chromosome

1. A threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries the genes and functions in the transmission of hereditary information.
2. A circular strand of DNA in bacteria that contains the hereditary information necessary for cell life.

Any of the organized components of each cell which carry the individual's hereditary material, deoxyribonucleic acid (DNA). Chromosomes are found in all organisms with a cell nucleus (eukaryotes) and are located within the nucleus. Each chromosome contains a single extremely long DNA molecule that is packaged by various proteins into a compact domain. A full set, or complement, of chromosomes is carried by each sperm or ovum in animals and each pollen grain or ovule in plants. This constitutes the haploid (n) genome of that organism and contains a complete set of the genes characteristic of that organism. Sexually reproducing organisms in both the plant and animal kingdoms begin their development by the fusion of two haploid germ cells and are thus diploid (2n), with two sets of chromosomes in each body cell. These two sets of chromosomes carry virtually all the thousands of genes of each cell, with the exception of the tiny number in the mitochrondria (in animal), and a few plant chloroplasts. See also Deoxyribonucleic acid (DNA); Gene.

Chromosomes can change their conformation and degree of compaction throughout the cell cycle. During interphase, the major portion of the cycle, chromosomes are not visible under the light microscope because, although they are very long, they are extremely thin. However, during cell division (mitosis or meiosis), the chromosomes become compacted into shorter and thicker structures that can be seen under the microscope. At this time they appear as paired rods with defined ends, called telomeres, and they remain joined at a constricted region, the centromere, until the beginning of anaphase of cell division. See also Cell cycle; Meiosis; Mitosis.

Chromosomes are distinguished from one another by length and position of the centromere. They are metacentric (centromere in the middle of the chromosome), acrocentric (centromere close to one end), or telocentric (centromere at the end, or telomere). The centromere thus usually lies between two chromosome arms, which contain the genes and their regulatory regions, as well as other DNA sequences that have no known function. In many species, regional differences in base composition and in the time at which the DNA is replicated serve as the basis for special staining techniques that make visible a series of distinctive bands on each arm, and these can be used to identify the chromosome.

Compaction

Each nucleus in the cell of a human or other mammal contains some 6 billion base pairs of DNA which, if stretched out, would form a very thin thread about 6 ft (2 m) long. This DNA has to be packaged into the chromosome within a nucleus that is much smaller than a printed dot. Each chromosome contains a single length of DNA comprising a specific portion of the genetic material of the organism. Tiny stretches of DNA, about 140 base pairs long and containing acidic phosphate groups, are individually wrapped around an octamer consisting of two molecules of each of the four basic histone proteins H2a, H2b, H3, and H4. This arrangement produces small structures called nucleosomes and results in a sevenfold compaction of the DNA strand. Further compaction is achieved by binding the histone protein H1 and several nonhistone proteins, resulting in a supercoiled structure in which the chromosome is shortened by about 1600-fold in the interphase nucleus and by about 8000-fold during metaphase and anaphase, where the genetic material must be fully compacted for transport to the two daughter cells. At the point of maximum compaction, human chromosomes range in size from about 2 to 10 micrometers in length, that is, less than 0.0004 in. See also Nucleosome.

Number and size

Each diploid (2n) organism has a characteristic number of chromosomes in each body (somatic) cell, which can vary from two in a nematode worm and one species of ant, to hundreds in some butterflies, crustaceans, and plants. The diploid number of chromosomes includes a haploid (n) set from each parent. Many one-celled organisms are haploid throughout most of their life cycle. The human diploid number is 46.

There is some relationship between the number of chromosomes and their size. Some of the chromosomes in certain classes of organisms with large numbers of chromosomes are very tiny, and have been called microchromosomes. In birds and some reptiles, there are about 30–40 pairs of microchromosomes in addition to 5–7 or so pairs of regular-sized macrochromosomes. The number of microchromosomes is constant in any species carrying them, and only their size distinguishes them from the widespread macrochromosomes. At least seven microchromosomes in birds have been shown to contain genes, and all are thought to.

In some species of insects, plants, flatworms, snails, and rarely vertebrates (such as the fox), the number of chromosomes can vary because of the presence of a variable number of accessory chromosomes, called B chromosomes. It is not clear what role, if any, B chromosomes play, but they appear to be made primarily of DNA that neither contains functional genes nor has much effect on the animal or plant even when present in multiple copies.

Structure

A telomere caps each end of every chromosome and binds specific proteins that protect it from being digested by enzymes (exonucleases) present in the same cell. Most important, the telomere permits DNA replication to continue to the very end of the chromosome, thus assuring its stability. The telomere is also involved in attachment of the chromosome ends to the nuclear membrane and in pairing of homologous chromosomes during meiosis. The structure of telomeric DNA is very similar in virtually all eukaryotic organisms except the fruit fly (Drosophila). One strand of the DNA is rich in guanine and is oriented toward the end of the chromosome, and the other strand is rich in cytosine and is oriented toward the centromere. In most organisms, the telomere consists of multiple copies of a very short DNA repeat.

The centromere is responsible for proper segregation of each chromosome pair during cell division. The chromatids in mitosis and each pair of homologous chromosomes in meiosis are held together at the centromere until anaphase, when they separate and move to the spindle poles, thus being distributed to the two daughter cells. The kinetochore, which is the attachment site for the microtubules that guide the movement of the chromosomes to the poles, is organized around the centromere. The molecular structures of centromeres in most species are still unclear. The repetitive DNA making up and surrounding the centromere is called heterochromatin because it remains condensed throughout the cell cycle and hence stains intensely.

One or more pairs of chromosomes in each species have a region called a secondary constriction which does not stain well. This region contains multiple copies of the genes that transcribe, within the nucleolus, the ribosomal RNA (rRNA). The number of active rRNA genes may be regulated, and an organism that has too few copies of the rRNA genes may develop abnormally or not survive. See also Ribosomes.

Staining

Staining with quinacrine mustard produces consistent, bright and less bright fluorescence bands (Q bands) along the chromosome arms because of differences in the relative amounts of CG (cytosine-guanine) or AT (adenine-thymine) base pairs. The distinctive Q-band pattern of each chromosome makes it possible to identify every chromosome in the human genome. Quinacrine fluorescence can also reveal a difference in the amount or type of heterochromatin on the two members of a homologous pair of chromosomes, called heteromorphism or polymorphism. Such differences can be used to identify the parental origin of a specific chromosome, such as the extra chromosome in individuals who have trisomy 21. Two other methods involve treating chromosomes in various ways before staining with Giemsa. Giemsa or G-band patterns are essentially identical to Q-band patterns; reverse Giemsa or R-band patterns are the reverse, or reciprocal, of those seen with Q or G banding. In humans, most other mammals, and birds (macrochromosomes only), the Q-, G-, and R-banding patterns are so distinctive that each chromosome pair can be individually identified, making it possible to construct a karyotype, or organized array of the chromosome pairs from a single cell (Fig. 1). The chromosomes are identified on the basis of the banding patterns, and the pairs are arranged and numbered in some order, often based on length. In the human karyotype, the autosomes are numbered 1 through 22, and the sex chromosomes are called X and Y. The short arm of a chromosome is called the p arm, and the long arm is called the q arm; a number is assigned to each band on the arm. Thus, band 1q23 refers to band 23 on the long arm of human chromosome 1.

G-banded metaphase karyotype of a human male cell. Every chromosome pair can be identified by its banding pattern. Chromosome 1 is about 12 μm long.

Imprinting

A chromosome carries the same complement of genes whether it is transmitted from the father or the mother, and most of these genes appear to be functionally the same. However, a small number of mammalian genes are functionally different depending on whether they were transmitted by the egg or by the sperm. This phenomenon is known as imprinting. It appears to be caused by the inactivation of certain genes in sperm or ova, probably by methylation of cytosine residues within the regulatory (promotor) region of the imprinted gene. As a result of imprinting, normal development of the mammalian embryo requires the presence of both a maternal and a paternal set of chromosomes. Parthenogenesis, the formation of a normal individual from two sets of maternal chromosomes, is therefore not possible in mammals.

Sex chromosomes

In most mammals, the sex of an individual is determined by whether or not a Y chromosome is present because the Y chromosome carries the male-determining SRY gene. Thus XX and the rare XO individuals are female, while XY and the uncommon XXY individuals are male. In contrast, sex in the fruit fly depends on the balance of autosomes (non-sex chromosomes) and X chromosomes. Thus, in diploids, XX and the rare XXY flies are female, while XY and the rare XO flies are male. In both mammals and fruit flies, males are the heterogametic sex, producing gametes that contain either an X or a Y chromosome; and females are the homogametic sex, producing only gametes containing an X. In birds and butterflies, however, females are the heterogametic sex and males the homogametic sex. Other sex-determining systems are used by some classes of organisms, while sex in some species is determined by a single gene or even by environmental factors such as temperature (some turtles and alligators) or the presence of a nearby female (Bonellia, a marine worm) rather than by a chromosome-mediated mechanism.

More than 900 gene loci have been mapped to the human X chromosome. If the genes on both X chromosomes were fully expressed in female mammalian cells, then male cells, which have only one X, would exhibit only half as much gene product as female cells. However, dosage compensation is achieved, because genes on only one X chromosome are expressed, and genes on any additional X chromosomes are inactivated. This X inactivation randomly occurs during an early stage in embryonic development, and is transmitted unchanged to each of the daughter cells. Mammalian females are therefore mosaics of two types of cells, those with an active maternally derived X and those with an active paternally derived X. Species other than mammals do not show this type of dosage compensation mechanism for sex-linked genes, and some show none at all.

The Y chromosome is one of the smallest chromosomes in the genome in most mammalian species. Usually the mammalian Y chromosome has a very high proportion of heterochromatin, as does the large Y chromosome in Drosophila. Very few genes are located on the Y chromosome in mammals or in Drosophila, and most of these genes are concerned with either sex determination or the production of sperm. In some species of insects and other invertebrates, no Y chromosome is present, and sex in these species is determined by the X:autosome balance (XX female, XO male). See also Cell nucleus; Genetics; Human genetics; Sex determination; Sex-linked inheritance.

One of a number of small, dark-staining, and more or less rod-shaped bodies situated in the nucleus of a cell. At the time of cell division, chromosomes divide and distribute equally to the daughter cells. They contain genes arranged along their length. The number of chromosomes in the somatic cells of an individual is constant (the diploid number), whereas just half this number (the haploid number) appears in germ cells.

(click to enlarge)
During the first stages of cell division, the recognizable double-stranded chromosome is formed by … (credit: © Merriam-Webster Inc.)

Microscopic, threadlike part of a cell that carries hereditary information in the form of genes. The structure and location of chromosomes differentiate prokaryotic cells from eukaryotic cells (see prokaryote, eukaryote). Every species has a characteristic number of chromosomes; humans have 23 pairs (22 pairs of autosomal, or nonsex, chromosomes and one pair of sex chromosomes). Human chromosomes consist primarily of DNA. During cell division (see meiosis, mitosis), chromosomes are distributed evenly among daughter cells. In sexually reproducing organisms, the number of chromosomes in somatic (nonsex) cells is diploid, while gametes or sex cells (egg and sperm) produced by meiosis are haploid (see ploidy). Fertilization restores the diploid set of chromosomes in the zygote.

For more information on chromosome, visit Britannica.com.

The threadlike bodies in the nucleus of a cell that contain the strands of DNA which carry genes. In sexual reproduction, chromosomes pair up and divide in ways which randomly allocate genes to the resulting gametes, ensuring inherited diversity in the resulting progeny.

Coiled structure in the nucleus of cells, consisting of DNA and proteins.

A chromosome is the threadlike part of a cell that contains DNA and carries the genetic material of a cell. In prokaryotic cells chromosomes consist entirely of DNA and are not enclosed in a nuclear membrane. In eukaryotic cells the chromosomes are found within the nucleus and contain both DNA and RNA.

The small bodies in the nucleus of a cell that carry the chemical “instructions” for reproduction of the cell. They consist of strands of DNA wrapped in a double helix around a core of proteins. Each species of plant or animal has a characteristic number of chromosomes. For human beings, for example, it is forty-six.

In animal cells, a structure in the nucleus, containing a linear thread of deoxyribonucleic acid (DNA), which transmits genetic information and is associated with ribonucleic acid and histones.
During cell division the material composing the chromosome is compactly coiled, making it visible with appropriate staining and permitting its movement in the cell with minimal entanglement. Each organism of a species is normally characterized by the same number of chromosomes in its somatic cells. The diploid numbers (number of total chromosomes per cell) are cattle—60, sheep—54, horse—64, donkey—62, pig—38, dog—78, cat—38, human—46. The chromosomes are arranged in pairs and one of the pairs is the sex chromosomes (XX or XY), which determines the sex of the organism. See also heredity.

• compound c. — a genetic engineering procedure which produces two chromosomes in one of which the left arms of the two original chromosomes are joined together and the two original right arms are also joined together; used in genetic control of insect populations.
• homologous c's — the chromosomes of a matching pair in the diploid complement that contain alleles of specific genes.
• lampbrush c. — so named because of the bristling appearance given them by many open loops of chromatin along the extended chromosome.
• ring c. — a chromosome in which both ends have been lost (deletion) and the two broken ends have reunited to form a ring-shaped figure.
• sex c's — the chromosomes responsible for determination of the sex of the individual that develops from a zygote, in mammals constituting an unequal pair, the X and the Y chromosome.
• somatic c. — autosome.
• submetacentric c. — see submetacentric.
• W c. — sex chromosome in animals such as poultry in which the female is the heterogametic state, the male has the ZZ genotype and the female the ZW genotype.
• X c. — the female sex chromosome, being carried by half the male gametes and all female gametes; female diploid cells have two X chromosomes, the male has the XY genotype.
• Y c. — the male sex chromosome, being carried by half the male gametes and none of the female gametes; male diploid cells have an X and a Y chromosome; females carry the XX genotype.
• Z c. — sex chromosome in animals, such as poultry, in which the female is the heterogametic sex; the male has the ZZ genotype and the female the ZW genotype.

For a non-technical introduction to the topic, see Introduction to genetics.

Diagram of a duplicated and condensed (metaphase) eukaryotic chromosome. (1) Chromatid – one of the two identical parts of the chromosome after S phase. (2) Centromere – the point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm.

A chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα (chroma, color) and σῶμα (soma, body) due to their property of being very strongly stained by particular dyes. Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear, and can be composed of 10,000 to 1,000,000,000^[1] nucleotides in a long chain. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example, mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.

In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes are the essential unit for cellular division and must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, whereas duplicated chromosomes (copied during synthesis phase) contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure (pictured to the right). Chromosomal recombination plays a vital role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may aberrantly evade apoptosis leading to the progression of cancer.

However, in practice "chromosome" is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a plasmid or a small chromosome. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest chromosomes are found in viruses: these DNA or RNA molecules are short linear or circular chromosomes that often lack any structural proteins.

Contents 1 History 1.1 Nucleus as the seat of heredity 1.2 Chromosomes as vectors of heredity 2 Chromosomes in eukaryotes 2.1 Chromatin 2.1.1 Interphase chromatin 2.1.2 Metaphase chromatin and division 3 Chromosomes in prokaryotes 3.1 Structure in sequences 3.2 DNA packaging 4 Number of chromosomes in various organisms 4.1 Eukaryotes 4.2 Prokaryotes 5 Karyotype 5.1 Historical note 6 Chromosomal aberrations 7 Human chromosomes 8 See also 9 External links 10 References

History

Nucleus as the seat of heredity

The origin of this groundbreaking idea lies in a few sentences tucked away in Ernst Haeckel's Generelle Morphologie of 1866.^[2] The evidence for this insight gradually accumulated until, after twenty or so years, two of the greatest in a line of great German scientists^{[citation needed]} spelled out the concept. August Weismann proposed that the germ line is separate from the soma, and that the cell nucleus is the repository of the hereditary material, which, he proposed, is arranged along the chromosomes in a linear manner. Further, he proposed that at fertilisation a new combination of chromosomes (and their hereditary material) would be formed. This was the explanation for the reduction division of meiosis (first described by van Beneden).

Chromosomes as vectors of heredity

In a series of experiments, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity. His two principles were based upon the continuity of chromosomes and the individuality of chromosomes^{[citation needed]}.

It is the second of these principles that was so original^{[citation needed]}. Boveri was able to test the proposal put forward by Wilhelm Roux, that each chromosome carries a different genetic load, and showed that Roux was right. Upon the rediscovery of Mendel, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. It is interesting to see that Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Walter Sutton and Theophilus Painter were all influenced by Boveri (Wilson and Painter actually worked with him).

In his famous textbook The Cell, Wilson linked Boveri and Sutton together by the Boveri-Sutton theory. Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, 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.^[3]

Chromosomes in eukaryotes

It has been suggested that Eukaryotic chromosome fine structure be merged into this article or section. (Discuss)

Eukaryotes (cells with nuclei such as plants, yeast, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes.

In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around histones (structural proteins), forming a composite material called chromatin.

Chromatin

Main article: Chromatin

Fig. 2: The major structures in DNA compaction; DNA, the nucleosome, the 10nm "beads-on-a-string" fibre, the 30nm fibre and the metaphase chromosome.

Chromatin is the complex of DNA and protein found in the eukaryotic nucleus which packages chromosomes. The structure of chromatin varies significantly between different stages of the cell cycle, according to the requirements of the DNA.

Interphase chromatin

During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished:

• Euchromatin, which consists of DNA that is active, e.g., being expressed as protein.
• Heterochromatin, which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:
  • Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences.
  • Facultative heterochromatin, which is sometimes expressed.

Individual chromosomes cannot be distinguished at this stage – they appear in the nucleus as a homogeneous tangled mix of DNA and protein.

Metaphase chromatin and division

See also: mitosis and meiosis

Human chromosomes during metaphase.

In the early stages of mitosis or meiosis (cell division), the chromatin strands become more and more condensed. They cease to function as accessible genetic material (transcription stops) and become a compact transportable form. This compact form makes the individual chromosomes visible, and they form the classic four arm structure, a pair of sister chromatids attached to each other at the centromere. The shorter arms are called p arms (from the French petit, small) and the longer arms are called q arms (q follows p in the Latin alphabet). This is the only natural context in which individual chromosomes are visible with an optical microscope.

During divisions, long microtubules attach to the centromere and the two opposite ends of the cell. The microtubules then pull the chromatids apart, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and can function again as chromatin. In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus (Fig. 2).

The self-assembled microtubules form the spindle, which attaches to chromosomes at specialized structures called kinetochores, one of which is present on each sister chromatid. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region.

Chromosomes in prokaryotes

The prokaryotes – bacteria and archaea – typically have a single circular chromosome, but many variations do exist.^[4] Most bacteria have a single circular chromosome that can range in size from only 160,000 base pairs in the endosymbiotic bacteria Candidatus Carsonella ruddii,^[5] to 12,200,000 base pairs in the soil-dwelling bacteria Sorangium cellulosum.^[6] Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.^[7]

Structure in sequences

Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a single point (the origin of replication) from which replication starts, whereas some archaea contain multiple replication origins.^[8] The genes in prokaryotes are often organized in operons, and do not usually contain introns, unlike eukaryotes.

DNA packaging

Prokaryotes do not possess nuclei. Instead, their DNA is organized into a structure called the nucleoid.^[9] 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.^[10] In archaea, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.^[11][12]

Bacterial chromosomes tend to be tethered to the plasma membrane of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of the membranes (and the attached DNA).

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled. The DNA must first be released into its relaxed state for access for transcription, regulation, and replication.

Number of chromosomes in various organisms

Main article: List of number of chromosomes of various organisms

Eukaryotes

These tables give the total number of chromosomes (including sex chromosomes) in a cell nucleus. For example, human cells are diploid and have 22 different types of autosome, each present as two copies, and two sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosomes, such as bread wheat, which is hexaploid and has six copies of seven different chromosomes – 42 chromosomes in total.

Chromosome numbers in some plants Plant Species # Arabidopsis thaliana (diploid)[13] 10 Rye (diploid)[14] 14 Maize (diploid)[15] 20 Einkorn wheat (diploid)[16] 14 Durum wheat (tetraploid)[16] 28 Bread wheat (hexaploid)[16] 42 Potato (tetraploid)[17] 48 Cultivated tobacco (diploid)[18] 48 Adder's Tongue Fern (diploid)[19] approx 1,400

Chromosome numbers (2n) in some animals Species # Species # Common fruit fly 8 Guinea Pig[20] 64 Dove[citation needed] 78 Garden snail[21] 54 Earthworm Octodrilus complanatus[22] 36 Tibetan fox 36 Domestic cat[23] 38 Domestic pig 38 Laboratory mouse 40 Laboratory rat 42 Rabbit[citation needed] 44 Syrian hamster 44 Hare[citation needed] 46 Human[24] 46 Gorillas, Chimpanzees[24] 48 Domestic sheep 54 Elephants[25] 56 Cow 60 Donkey 62 Horse 64 Dog[26] 78 Kingfisher[27] 132 Goldfish[28] 100-104 Silkworm[29] 56

Chromosome numbers in other organisms Species Large Chromosomes Intermediate Chromosomes Small Chromosomes Trypanosoma brucei 11 6 ~100 Chicken[30] 8 2 sex chromosomes 60 Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes (see the table). Other eukaryotic chromosomes, i.e., mitochondrial and plasmid-like small chromosomes, are much more variable in number, and there may be thousands of copies per cell. The 24 human chromosome territories during prometaphase in fibroblast cells. Asexually reproducing species have one set of chromosomes, which is the same in all body cells. Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes, one from the mother and one from the father. Gametes, reproductive cells, are haploid [n]: They have one set of chromosomes. Gametes are produced by meiosis of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed. Some animal and plant species are polyploid [Xn]: They have more than two sets of homologous chromosomes. Plants important in agriculture such as tobacco or wheat are often polyploid, compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some cultivars as well as the wild progenitors. The more-common pasta and bread wheats are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in the wild wheat.[31] Prokaryotes Prokaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies.[32] For example, Buchnera, a symbiont of aphids has multiple copies of its chromosome, ranging from 10–400 copies per cell.[33] However, in some large bacteria, such as Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present.[34] Plasmids and plasmid-like small chromosomes are, as in eukaryotes, very 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, and vice versa. Karyotype Main article: Karyotype Figure 3: Karyogram of a human male In general, the karyotype is the characteristic chromosome complement of a eukaryote species.[35] The preparation and study of karyotypes is part of cytogenetics. Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are often highly variable. There may be variation between species in chromosome number and in detailed organization. In some cases, there is significant variation within species. Often there is 1. variation between the two sexes; 2. variation between the germ-line and soma (between gametes and the rest of the body); 3. variation between members of a population, due to balanced genetic polymorphism; 4. geographical variation between races; 5. mosaics or otherwise abnormal individuals. Also, variation in karyotype may occur during development from the fertilised 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: Fig. 3. Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males. Historical note Investigation into the human karyotype took many years to settle the most basic question. How many chromosomes does a normal diploid human cell contain? In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism.[36] Painter in 1922 was not certain whether the diploid number of man is 46 or 48, at first favouring 46.[37] He revised his opinion later from 46 to 48, and he correctly insisted on man's having an XX/XY system.[38] New techniques were needed to definitively solve the problem: 1. Using cells in culture 2. Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes 3. Arresting mitosis in metaphase by a solution of colchicine 4. Squashing the preparation on the slide forcing the chromosomes into a single plane 5. Cutting up a photomicrograph and arranging the result into an indisputable karyogram. It took until the mid-1950s until it became generally accepted that the human karyotype include only 46 chromosomes. Considering the techniques of Winiwarter and Painter, their results were quite remarkable.[39][40] Chimpanzees (the closest living relatives to modern humans) have 48 chromosomes. Chromosomal aberrations Main articles: Chromosome abnormalities and aneuploidy The three major single chromosome mutations; deletion (1), duplication (2) and inversion (3). The two major two-chromosome mutations; insertion (1) and translocation (2). In Down syndrome, there are three copies of chromosome 21 Chromosomal aberrations are disruptions in the normal chromosomal content of a cell, and are a major cause of genetic conditions in humans, such as Down syndrome. Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of birthing a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets, aneuploidy, may be lethal or give rise to genetic disorders. Genetic counseling is offered for families that may carry a chromosome rearrangement. The gain or loss of DNA from chromosomes can lead to a variety of genetic disorders. Human examples include: Cri du chat, which is caused by the deletion of part of the short arm of chromosome 5. "Cri du chat" means "cry of the cat" in French, and 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, and are moderately to severely mentally retarded and very short. Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4. It is characterized by severe growth retardation and severe to profound mental retardation. Down's syndrome, usually is 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 mental retardation.[41] Edwards syndrome, which is the second-most-common trisomy; Down syndrome is the most common. It is a trisomy of chromosome 18. Symptoms include mental and motor retardation and numerous congenital anomalies causing serious health problems. Ninety percent die in infancy; however, those that live past their first birthday usually are quite healthy thereafter. They have a characteristic clenched hands and overlapping fingers. Patau Syndrome, also called D-Syndrome or trisomy-13. Symptoms are somewhat similar to those of trisomy-18, but they do not have the characteristic hand shape. Idic15, abbreviation for Isodicentric 15 on chromosome 15; also called the following names due to various researches, but they all mean the same; IDIC(15), Inverted dupliction 15, extra Marker, Inv dup 15, partial tetrasomy 15 Jacobsen syndrome, also called the terminal 11q deletion disorder.[42] This is a very rare disorder. Those affected have normal intelligence or mild mental retardation, with poor expressive language skills. Most have a bleeding disorder called Paris-Trousseau syndrome. Klinefelter's syndrome (XXY). Men with Klinefelter syndrome are usually sterile, and tend to have longer arms and legs and to be taller than their peers. Boys with the syndrome are often shy and quiet, and have a higher incidence of speech delay and dyslexia. During puberty, without testosterone treatment, some of them may develop gynecomastia. Turner syndrome (X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. People with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development and a "caved-in" appearance to the chest. XYY syndrome. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are somewhat more likely to have learning difficulties. Triple-X syndrome (XXX). XXX girls tend to be tall and thin. They have a higher incidence of dyslexia. Small supernumerary marker chromosome. This means there is an extra, abnormal chromosome. Features depend on the origin of the extra genetic material. Cat-eye syndrome and isodicentric chromosome 15 syndrome (or Idic15) are both caused by a supernumerary marker chromosome, as is Pallister-Killian syndrome. Chromosomal mutations produce changes in whole chromosomes (more than one gene) or in the number of chromosomes present. Deletion – loss of part of a chromosome Duplication – extra copies of a part of a chromosome Inversion – reverse the direction of a part of a chromosome Translocation – part of a chromosome breaks off and attaches to another chromosome Most mutations are neutral – have little or no effect A detailed graphical display of all human chromosomes and the diseases annotated at the correct spot may be found at[43]. Human chromosomes Human cells have 23 pairs of large linear nuclear chromosomes, giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the mitochondrial genome. Sequencing of the human genome has provided a great deal of information about each of the chromosomes. Below is a table compiling statistics for the chromosomes, based on the Sanger Institute's human genome information in the Vertebrate Genome Annotation (VEGA) database.[44] Number of genes is an estimate as it is in part based on gene predictions. Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions. Chromosome Genes Total bases Sequenced bases[45] 1 4,220 247,199,719 224,999,719 2 1,491 242,751,149 237,712,649 3 1,550 199,446,827 194,704,827 4 446 191,263,063 187,297,063 5 609 180,837,866 177,702,766 6 2,281 170,896,993 167,273,993 7 2,135 158,821,424 154,952,424 8 1,106 146,274,826 142,612,826 9 1,920 140,442,298 120,312,298 10 1,793 135,374,737 131,624,737 11 379 134,452,384 131,130,853 12 1,430 132,289,534 130,303,534 13 924 114,127,980 95,559,980 14 1,347 106,360,585 88,290,585 15 921 100,338,915 81,341,915 16 909 88,822,254 78,884,754 17 1,672 78,654,742 77,800,220 18 519 76,117,153 74,656,155 19 1,555 63,806,651 55,785,651 20 1,008 62,435,965 59,505,254 21 578 46,944,323 34,171,998 22 1,092 49,528,953 34,893,953 X (sex chromosome) 1,846 154,913,754 151,058,754 Y (sex chromosome) 454 57,741,652 25,121,652 Total 32,185 3,079,843,747 2,857,698,560 See also Locus (explains gene location nomenclature) Sex-determination system XY sex-determination system X chromosome X-inactivation Y chromosome Y-chromosomal Adam Y-chromosomal Aaron Genetic genealogy Genealogical DNA test Genetic deletion List of number of chromosomes of various organisms For information about chromosomes in genetic algorithms, see chromosome (genetic algorithm) External links Wikimedia Commons has media related to: Chromosomes An Introduction to DNA and Chromosomes from HOPES: Huntington's Outreach Project for Education at Stanford Chromosome Abnormalities at AtlasGeneticsOncology 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 Kimballs Chromosome pages Chromosome News from Genome News Network Eurochromnet, European network for Rare Chromosome Disorders on the Internet http://www.ensembl.org Ensembl project, presenting chromosomes, their genes and syntenic loci graphically via the web Genographic Project Home reference on Chromosomes from the U.S. National Library of Medicine References ^ Paux E, Sourdille P, Salse J, et al. 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See http://www.ncbi.nlm.nih.gov/genome/seq/ for more information on the Human Genome Project. v • d • e Genetics: chromosomes General Karyotype · Ploidy · Meiosis Classification Autosome · Sex chromosome Evolution Chromosomal inversion · Chromosomal translocation · Polyploidy · Paleopolyploidy Structure Nucleosome · Telomere · Chromatid Chromatin Euchromatin · Heterochromatin Histone H1 · H2A · H2B · H3 · H4 Centromere A · B · C1 · C2 · E · F · H · I · J · K · M · N · O · P · Q · T

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Dansk (Danish)
n. - kromosom

Nederlands (Dutch)
chromosoom

Français (French)
n. - chromosome

Deutsch (German)
n. - Chromosom, Erbträger, Kernschleife

Ελληνική (Greek)
n. - (βιολ.) χρωμόσωμα, χρωματόσωμα

Italiano (Italian)
cromosoma

Português (Portuguese)
n. - cromossomo (m) (Citol.)

Русский (Russian)
хромосома

Español (Spanish)
n. - cromosoma

Svenska (Swedish)
n. - kromosom

中文（简体）(Chinese (Simplified))
染色体

中文（繁體）(Chinese (Traditional))
n. - 染色體

한국어 (Korean)
n. - 염색체

日本語 (Japanese)
n. - 染色体

العربيه (Arabic)
‏(الاسم) كروموسوم, جسم صبغي في نواة الخليه, العامل في نقل الصفات الوراثيه‏

עברית (Hebrew)
n. - ‮כל אחד ממבני ה-AND הנושאים מידע גנטי, כרומוזום‬

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