi'' is a bacterial Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganism A microorganism, or microbe,, ''mikros'', " ...
'', the cause of
Lyme disease Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the ''Borrelia'' bacterium which is spread by ticks in the genus ''Ixodes''. The most common sign of infection is an expanding red rash, known as erythema migrans ...
, containing a single ''linear'' chromosome.


Structure in sequences

Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the
origin of replication The origin of replication (also called the replication origin) is a particular sequence in a genome at which replication is initiated. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by se ...
) from which replication starts, whereas some archaea contain multiple replication origins. The genes in prokaryotes are often organized in
operons In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...
, and do not usually contain
intron An intron (for ''intragenic region'') is any Nucleic acid sequence, nucleotide sequence within a gene that is removed by RNA splicing during Post-transcriptional modification, maturation of the final RNA product. In other words, introns are non-c ...

intron
s, unlike eukaryotes.


DNA packaging

Prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
s do not possess nuclei. Instead, their DNA is organized into a structure called the
nucleoid The nucleoid (meaning ''nucleus-like'') is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. The chromosome A chromosome is a long DNA molecule with part or all of the genetic mate ...
. 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 ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
, 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 Extrachromosomal DNA (abbreviated ecDNA) is any DNA that is found off the chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryo ...
s or other
extrachromosomal DNA Extrachromosomal DNA (abbreviated ecDNA) is any DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carb ...
. These are circular structures in the
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
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 unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
. In prokaryotes (see nucleoids) and viruses, the DNA is often densely packed and organized; in the case of
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...
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 DNA supercoiling refers to the over- or under-winding of a DNA strand, and is an expression of the strain on that strand. Supercoiling is important in a number of biological processes, such as compacting DNA, and by regulating access to the gene ...
. The DNA must first be released into its relaxed state for access for transcription, regulation, and .


Eukaryotes

Each eukaryotic chromosome consists of a long linear DNA molecule associated with proteins, forming a compact complex of proteins and DNA called ''
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
.'' Chromatin contains the vast majority of the DNA of an organism, but a inherited maternally, can be found in the
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
. It is present in most
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
, with a few exceptions, for example,
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...

red blood cell
s.
Histone In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
s are responsible for the first and most basic unit of chromosome organization, the
nucleosome A nucleosome is the basic structural unit of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon ...

nucleosome
.
Eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
(
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
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 In this diagram of a duplicated chromosome, (2) identifies the centromere—the region that joins the two sister chromatids, or each half of the chromosome. In prophase of mitosis, specialized regions on centromeres called kinetochores attach chr ...
, 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 A mitochondrion (, plural mitochondria) is a double membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the ide ...

mitochondrial
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
, and some eukaryotes may have additional small circular or linear
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
ic chromosomes. In the nuclear chromosomes of
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
s, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around
histone In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
s (structural
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
s), forming a composite material called
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
.


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 Interphase is the portion of the cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA ...

interphase
(the period of the
cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication In , DNA replication is the of pro ...

cell cycle
where the cell is not dividing), two types of
chromatin Chromatin is a complex of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecu ...
can be distinguished: *
Euchromatin Image:Diagram human cell nucleus.svg, 350px, The nucleus of a human cell showing the location of euchromatin Euchromatin is a lightly packed form of chromatin (DNA, RNA, and protein) that is enriched in gene, genes, and is often (but not always) und ...
, which consists of DNA that is active, e.g., being expressed as protein. *
Heterochromatin Heterochromatin is a tightly packed form of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon r ...
, 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 In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
or
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

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. The loops of 30-nm chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells. The DNA is thus condensed about 10,000 fold. The chromosome scaffold, which is made of proteins such as
condensin Condensins are large protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellē ...
,
TOP2A DNA topoisomerase 2-alpha is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and ...
and KIF4, plays an important role in holding the chromatin into compact chromosomes. Loops of 30 nm 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 In the diagram, (1) refers to a chromatid: 1-half of two identical threadlike strands of a replicated sister chromatid pair") are joined at the region called the centromere">ister_chromatids.html" ;"title="chromosome. During cell division, the ide ...
attached to each other at the
centromere In this diagram of a duplicated chromosome, (2) identifies the centromere—the region that joins the two sister chromatids, or each half of the chromosome. In prophase of mitosis, specialized regions on centromeres called kinetochores attach chr ...
. The shorter arms are called '' p arms'' (from the French ''petit'', small) and the longer arms are called ''
q arm In genetics, a locus (plural 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 ...
s'' (''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 (from grc, μικρός ''mikrós'' 'small' and ''skopeîn'' 'to look (at); examine, inspect') is a laboratory instrument used to examine objects that are too small to be seen by the naked eye Naked eye, also called bare ...

microscope
. Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops. During mitosis,
microtubule Microtubules are polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules, or macromolecules, composed of many Repeat unit, rep ...

microtubule
s grow from centrosomes located at opposite ends of the cell and also attach to the centromere at specialized structures called
kinetochores A kinetochore (, ) is a disc-shaped protein structure associated with duplicated chromatids in eukaryote, eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centr ...
, one of which is present on each sister
chromatid A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chromat ...
. 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:
autosome An autosome is any chromosome that is not a sex chromosome (an allosome). The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome pairs which may have different structures. The DNA in autosomes is coll ...
s (body chromosome(s)) and allosome (
sex chromosome A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organi ...
(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 Mitochondrial DNA (mtDNA or mDNA) is the DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five car ...
.
Sequencing In genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor Mendel, Moravia, Moravian scientist ...

Sequencing
of the
human genome The human genome is a complete set of nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic molecules c ...

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 The Wellcome Sanger Institute, previously known as The Sanger Centre and Wellcome Trust Sanger Institute, is a non-profit A nonprofit organization (NPO), also known as a non-business entity, not-for-profit organization, or nonprofit institution ...
'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 prediction In computational biology Computational biology involves the development and application of data-analytical and theoretical methods, mathematical modelling and computational simulation techniques to the study of biological, ecological, behavioural, ...
s. Total chromosome length is an estimate as well, based on the estimated size of unsequenced
heterochromatin Heterochromatin is a tightly packed form of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon r ...
regions.


Number in various organisms


In eukaryotes

The number of chromosomes in eukaryotes is highly variable (see table). In fact, chromosomes can fuse or break and thus evolve into novel karyotypes. Chromosomes can also be fused artificially. For example, the 16 chromosomes of
yeast Yeasts are eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...

yeast
have been fused into one giant chromosome and 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 example, most
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
are
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
, like
humans Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

humans
who have 22 different types of
autosome An autosome is any chromosome that is not a sex chromosome (an allosome). The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome pairs which may have different structures. The DNA in autosomes is coll ...
s, each present as two homologous pairs, and two
sex chromosomes A sex chromosome, (also referred to as an allosome, heterotypical chromosome, gonosome, or heterochromosome, or idiochromosome) is a chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. M ...
. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosome types, such as
bread wheat Common wheat (''Triticum aestivum''), also known as bread wheat, is a cultivated wheat Wheat is a grass widely Agriculture, cultivated for its seed, a cereal grain which is a worldwide staple food. The Taxonomy of wheat, many species of whea ...
, which is ''hexaploid'' and has six copies of seven different chromosome types – 42 chromosomes in total. Normal members of a particular eukaryotic
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

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. 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 cell A somatic cell (from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ...
s (body cells), which are
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
having two sets of chromosomes (23 pairs in humans), one set from the mother and one from the father.
Gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply ...
s, reproductive cells, are
haploid Ploidy () is the number of complete sets of chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by ...
They have one set of chromosomes. Gametes are produced by
meiosis Meiosis (; , because it is a reductional division) is a special type of of in organisms used to produce the , such as or . It involves two rounds of division that ultimately result in four cells with only one copy of each (). Additionall ...

meiosis
of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (
crossover CrossOver is a Microsoft Windows Microsoft Windows, commonly referred to as Windows, is a group of several Proprietary software, proprietary graphical user interface, graphical operating system families, all of which are developed and markete ...
), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilization
), a new diploid organism is formed. Some animal and plant species are
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
They have more than two sets of
homologous chromosome A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome A chromosome is a long DNA The structure of part of a DNA double helix Deoxyribonucleic acid (; DNA) is a molecule File:Pen ...
s. Plants important in agriculture 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 type of plant that people have bred for desired traits, which are reproduced in each new generation by a method such as grafting, tissue culture or carefully controlled seed production. Most cultivars arise from purposeful human ...
s 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.


In prokaryotes

Prokaryote A prokaryote () is a single-celled organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. For example, '' Buchnera'', a
symbiont Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biology ...

symbiont
of
aphid Aphids are small sap-sucking insects and members of the Taxonomic rank, superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Erio ...

aphid
s 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

In general, the
karyotype A karyotype is a preparation of the complete set of metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies ...

karyotype
is the characteristic chromosome complement of a
eukaryote Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryote
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
. The preparation and study of karyotypes is part of
cytogenetics Cytogenetics is essentially a branch of genetics Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) Though heredity had been observed for millennia, Gregor ...
. Although the and transcription of
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
is highly standardized in
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...
, 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
and Somatic cell, soma (between gametes and the rest of the body) :3. variation between members of a population, due to polymorphism (biology), balanced genetic polymorphism :4. allopatric speciation, geographical variation between Race (classification of human beings), races :5. mosaic (genetics), 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 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 XY sex-determination system, gonosomes (sex chromosomes, in contrast to
autosome An autosome is any chromosome that is not a sex chromosome (an allosome). The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome pairs which may have different structures. The DNA in autosomes is coll ...
s). 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 Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mate ...
human cell contain?'' In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XO sex-determination system, XX/XO Sex-determination system, sex determination mechanism. Theophilus Painter, 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 XY sex-determination system, XX/XY system. New techniques were needed to definitively solve the problem: # Using cells in culture # Arresting
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
in
metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical s ...

metaphase
by a solution of colchicine # Pretreating cells in a Hypotonicity, 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 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. Pan (genus), 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 (human), chromosome 2.


Aberrations

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 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 disorders. Human examples include: * Cri du chat, which is caused by the Genetic deletion, 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.European Chromosome 11 Network
/ref> 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 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 genetic material. Cat-eye syndrome and Isodicentric 15, 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, 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 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, insecticides, and perfluorinated compounds. Increased aneuploidy is often associated with increased DNA damage in spermatozoa.


See also

* Aneuploidy * Chromomere * Chromosome segregation * Cohesin * Condensin *
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
* Deletion (genetics), Genetic deletion * Epigenetics * For information about chromosomes in genetic algorithms, see chromosome (genetic algorithm) * Genetic genealogy ** Genealogical DNA test * Lampbrush chromosome * List of number of chromosomes of various organisms * Locus (genetics), Locus (explains gene location nomenclature) * Maternal influence on sex determination * Non-disjunction * Sex-determination system ** XY sex-determination system *** X chromosome, X-chromosome **** X-inactivation *** Y chromosome, Y-chromosome **** Y-chromosomal Aaron **** Y-chromosomal Adam * Polytene chromosome * Protamine * Neochromosome * Parasitic chromosome


Notes and references


External links


An Introduction to DNA and Chromosomes
from HOPES: Huntington's Outreach Project for Education at Stanford
Chromosome Abnormalities at AtlasGeneticsOncology

On-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 genes and synteny, syntenic loci graphically via the web
Genographic Project

Home reference on Chromosomes
from the U.S. National Library of Medicine
Visualisation of human chromosomes
and comparison to other species
Unique – The Rare Chromosome Disorder Support Group
Support for people with rare chromosome disorders {{Authority control Chromosomes, Nuclear substructures Cytogenetics