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A chromosome is a long DNA molecule with part or all of the
genetic material Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cla ...
of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in
eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
cells the most important of these proteins are the
histone In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn a ...
s. These proteins, aided by chaperone proteins, bind to and
condense Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
the DNA molecule to maintain its integrity. These chromosomes display a complex three-dimensional structure, which plays a significant role in
transcriptional regulation In molecular biology and genetics, transcriptional regulation is the means by which a cell regulates the conversion of DNA to RNA (transcription), thereby orchestrating gene activity. A single gene can be regulated in a range of ways, from alt ...
. 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 light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsco ...
only during the
metaphase Metaphase ( and ) is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed in anaphase). These chromosomes, carrying genetic information, align ...
of
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
(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 in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during ...
), and both copies are joined by a
centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
, resulting either in an X-shaped structure (pictured above), if the
centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
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 () is the stage of mitosis after the process of metaphase, when replicated chromosomes are split and the newly-copied chromosomes (daughter chromatids) are moved to opposite poles of the cell. Chromosomes also reach their overall maxim ...
during
chromosome segregation Chromosome 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 nucleus. This segregati ...
. Chromosomal recombination during
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
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 ...
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 and can be attributed to the span of survival for a species. It is dis ...
. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo
mitotic catastrophe Mitotic Catastrophe has been defined as either a cellular mechanism to prevent potentially cancerous cells from proliferating or as a mode of cellular death that occurs following improper cell cycle progression or entrance. Mitotic catastrophe can ...
. Usually, this will make the cell initiate
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
. Some use the term chromosome in a wider sense, to refer to the individualized portions of
chromatin Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
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 Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
(''chroma'', "colour") and (''soma'', "body"), describing their strong staining by particular
dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...
s. The term was coined by the German anatomist Heinrich Wilhelm Waldeyer, referring to the term
chromatin Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
, which was introduced by
Walther Flemming Walther Flemming (21 April 1843 – 4 August 1905) was a German biologist and a founder of cytogenetics. He was born in Sachsenberg (now part of Schwerin) as the fifth child and only son of the psychiatrist Carl Friedrich Flemming (1799–18 ...
, the discoverer of
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
. 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, and has 12,998 inhabitants as of 30 June 2017. Schleiden is connected by a tourist railway to Kall, on the Eifel Railway between ...
, Virchow 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, comparative anatomist and co-founder of modern cytology. He was notable for the first hypothesis regarding cellular processes that cause cancer, and for descr ...
gave definitive contributions to elucidating that chromosomes are the vectors 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 zoologist and pioneer of experimental embryology. Early life Roux was born and educated in Jena, Germany where he attended university and studied under Ernst Haeckel. He also attended ...
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, Augustinians, OSA (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a biologist, meteorologist, mathematician, Augustinians, Augustinian friar and abbot of St Thomas's Abbey, Brno, St. Thomas' Abbey in Br ...
'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 (October 19, 1856 – March 3, 1939) was a pioneering American zoologist and geneticist. He wrote one of the most influential textbooks in modern biology, ''The Cell''. Career Wilson was born in Geneva, Illinois, the so ...
,
Nettie Stevens Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an American geneticist who discovered sex chromosomes. In 1905, soon after the rediscovery of Mendel's paper on genetics in 1900, she observed that male mealworms produced two kinds of sper ...
,
Walter Sutton Walter Stanborough Sutton (April 5, 1877 – November 10, 1916) was an American geneticist and physician whose most significant contribution to present-day biology was his theory that the Mendelian laws of inheritance could be applied to chrom ...
and
Theophilus Painter Theophilus Shickel Painter (August 22, 1889 – October 5, 1969) was an American zoologist best known for his work on the structure and function of chromosomes, especially the sex-determination genes X and Y in humans. He was the first to discove ...
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 Boveri–Sutton chromosome theory (also known as the chromosome theory of inheritance or the Sutton–Boveri theory) is a fundamental unifying theory of genetics which identifies chromosomes as the carriers of genetic material.< ...
(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. He was also a renowned Taxonomy (biology), taxonomist, tropical explorer, ornithologist, Philosophy of biology, philosopher o ...
remarks that the theory was hotly contested by some famous geneticists:
William Bateson William Bateson (8 August 1861 – 8 February 1926) was an English 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 rediscove ...
,
Wilhelm Johannsen Wilhelm Johannsen (3 February 1857 – 11 November 1927) was a Danish pharmacist, botanist, plant physiologist, and geneticist. He is best known for coining the terms gene, phenotype and genotype, and for his 1903 "pure line" experiments in g ...
,
Richard Goldschmidt Richard Benedict Goldschmidt (April 12, 1878 – April 24, 1958) was a German-born American geneticist. He is considered the first to attempt to integrate genetics, development, and evolution. He pioneered understanding of reaction norms, gen ...
and
T.H. Morgan Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, Embryology, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating t ...
, 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 Theophilus Shickel Painter (August 22, 1889 – October 5, 1969) was an American zoologist best known for his work on the structure and function of chromosomes, especially the sex-determination genes X and Y in humans. He was the first to discove ...
. 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
Joe Hin Tjio Joe Hin Tjio (2 November 1919 – 27 November 2001), was an Indonesian-born American cytogeneticist. He was renowned as the first person to recognize the normal number of human chromosomes on December 22, 1955 at the Institute of Genetics of the ...
.


Prokaryotes

The
prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
s – bacteria and
archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
 – typically have a single
circular chromosome A circular chromosome is a chromosome in bacteria, archaea, mitochondria, and chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromosome of most eukaryotes. Most prokaryote chromosomes contain a circular DNA molecu ...
, 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 of a prokaryote is circular, and its length is very large compared to the cell d ...
s, 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 An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
bacteria '' Candidatus Hodgkinia cicadicola'' and '' Candidatus Tremblaya princeps'', to more than 14,000,000 base pairs in the soil-dwelling bacterium ''
Sorangium cellulosum ''Sorangium cellulosum'' is a soil-dwelling Gram-negative bacterium of the group myxobacteria. It is motile and shows gliding motility. Under stressful conditions this motility, as in other myxobacteria, the cells congregate to form fruiting bodi ...
''.
Spirochaete A spirochaete () or spirochete is a member of the phylum Spirochaetota (), (synonym Spirochaetes) which contains distinctive diderm (double-membrane) gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or s ...
s of the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''
Borrelia ''Borrelia'' is a genus of bacteria of the spirochete phylum. Several species cause Lyme disease, also called Lyme borreliosis, a zoonotic, vector-borne disease transmitted by ticks. Other species of ''Borrelia'' cause relapsing fever, and are tr ...
'' are a notable exception to this arrangement, with bacteria such as ''
Borrelia burgdorferi ''Borrelia burgdorferi'' is a bacterial species of the spirochete class in the genus ''Borrelia'', and is one of the causative agents of Lyme disease in humans. Along with a few similar genospecies, some of which also cause Lyme disease, it make ...
'', 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 migran ...
, 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 semi ...
) from which replication starts, whereas some archaea contain multiple replication origins. The genes in prokaryotes are often organized in
operons In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...
, and do not usually contain
intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene. ...
s, unlike eukaryotes.


DNA packaging

Prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
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 ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
, 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; how ...
s or other extrachromosomal DNA. These are circular structures in the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
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 organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...
. In prokaryotes (see
nucleoids 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 of a prokaryote is circular, and its length is very large compared to the cell d ...
) and viruses, the DNA is often densely packed and organized; in the case of
archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...
, 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 that separates and protects the interior of all cells from the outside environment (t ...
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 Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...
, regulation, and replication.


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 and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
.'' Chromatin contains the vast majority of the DNA of an organism, but a small amount inherited maternally, can be found in the
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
. It is present in most cells, 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 ''erythros'' for "red" and ''kytos'' for "holl ...
s.
Histone In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn a ...
s 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 proteins and resembles thread wrapped around a spool. The nucleosome is the fundamen ...
.
Eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
( 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 The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
, 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 (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is use ...
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
, and some eukaryotes may have additional small circular or linear
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
ic chromosomes. In the nuclear chromosomes of
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
s, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around
histone In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn a ...
s (structural
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s), forming a composite material called
chromatin Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
.


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 that is not accompanied by visible changes under the microscope, and includes the G1, S and G2 phases. During interphase, the cell grows (G1), replicates its DNA (S) and prepares for mitosis (G2). A c ...
(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) and some of its organelles, and subs ...
where the cell is not dividing), two types of
chromatin Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
can be distinguished: *
Euchromatin Euchromatin (also called "open chromatin") is a lightly packed form of chromatin ( DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active transcription. Euchromatin stands in contrast to heterochromatin, whic ...
, which consists of DNA that is active, e.g., being expressed as protein. *
Heterochromatin Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
, 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, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
or
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
(cell division), the chromatin double helix become more and more condensed. They cease to function as accessible genetic material (
transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...
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 complexes that play a central role in chromosome assembly and segregation during mitosis and meiosis (Figure 1). Their subunits were originally identified as major components of mitotic chromosomes assembled in ''Xeno ...
,
TOP2A DNA topoisomerase IIα is a human enzyme encoded by the ''TOP2A'' gene. Topoisomerase IIα relives topological DNA stress during transcription, condenses chromosomes, and separates chromatids. It catalyzes the transient breaking and rejoining of ...
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 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 chro ...
attached to each other at the
centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
. The shorter arms are called ''
p 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'' (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 () is a 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 means being invisibl ...
. Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops. During mitosis,
microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27  nm and have an inner diameter between 11 an ...
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 eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centromere and ...
, 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 chro ...
. 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. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome, allosomal (sex chromosome) pairs, which may have different structures. The DNA in au ...
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 that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
(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 located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...
.
Sequencing In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which succ ...
of the
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the n ...
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 organisation, non-profit British genomics and genetics research institute, primarily funded by the Wellcome Trust. It is l ...
'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, gene prediction or gene finding refers to the process of identifying the regions of genomic DNA that encode genes. This includes protein-coding genes as well as RNA genes, but may also include prediction of other functiona ...
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 or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
regions. Based on the micrographic characteristics of size, position of the
centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
and sometimes the presence of a
chromosomal satellite Satellite or SAT chromosomes are chromosomes that contain secondary constructs that serve as identification. They are observed in Acrocentric chromosomes. In addition to the centromere, one or more secondary constrictions can be observed in so ...
, 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 metaphase 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 disce ...
is the characteristic chromosome complement of a
eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
species In biology, a species is the basic unit of 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 individuals of the appropriate s ...
. The preparation and study of karyotypes is part of
cytogenetics Cytogenetics is essentially a branch of genetics, but is also a part of cell biology/cytology (a subdivision of human anatomy), that is concerned with how the chromosomes relate to cell behaviour, particularly to their behaviour during mitosis an ...
. Although the replication and
transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...
of DNA is highly standardized in
eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
, 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 and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. Th ...
and
soma Soma may refer to: Businesses and brands * SOMA (architects), a New York–based firm of architects * Soma (company), a company that designs eco-friendly water filtration systems * SOMA Fabrications, a builder of bicycle frames and other bicycle ...
(between
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
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'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
(in a reaction vial) with
colchicine Colchicine is a medication used to treat gout and Behçet's disease. In gout, it is less preferred to NSAIDs or steroids. Other uses for colchicine include the management of pericarditis and familial Mediterranean fever. Colchicine is taken ...
. 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
autosome An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome, allosomal (sex chromosome) pairs, which may have different structures. The DNA in au ...
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, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
human cell contain?'' In 1912, Hans von Winiwarter reported 47 chromosomes in
spermatogonia A spermatogonium (plural: ''spermatogonia'') is an undifferentiated male germ cell. Spermatogonia undergo spermatogenesis to form mature spermatozoa in the seminiferous tubules of the testis. There are three subtypes of spermatogonia in humans: ...
and 48 in
oogonia An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes. In the mammalian fetus Oogonia are formed in lar ...
, concluding an XX/XO sex determination mechanism.
Painter Painting is the practice of applying paint, pigment, color or other medium to a solid surface (called the "matrix" or "support"). The medium is commonly applied to the base with a brush, but other implements, such as knives, sponges, and ai ...
in 1922 was not certain whether the diploid number of man is 46 or 48, at first favouring 46. He revised his opinion later from 46 to 48, and he correctly insisted on humans having an XX/XY system. New techniques were needed to definitively solve the problem: # Using cells in culture # Arresting
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
in
metaphase Metaphase ( and ) is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed in anaphase). These chromosomes, carrying genetic information, align ...
by a solution of
colchicine Colchicine is a medication used to treat gout and Behçet's disease. In gout, it is less preferred to NSAIDs or steroids. Other uses for colchicine include the management of pericarditis and familial Mediterranean fever. Colchicine is taken ...
# Pretreating cells in a
hypotonic solution In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane-i ...
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.
Chimpanzee The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative th ...
s, the closest living relatives to modern humans, have 48 chromosomes as do the other
great apes The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); ''Gorilla'' (the east ...
: in humans two chromosomes fused to form
chromosome 2 Chromosome 2 is one of the twenty-three pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 2 is the second-largest human chromosome, spanning more than 242 million base pairs and representing almost e ...
.


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 Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with physical growth delays, mild to moderate intellectual dis ...
, although most aberrations have little to no effect. Some chromosome abnormalities do not cause disease in carriers, such as
translocations In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translo ...
, or
chromosomal inversions An inversion is a chromosome rearrangement in which a segment of a chromosome becomes inverted within its original position. An inversion occurs when a chromosome undergoes a two breaks within the chromosomal arm, and the segment between the two br ...
, 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 Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any ...
, 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 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 chromosomal abnormality. Although polygenic disorders ...
. Human examples include: *
Cri du chat Cri du chat syndrome is a rare genetic disorder due to a partial chromosome deletion on chromosome 5. Its name is a French term ("cat-cry" or " call of the cat") referring to the characteristic cat-like cry of affected children. It was first de ...
, which is caused by the deletion of part of the short arm of chromosome 5. "Cri du chat" means "cry of the cat" in French; the condition was so-named because affected babies make high-pitched cries that sound like those of a cat. Affected individuals have wide-set eyes, a small head and jaw, moderate to severe mental health problems, and are very short. *
Down syndrome Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with physical growth delays, mild to moderate intellectual dis ...
, the most common trisomy, usually caused by an extra copy of chromosome 21 (
trisomy 21 A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two. A trisomy is a type of aneuploidy (an abnormal number of chromosomes). Description and causes Most organisms that reprodu ...
). Characteristics include decreased muscle tone, stockier build, asymmetrical skull, slanting eyes and mild to moderate developmental disability. *
Edwards syndrome Edwards syndrome, also known as trisomy 18, is a genetic disorder caused by the presence of a third copy of all or part of chromosome 18. Many parts of the body are affected. Babies are often born small and have heart defects. Other features in ...
, 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 Isodicentric 15, also called marker chromosome 15 syndrome, idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15), is a chromosome abnormality in which a child is born with extra genetic material from chromosome 15. People wit ...
, also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15). *
Jacobsen syndrome Jacobsen syndrome is a rare Chromosome abnormality, chromosomal disorder resulting from Deletion (genetics), deletion of genes from chromosome 11 that includes band 11q24.1. It is a congenital disorder. Since the deletion takes place on the q arm ...
, which is very rare. It is also called the terminal 11q deletion disorder. Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called
Paris-Trousseau syndrome Paris-Trousseau syndrome (PTS) is an inherited disorder characterized by mild hemorrhagic tendency associated with 11q chromosome deletion. It manifests as a granular defect within an individual's platelets. It is characterized by thrombocytes w ...
. *
Klinefelter syndrome Klinefelter syndrome (KS), also known as 47,XXY, is an aneuploid genetic condition where a male has an additional copy of the X chromosome. The primary features are infertility and small, poorly functioning testicles. Usually, symptoms are su ...
(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 Speech delay, also known as alalia, refers to a delay in the development or use of the mechanisms that produce speech. Speech – as distinct from language – is the actual process of making sounds, using such organs and structures as the lungs, ...
and
dyslexia Dyslexia, also known until the 1960s as word blindness, is a disorder characterized by reading below the expected level for one's age. Different people are affected to different degrees. Problems may include difficulties in spelling words, r ...
. Without testosterone treatment, some may develop
gynecomastia Gynecomastia (also spelled gynaecomastia) is the abnormal non-cancerous enlargement of one or both breasts in males due to the growth of breast tissue as a result of a hormone imbalance between estrogens and androgens. Updated by Brent Wisse (1 ...
during puberty. *
Patau Syndrome Patau syndrome is a syndrome caused by a chromosome, chromosomal abnormality, in which some or all of the Cell (biology), cells of the body contain Trisomy, extra genetic material from chromosome 13 (human), chromosome 13. The extra genetic mater ...
, 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 A small supernumerary marker chromosome (sSMC) is an abnormal extra chromosome. It contains copies of parts of one or more normal chromosomes and like normal chromosomes is located in the cell's nucleus, is replicated and distributed into each d ...
. 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 Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic condition in which a female is partially or completely missing an X chromosome. Signs and symptoms vary among those affected. Often, a short and webbed neck, low-set ears, low hair ...
(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 Wolf–Hirschhorn syndrome (WHS) is a chromosomal deletion syndrome resulting from a partial deletion on the short arm of chromosome 4 (del(4p16.3)). Features include a distinct craniofacial phenotype and intellectual disability. Signs and sympto ...
, 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 syndrome, also known as Jacobs syndrome, is an aneuploid genetic condition in which a male has an extra Y chromosome. There are usually few symptoms. These may include being taller than average, acne, and an increased risk of learning disa ...
. 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.


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, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...
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 organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
are
diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
, like
humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
who have 22 different types of
autosome An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome, allosomal (sex chromosome) pairs, which may have different structures. The DNA in au ...
s, each present as two homologous pairs, and two
sex chromosomes A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
. 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 species. About 95% of wheat produced worldwide is common wheat; it is the most widely grown of all crops and the cereal with the highest monetary yield. Ta ...
, 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 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 individuals of the appropriate s ...
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 σῶμα ''sôma'', meaning "body"), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Such cells compo ...
s (body cells), which are
diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
nhaving two sets of chromosomes (23 pairs in humans), one set from the mother and one from the father.
Gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...
s, reproductive cells, are
haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
They have one set of chromosomes. Gametes are produced by
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
of a diploid
germ line In biology and genetics, the germline is the population of a multicellular organism's cells that pass on their genetic material to the progeny (offspring). In other words, they are the cells that form the egg, sperm and the fertilised egg. They ...
cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (
crossover Crossover may refer to: Entertainment Albums and songs * ''Cross Over'' (Dan Peek album) * ''Crossover'' (Dirty Rotten Imbeciles album), 1987 * ''Crossover'' (Intrigue album) * ''Crossover'' (Hitomi Shimatani album) * ''Crossover'' (Yoshino ...
), 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 spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
), a new diploid organism is formed. Some animal and plant species are
polyploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei ( eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contain ...
n 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 that pair up with each other inside a cell during fertilization. Homologs have the same genes in the same loci where they provide points alon ...
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 cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture, ...
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 that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
species In biology, a species is the basic unit of 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 individuals of the appropriate s ...
generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. For example, '' Buchnera'', a
symbiont Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
of
aphid Aphids are small sap-sucking insects and members of the superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white woolly aphids. A t ...
s has multiple copies of its chromosome, ranging from 10–400 copies per cell. However, in some large bacteria, such as ''
Epulopiscium fishelsoni "''Candidatus'' Epulonipiscium" is a genus of Gram-positive bacteria that have a symbiotic relationship with surgeonfish. These bacteria are known for their unusually large size, many ranging from 200–700 μm in length. Until the discovery of ...
'' 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

*
Aneuploidy Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any ...
* Chromomere *
Chromosome segregation Chromosome 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 nucleus. This segregati ...
*
Cohesin Cohesin is a protein complex that mediates sister chromatid cohesion, homologous recombination, and DNA looping. Cohesin is formed of SMC3, SMC1, SCC1 and SCC3 ( SA1 or SA2 in humans). Cohesin holds sister chromatids together after DNA rep ...
*
Condensin Condensins are large protein complexes that play a central role in chromosome assembly and segregation during mitosis and meiosis (Figure 1). Their subunits were originally identified as major components of mitotic chromosomes assembled in ''Xeno ...
* DNA *
Genetic deletion In genetics, a deletion (also called gene deletion, deficiency, or deletion mutation) (sign: Δ) is a mutation (a genetic aberration) in which a part of a chromosome or a sequence of DNA is left out during DNA replication. Any number of nucleoti ...
*
Epigenetics In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are "o ...
* For information about chromosomes in
genetic algorithm In computer science and operations research, a genetic algorithm (GA) is a metaheuristic inspired by the process of natural selection that belongs to the larger class of evolutionary algorithms (EA). Genetic algorithms are commonly used to gene ...
s, see
chromosome (genetic algorithm) In genetic algorithms, a chromosome (also sometimes called a genotype) is a set of parameters which define a proposed solution to the problem that the genetic algorithm is trying to solve. The set of all solutions is known as the ''population''. T ...
*
Genetic genealogy Genetic genealogy is the use of genealogical DNA tests, i.e., DNA profiling and DNA testing, in combination with traditional genealogical methods, to infer genetic relationships between individuals. This application of genetics came to be used b ...
**
Genealogical DNA test A genealogical DNA test is a DNA-based test used in genetic genealogy that looks at specific locations of a person's genome in order to find or verify ancestral genealogical relationships, or (with lower reliability) to estimate the ethnic mixt ...
* Lampbrush chromosome * List of number of chromosomes of various organisms *
Locus Locus (plural loci) is Latin for "place". It may refer to: Entertainment * Locus (comics), a Marvel Comics mutant villainess, a member of the Mutant Liberation Front * ''Locus'' (magazine), science fiction and fantasy magazine ** ''Locus Award' ...
(explains gene location nomenclature) *
Maternal influence on sex determination The XY sex-determination system is a sex-determination system used to classify many mammals, including humans, some insects (''Drosophila''), some snakes, some fish (guppy, guppies), and some plants (''Ginkgo'' tree). In this system, the sex of ...
*
Microchromosome A microchromosome (μChr) is a type of very small chromosome which is a typical component of the karyotype of birds, some reptiles, fish, and amphibians; they have yet to be found in mammals. They are less than 20 Mb in size; chromosomes which a ...
*
Minichromosome A minichromosome is a small chromatin-like structure resembling a chromosome and consisting of centromeres, telomeres and replication origins but little additional genetic material. They replicate autonomously in the cell during cellular division. ...
*
Non-disjunction 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 ...
*
Secondary chromosome Secondary chromosomes (recently renamed chromids) are a class of bacterial replicons (replicating DNA molecules). These replicons are called "chromids" because they have characteristic features of both ''chrom''osomes and plasm''id''s. Early on, ...
*
Sex-determination system A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. In some species there are hermap ...
**
XY sex-determination system The XY sex-determination system is a sex-determination system used to classify many mammals, including humans, some insects (''Drosophila''), some snakes, some fish (guppies), and some plants (''Ginkgo'' tree). In this system, the sex of an indi ...
***
X-chromosome The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex- ...
****
X-inactivation X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into ...
***
Y-chromosome The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abs ...
****
Y-chromosomal Aaron Y-chromosomal Aaron is the name given to the hypothesized most recent common ancestor of the patrilineal Jewish priestly caste known as ''Kohanim'' (singular "Kohen", also spelled "Cohen"). According to the traditional understanding of the Hebrew ...
****
Y-chromosomal Adam In human genetics, the Y-chromosomal most recent common ancestor (Y-MRCA, informally known as Y-chromosomal Adam) is the patrilineal most recent common ancestor (MRCA) from whom all currently living humans are descended. He is the most recent mal ...
**
ZO sex-determination system The ZO sex-determination system is a system that determines the sex of offspring in several moths. In those species, there is one sex chromosome, Z. Males have two Z chromosomes, whereas females have one Z. Males are ZZ, while females are ZO. S ...
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ZW sex-determination system The ZW sex-determination system is a chromosomal system that determines the sex of offspring in birds, some fish and crustaceans such as the giant river prawn, some insects (including butterflies and moths), the schistosome family of flatworms, ...
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XO sex-determination system The XO sex-determination system (sometimes X0 sex-determination system) is a system that some species of insects, arachnids, and mammals use to determine the sex of offspring. In this system, there is only one sex chromosome, referred to as X. Male ...
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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 only observed in reptiles and teleost fish ...
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Haplodiploid sex-determination system Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky. Haplodiploidy determines the sex ...
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Polytene chromosome Polytene chromosomes are large chromosomes which have thousands of DNA strands. They provide a high level of function in certain tissues such as salivary glands of insects. Polytene chromosomes were first reported by E.G.Balbiani in 1881. Pol ...
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Protamine Protamines are small, arginine-rich, cell nucleus, nuclear proteins that replace histones late in the haploid phase of spermatogenesis and are believed essential for sperm head condensation and DNA stabilization. They may allow for denser packagin ...
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Neochromosome A neochromosome is a chromosome that is not normally found in nature. Cancer-associated neochromosomes are found in some cancer cells. Neochromosomes have also been created using genetic engineering techniques. Cancer-associated neochromosomes ...
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Parasitic chromosome Parasitic chromosomes are "selfish" chromosomes that propagate throughout cell divisions, even if they confer no benefit to the overall organism's survival. Parasitic chromosomes can persist even if slightly detrimental to survival, as is characteri ...


Notes and references


External links


An Introduction to DNA and Chromosomes
from
HOPES The Huntington's disease Outreach Project for Education at Stanford (HOPES) is a student-run project at Stanford University dedicated to making scientific information about Huntington's disease (HD) more readily accessible to patients and the publ ...
: 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 Ensembl genome database project is a scientific project at the European Bioinformatics Institute, which provides a centralized resource for geneticists, molecular biologists and other researchers studying the genomes of our own species and other v ...
project, presenting chromosomes, their
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s and
syntenic In genetics, the term synteny refers to two related concepts: * In classical genetics, ''synteny'' describes the physical co-localization of genetic loci on the same chromosome within an individual or species. * In current biology, ''synteny'' mo ...
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 Nuclear substructures Cytogenetics