chromosome

enUK

chromosome

(krō`məsōm'), structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome is usually reserved for the structure when it is condensed and readily visible during cell division (see mitosismitosis
, process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei.
..... Click the link for more information. ). At other times the chromosome appears as a fibrous structure, called the chromonema, consisting of accumulations (called chromomeres) of chromatin, the dye-absorbing material. During nuclear division, when each chromosome splits, each of the duplicate chromosomes is called a chromatid. A certain number of chromosomes is characteristic of each species of plant and animal; e.g., the human has 46 chromosomes, the potato has 48, and the fruit fly Drosophila has 8. Each of these chromosome numbers is the so-called diploid number, i.e., the number found in the somatic (body) cells and in the germ cells that give rise to the gametes, or reproductive cells. When the germ cells divide in the two-step process of meiosismeiosis
, process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed.
..... Click the link for more information. , the chromosomes are separated in such a way that each daughter cell receives a haploid (half the diploid) number of chromosomes. Fusion of the male and female gametes in fertilization restores the diploid number in the fertilized egg, or zygote, which thus contains two sets of homologous chromosomes, one from each parent. The principal constituents of the chromosomes are nucleoproteins containing deoxyribonucleic acid, or DNA (see nucleic acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. ). Chromosomes appear microscopically as a linear arrangement of genes, the factors that determine the inherited characteristics of all living organisms. The very large chromosomes in the salivary gland cells of Drosophila and other insects have furnished valuable material for the study of geneticsgenetics,
scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
..... Click the link for more information. .

Chromosome

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 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 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 (Fig. 1). 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 Nucleosome

Organization of DNA into chromosomes

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 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. 2). 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

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 Cell nucleus, Genetics, Human genetics, Sex determination, Sex-linked inheritance

Chromosome

an organoid in a cell nucleus. The aggregate of chromosomes determines the main hereditary characteristics of cells and organisms. The complete chromosome set of a given organism is called the karyotype. In most animals and plants, every chromosome in every cell is represented twice. One chromosome of each pair is received from the father and the other from the mother when the nuclei of sex cells fuse during fertilization. Such chromosomes are called homologous, and the set of homologous chromosomes is termed a diploid set. The chromosome set in the cells of dioecious organisms contains one or more pairs of sex chromosomes, which usually differ in morphological characters according to sex; the chromosomes other than sex chromosomes are called autosomes. In mammals the genes that determine sex are localized in the sex chromosomes, whereas in the fruit fly Drosophila sex is determined by the ratio of the sex chromosomes to the autosomes, a concept expressed by the balance theory of sex determination.

The first description of chromosomes was made in 1888 by the German geneticist W. von Waldeyer-Hartz, who described them as solid bodies that stain strongly with basic dyes. It later became apparent, however, that the external appearance of chromosomes changes substantially during the different stages of the cell cycle. As compact formations with a characteristic morphology, chromosomes may be clearly distinguished in a light microscope only during cell division—in the metaphase of mitosis and meiosis.

The basic elements of chromosomes at all stages of the cell cycle are chromonemata, that is, threadlike structures that are tightly coiled during cell division, causing spiralization of the chromosomes, but that remain uncoiled (despiralized) in nondividing cells. After cell division is completed, the chromosomes that moved to the cells’ poles disintegrate and become surrounded by a nuclear membrane. During the period between two cell divisions—the interphase—the despiralization of the chromosomes continues and the chromosomes are no longer visible in the light microscope.

The morphology of the chromosomes of eukaryotes differs substantially from that of prokaryotes and viruses. Prokaryotes and viruses generally contain a single linear or circular chromosome that does not have a supramolecular structure and that is not separated from the cytoplasm by a nuclear membrane. The concept of chromosomes may be applied to the genetic material of prokaryotes only provisionally: this concept, formulated during the study of eukaryotic chromosomes, assumes that chromosomes have a complex of biopolymers (nucleic acids and proteins) as well as a specific supramolecular structure. Consequently, only eukaryotic chromosomes are described below.

The changes in the external appearance of chromosomes during the cellular and life cycles are caused by the chromosomes’ functional characteristics. However, the principles underlying the chromosomes’ organization, individuality, and continuity over several generations of cells and in different organisms remain unaltered, as shown by biochemical, cytological, and genetic studies on the chromosomes of different organisms. These studies have served as the basis of the chromosomes theory of heredity.

Molecular basis of chromosome structure. The significance of chromosomes as cellular organoids capable of storing, reproducing, and effecting hereditary information results from the characteristics of their constituent biopolymers. The first molecular model of a chromosome was devised in 1928 by N. K. Kol’tsov, who hypothesized the principles of the chromosome’s organization. Hereditary information in chromosomes is recorded by the deoxyribonucleic acid (DNA) molecule and by its genetic code. Approximately 99 percent of all the DNA in a cell is concentrated in the chromosomes. The remaining DNA is located in other cellular organoids and determines cytoplasmic inheritance. The DNA in eukaryotic chromosomes is combined with basic proteins (histones) and with nonhistone proteins. The nonhistone proteins effect the complex assemblage of DNA in the chromosomes and regulate the DNA’s transcription, that is, the DNA’s ability to synthesize ribonucleic acid (RNA).

Chromosomes during the interphase. Since chromosomes perform their main functions—reproduction and transcription— during the interphase, their structure at this stage of the cell’s cycle is of particular interest. Chromosomes are barely discernible during the interphase since the euchromatin, which comprises many areas of the chromosomes, is completely uncoiled owing to the active synthesis of RNA. The heterochromatin in the chromosomes, however, is not involved in RNA synthesis and continues to retain its solid structure. The euchromatin contains elementary deoxyribonucleoprotein (DNP) threads, as well as ribonucleoprotein particles 200–500 angstroms in diameter that are called ribonucleoprotein (RNP) granules, intergranules, or perichromatin granules. These particles, which represent an assemblage of RNA synthesized on the chromosomes and combined with protein, aid in the development of messenger RNA and its transfer to the cytoplasm.

Interphasic chromosomes are studied by biochemical methods of isolating chromatin—the chromosomal material in a nucleus— and separating it into euchromatin and heterochromatin. These chromosomes are also studied by electron microscopy of intact nuclei and of isolated chromatin. Giant lampbrush chromosomes from the oocytes of animals, and multifilamentous (polytene) chromosomes of dipterous insects, are used as models of interphasic chromosomes. In lampbrush chromosomes, the inactive regions have the appearance of tightly coiled structures. These are the chromomeres, which are also found in the chromosomes of somatic cells, particularly during the prophase of mitosis. The chromomeres are believed to be morphological, and perhaps functional, units of chromosomes. In those regions of chromosomes that actively synthesize RNA, the chromomeres are uncoiled and form lateral loops. In these loops the RNA molecules combine with protein to form ribonucleoproteins (RNP). These are particles that represent an assemblage of gene products; some of their lateral loops differ in size and in morphological characters. Polytene chromosomes appear in the tissues of dipterous insects and of some plants owing to repeated replication (doubling) of the original chromosomes without subsequent disjunction of the daughter chromosomes. The inactive parts of these polytene chromosomes have a disklike shape, whereas the active parts form swellings called chromosome puffs. Like lampbrush chromosomes, chromosome puffs contain RNP particles 200–500 angstroms in diameter. Electron microscopy and biochemical studies have indicated that the main structural unit in chromatin isolated from cells, in intact nuclei, and in giant chromosomes is a DNP strand 100–200 angstroms in diameter.

Studies on polytene chromosomes in different tissues and at different stages in the development of dipterous insects have demonstrated that the number and set of active chromosome puffs are individualized according to tissue and species. Consequently, although all the cells of a multicellular organism have an identical set of genes arranged linearly on each chromosome, the set of chromosome regions that are active and inactive in RNA synthesis differs in each type of cell and at each stage of development. That is, the same region is euchromatic in some tissues and heterochromatic in others. Certain chromosome regions are heterochromatic during the interphase of different types of cells and generally contain frequently repeating sequences of DNA. The nucleolus organizer—the region in the chromosome where the genes of ribosomal RNA are concentrated—functions constantly during the interphase of all types of cells. The nucleolus, long regarded as an independent cellular organoid, is formed in this region and is the site at which the precursors of the ribosomes are formed.

The chromosomes in the interphasic nucleus are separated from the cytoplasm by a nuclear membrane. The chromosomes are attached to this membrane in many places, mainly by means of telomeres and centromeres, and consequently each chromosome is believed to occupy a specific site in the nucleus. The chromosomes replicate at the time when the cells are preparing to divide during the interphase. Each chromosome creates its own copy on the basis of semiconservative replication of the DNA. The chromosomes of eukaryotes have many starting and ending points during replication, whereas prokaryotes have only a single starting point and a single ending point. This permits the nonsimultaneous replication of different chromosome regions during synthesis and also regulates the activity of the chromosomes.

Chromosomes during mitosis and meiosis. When cells begin to divide, the synthesis of DNA and RNA in the chromosomes ceases and the amount of material within the chromosomes increases. For example, in a single human chromosome a DNA chain 160 mm in length becomes contained within a volume of only 0.5 × 10 micrometers. The nuclear membrane disintegrates, and the chromosomes align themselves on the cell’s equator. They are most accessible for observation and morphological analysis at this time. The main structural unit of metaphasic chromosomes, like that of interphasic chromosomes, is a strand of DNP 100–200 angstroms in diameter coiled in a tight spiral. Several researchers have discovered that strands 100–200 angstroms in diameter form structures with a second level consisting of strands approximately 2,000 angstroms in diameter that also form the body of a metaphasic chromosome.

Every metaphasic chromosome consists of chromatids, which are produced by the replication of the original interphasic chromosome. The use of labeled and modified precursors of DNA facilitated the precise identification of differentially stained chromatids in chromosomes during the metaphase of mitosis. This led to the discovery that sister chromatids often exchange segments during replication—a phenomenon known as crossing over. In traditional cytology, the matrix of metaphasic chromosomes was considered to be of great importance and was regarded as an essential chromosomal component in which spiralized chromonemata were embedded. Modern cytologists believe that the matrix of metaphasic chromosomes is residual material from the disintegrating nucleolus. The matrix is often completely indiscernible.

The formation of sex cells in animals and plants is accompanied by a unique type of division, meiosis. Meiotic chromosomes differ from mitotic chromosomes in several ways. During meiosis, the daughter cells receive half the usual complement of chromosomes, owing to the conjugation of homologous chromosomes in the prophase of meiosis and as a result of two successive cell divisions during a single replication of DNA. During mitosis, on the other hand, the number of chromosomes remains the same. In addition, in meiotic chromosomes there is a temporary interruption during the prophase of meiosis. The meiotic chromosomes return to the interphasic state when the chromosomes begin actively synthesizing RNA. During this period most of the animals under analysis have lampbrush chromosomes. Finally, chromosomes in the metaphase of meiosis contain a larger amount of material.

Although extensive research has been devoted to chromosomes, the study of their structural and functional organization remains one of the most urgent tasks of modern biology. Chromosomes perform highly complex functions within the cell and have an intricate structure that has resisted complete elucidation. During the 1960’s and 1970’s, great progress has been achieved in understanding the molecular basis of chromosome structure owing to the development of molecular genetics. This progress has brilliantly confirmed and substantiated the principles of the chromosome theory of heredity.

REFERENCES

Wilson, E. Klelka i ee rol’ v razvitii i nasledstvennosti, vols. 1–2. Moscow-Leningrad, 1936–40. (Translated from English.)
Kol’tsov, N. K. Organizatsiia kletki. Moscow-Leningrad, 1936.
Prokof’eva-Bel’govskaia, A. A. “Stroenie khromosomy.” In Ionizi-ruiushchie izlucheniia i nasledstvennost’. (Itogi nauki: Biologicheskie nauki, fasc. 3.) Moscow, 1960.
Kiknadze, I. I. Funktsional’naia organizatsiia khromosom. Leningrad, 1972.
DeRobertis, E., V. Nowinski, and F. Saez. Biologiia kletki. Moscow, 1973. (Translated from English.)
Levitskii, G. A. Tsitologiia rastenii: Izbr. trudy. Moscow, 1976.
Darlington, C. D. Recent Advances in Cytology, 2nd ed. London, 1937.
Geitler, L. Chromosomenbau. (Protoplasma-Monographien, vol. 14.) Berlin, 1938.
Ris, H., and D. F. Kubai. “Chromosome Structure.” Annual Review of Genetics, 1970, vol. 4, pp. 236–94.
Handbook of Molecular Cytology. Edited by A. Lima-de-Faria. Amsterdam-London, 1969.
Chromosome Structure and Function. New York, 1974.

I. I. KIKNADZE

chromosome

[′krō·mə‚sōm] (genetics) A linear (usually) or circular structure containing deoxyribonucleic acid (DNA) complexed with histone and nonhistone proteins, a centromere, and a telomere at each end, if linear. Chromosomes are seen in animals, plants, and other eukaryotes during mitotic and meiotic cell divisions. The single DNA molecule in each chromosome carries a unique complement of linearly arranged genes.

chromosome

any of the microscopic rod-shaped structures that appear in a cell nucleus during cell division, consisting of nucleoprotein arranged into units (genes) that are responsible for the transmission of hereditary characteristics

Chromosome

enUK

chromosome

[kro´mo-sōm] 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. In bacterial genetics, a closed circle of double-stranded DNA that contains the genetic material of the cell and is attached to the cell membrane; the bulk of this material forms a compact bacterial nucleus. adj., adj chromoso´mal.
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, 46 being the number normally present in humans, including 22 pairs of autosomes and the two sex chromosomes (XX or XY), which determine the sex of the organism. (See also heredity.)Chromosome Analysis. This can be done on fetal cells obtained by amniocentesis or sampling" >chorionic villus sampling, on lymphocytes from a blood sample, on skin cells from a biopsy, or on cells from products of conception such as an aborted fetus. The cells are then cultured in the laboratory until they divide. Cell division is arrested in mid-metaphase by the drug Colcemid. The chromosomes can be stained by one of several techniques that produce a distinct pattern of light and dark bands along the chromosomes, and each chromosome can be recognized by its size and banding pattern. The chromosomal characteristics of an individual are referred to as the karyotype. It is also possible to make a photomicrograph of a cell nucleus, cut it apart, and rearrange it so that the individual chromosomes are in order and labeled. The autosomes are numbered 1–22, roughly in order of decreasing length. The chromosomes" >sex chromosomes are labeled X and Y. Karyotyping is useful in determining the presence of chromosome defects.
Before the chromosomes could be precisely identified they were placed in seven groups: A (chromosomes 1–3), B (4–5), C (6–12 and X), D (13–15), E (16–18), F (19–20), and G (21–22 and Y).Chromosomal Abnormalities. The prevalence of chromosomal disorders cannot be fully and accurately determined because many of these disorders do not permit full embryonic and fetal development and therefore end in spontaneous abortion. About one in every 100 newborn infants do, however, have a gross demonstrable chromosomal abnormality. A large majority of cytogenetic abnormalities can be identified by cytogenetic analysis either before birth, by means of sampling" >chorionic villus sampling or amniocentesis, or after birth.
Cytogenetic disorders with visible chromosomal abnormalities are evidenced by either an abnormal number of chromosomes or some alteration in the structure of one or more chromosomes. In the language of the geneticist, trisomy refers to the presence of an additional chromosome that is homologous with one of the existing pairs so that that particular chromosome is present in triplicate. An example of this type of disorder is a form of down syndrome (trisomy 21). Another example is patau's syndrome (trisomy 13), which produces severe anatomical malformations and profound mental retardation.
The term monosomy refers to the absence of one of a pair of homologous chromosomes. Monosomy involving an autosome usually results in the loss of too much genetic information to permit sufficient fetal development for a live birth. Either trisomy or monosomy involving the sex chromosomes yields relatively mild abnormalities.
A condition known as mosaicism results from an error in the distribution of chromosomes between daughter cells during an early embryonic cell division, producing two and sometimes three populations of cells with different chromosome numbers in the same individual. Mosaicism involving the sex chromosomes is not uncommon.
Other abnormal structural changes in the chromosome are consequences of some kind of chromosomal breakage, with either the loss or rearrangement of genetic material. translocation involves the transfer of a segment of one chromosome to another. inversion refers to a change in the sequence of genes along the chromosome, which occurs when there are two breaks in a chromosome and the segment between the breaks is reversed and reattached to the wrong ends. deletion occurs when a portion of a chromosome is lost. An example of this type of chromosomal abnormality is cri du chat syndrome, a deletion in the short arm of chromosome 5, marked by mental retardation and sometimes congenital heart defects. When deletion occurs at both ends of the chromosome, the two damaged ends can unite to form a circle and the rearrangement produces a chromosome" >ring chromosome. isochromosomes form when the centromere divides along the transverse plane rather than the normal long axis of the chromosome so that both arms are identical. All of the previously described structural abnormalities can affect both autosomal and sex chromosomes.
The causes of chromosomal errors are not completely understood. In some conditions such as Down syndrome, late maternal age seems to be a factor. Other factors may include the predisposition of chromosomes to nondisjunction (failure to separate during meiosis), exposure to radiation, and viruses.homologous c's the chromosomes of a matching pair in the diploid complement that contain alleles of specific genes.chromosome painting fluorescent in situ hybridization.Ph¹ chromosome (Philadelphia chromosome) an abnormality of chromosome 22, characterized by the translocation of genetic material from its long arm to chromosome 9, seen in the marrow cells of most patients with chronic myelogenous leukemia.ring chromosome 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 they are an unequal pair, the X and Y chromosomes.somatic chromosome autosome.X chromosome the female sex chromosome, being carried by half the male gametes and all female gametes; female diploid cells have two X chromosomes.Y chromosome 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.

chro·mo·some

(krō'mō-sōm), One of the bodies (normally 46 in somatic cells in humans) in the cell nucleus that is the bearer of genes, has the form of a delicate chromatin filament during interphase, contracts to form a compact cylinder segmented into two arms by the centromere during metaphase and anaphase stages of cell divison, and is capable of reproducing its physical and chemical structure through successive cell divisons. In bacteria and other prokaryotes, the chromosome is not enclosed within a nuclear membrane and not subject to a mitotic mechanism. Prokaryotes may have more than one chromosome. [chromo- + G. sōma, body]

chromosome

(krō′mə-sōm′)n.1. A 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 and archaea that contains the hereditary information necessary for cell life.

chro′mo·so′mal (-sō′məl), chro′mo·so′mic (-sō′mĭk) adj.chro′mo·so′mal·ly adv.

chromosome

adjective Referring to a chromosome. (Etymologically incorrect, but widely preferred to chromosomal.)
noun Any of a number of paired units of the self-replicating genetic material in the eukaryotic nucleus belonging to the organism’s genome, which allows the palette of phenotypic expression of individual organisms. Human chromosomes consist of 23 long (100–-300-million bp, each) paired DNA molecules that are associated with RNA and histone proteins, and most readily recognised during mitosis as they align themselves on the metaphase plate. Chromosomes are divided into structurally similar groups based on length from the centromere.
Human chromosome groups
A—Chromosomes 1–3.
B—Chromosomes 4, 5.
C—Chromosomes 6–12; X chromosome.
D—Chromosomes 13–15.
E—Chromosomes 16–18.
F—Chromosomes 19, 20.
G—Chromosomes 21, 22; Y chromosome.

chromosome

Genetics adjective Etymologically incorrect, but widely preferred noun Any of a number of paired units of the self-replicating genetic material in the eukaryotic nucleus, the 'master genetic database' containing the complete information present in a cell or virus which results in the palette of phenotypic expression of the individual; human chromosomes consist of 23 long–100-300 million bp, each–paired DNA or, in some organisms, RNA molecules that in humans are associated with RNA and histone proteins, and most readily recognized during mitosis as they align themselves on the metaphase plate; chromosomes are divided into structurally similar groups based on length from the centromere: group A–chromosomes 1-3; B–chromosomes 4, 5; C–chromosomes 6–12, X chromosome; D–chromosomes 13–15; E–chromosomes 16–18; F–chromosomes 19, 20; G–chromosomes 21, 22, Y chromosome. See Acentric chromosome, Accessory chromosome, Autosomal chromosome, B chromosome, Bacterial artificial chromosome, C banding, Christchurch chromosome, Eukaryote, Flow cytometry, G banding, Gene, Harlequin chromosome, Honorary chromosome, Homologous chromosome, Human genome project, Isochromosome, Lampbrush chromosome, Marker chromosome, Minichromosome chromosome, Nucleotide, Philadelphia chromosome, Ploidy analysis, Polytene chromosome, Protein, Q Banding, Ring chromosome, Sex chromosome, Translation, Transcription. ,Unbanded chromosome, X chromosome, Y chromosome.

chro·mo·some

(krō'mŏ-sōm) A body in the cell nucleus (of which there are normally 46 in humans) that is a bearer of genes, has the form of a delicate chromatin filament during interphase, contracts to form a compact cylinder segmented into two arms by the centromere during metaphase and anaphase stages of cell divison, and is capable of reproducing its physical and chemical structure through successive cell divisons.

chromosome

One of the discrete coiled DNA and protein structures, present in all animal and plant cells, which carry the genetic code for the construction of the body of the organism. Chromosomes are DNA bound to the protein histone in an enormously condensed manner. The packing ratio (DNA length divided by the chromosome length) may be as great as 7000. The code is represented by the GENES, of which there are about 100,000 in humans, strung along the chromosomes. Each normal human body cell contains 46 chromosomes, arranged in 23 pairs. A deficiency of chromosomes is incompatible with life but it is not uncommon for a person to have an extra copy of one of the chromosomes, invariably with undesirable effect. Chromosomes carry most, but not all, of the cell's DNA. Some of it is carried by the MITOCHONDRIA. The term arose when these ‘coloured bodies’ were first distinguished under the microscope by means of specific stains. So the term ‘chromosome’ actually refers to a characteristic not present in life.

chromosome

a coiled structure found in the nucleus of EUKARYOTE cells which contains DNA (the genetic material making up the genes), basic proteins called HISTONES, and nonhistone acidic proteins which may regulate the activity of DNA (see NUCLEOSOME). Each species of organism has a typical number of chromosomes (e.g. 46 in man, 20 in maize) which come in identical HOMOLOGOUS pairs in DIPLOID (1) types, although lower types such as some fungi have only one chromosome of each type (see HAPLOID, sense 1). Chromosomes are not visible during the interphase parts of the CELL CYCLE, but during MITOSIS and MEIOSIS they shorten and thicken and, after suitable preparation, may be observed under the microscope. Individual chromosomes can be recognized by their overall length and the position of the CENTROMERES. In PROKARYOTES, the chromosome consists of an intact DNA molecule lacking a centromere and is often circular. In viruses, the chromosomal material can be DNA or RNA. See NUCLEOID.

Chromosome

A structure composed of deoxyribonucleic acid (DNA) contained within a cell's nucleus (center) where genetic information is stored. Human have 23 pairs of chromosomes, each of which has recognizable characteristics (such as length and staining patterns) that allow individual chromosomes to be identified. Identification is assigned by number (1-22) or letter (X or Y).Mentioned in: Acoustic Neuroma, Amniocentesis, Birth Defects, Cerebral Amyloid Angiopathy, Cri Du Chat Syndrome, Down Syndrome, Edwards' Syndrome, Fragile X Syndrome, Gene Therapy, Genetic Testing, Hereditary Hemorrhagic Telangiectasia, Huntington Disease, Klinefelter Syndrome, Neurofibromatosis, Patau Syndrome, Retinoblastoma, Turner Syndrome, Von Willebrand Disease, Wilson Disease

chromosome

One of the thread-like structures located within the cell nucleus composed of an extremely long, double-stranded DNA (deoxyribonucleic acid) helix tightly folded around proteins called histones. Each chromosome carries genes that contain the hereditary material that controls the growth and characteristics of the body. There are 46 chromosomes in each human somatic cell organized in 23 pairs, of which 22 pairs are similar in appearance but differ at the molecular level. They are called autosomal chromosomes or autosomes and are designated by a number (with chromosome 1 being the longest, followed by chromosome 2, etc.). The other pair, the sex chromosomes determines the sex of the individual. In mammals the two sex chromosomes of females are alike (homologous) and are referred to as X chromosomes. Males carry one X chromosome along with a much shorter chromosome, the Y chromosome. Each chromosome has a centromere that divides it into two arms, the short arm 'p' and the long arm 'q'. Disorders of chromosome number in which the number of chromosomes is above or below the normal (46) are called aneuploidy. Common forms of aneuploidy are trisomy in which there is one extra chromosome and monosomy in which there is one less, than the normal 46. They rarely cause specific eye diseases but affected individuals present ocular manifestations. Examples: Down's syndrome (trisomy of chromosome 21), Edwards' syndrome (trisomy 18), Turner's syndrome (monosomy 45 XO). There are other chromosome abnormalities such as translocation (one segment of a chromosome is transferred to another chromosome) as may occur in congenital anterior polar cataract, deletion (a loss of a piece of chromosome) as in aniridia, choroideremia, retinoblastoma, etc. Other cases involve damage of a chromosome (e.g. fragile X syndrome). See defective colour vision; gene; mitosis; mutation.

chro·mo·some

(krō'mŏ-sōm) One of the bodies (normally 46 in somatic cells in humans) in the cell nucleus that is the bearer of genes, has the form of a delicate chromatin filament during interphase, contracts to form a compact cylinder segmented into two arms by the centromere during metaphase and anaphase stages of cell divison, and is capable of reproducing its physical and chemical structure through successive cell divisons.

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Compaction

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Structure

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Imprinting

Sex chromosomes

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REFERENCES

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Words related to chromosome

noun a threadlike strand of DNA in the cell nucleus that carries the genes in a linear order

Related Words