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KaryotypeA karyotype is the characteristic chromosome complement of a eukaryote species.[1][2] The preparation and study of karyotypes is part of cytology and, more specifically, cytogenetics.
In normal diploid organisms, autosomal chromosomes are present in two identical copies, though polyploid cells have multiple copies of chromosomes and haploid cells have single copies. The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size. The study of karyotypes is made possible by staining: usually a suitable dye is applied after cells have been arrested during cell division by a solution of colchicine. Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn foetus can be determined by observation of interphase cells (see amniotic centesis and Barr body). Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for women contain two X chromosomes and are denoted 46,XX; men have both an X and a Y chromosome denoted 46,XY. However, some individuals have other karyotypes with added or missing chromosomes, and in all such cases there are developmental abnormalities as a consequence. Karyotypes can be used for many purposes. for instance to study chromosomal aberrations, to study chromosome banding, or to compare the karyotypes of different species.
Additional recommended knowledge
Observations on karyotypesSix different characteristics of karyotypes are usually observed and compared: [3]
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information. Variation is often found: 1. between the two sexes 2. between the germ-line and soma (between gametes and the rest of the body) 3. between members of a population (chromosome polymorphism) 4. geographical variation between races 5. mosaics or otherwise abnormal individuals. [4] Historical noteLevitsky seems to have been the first to define the karyotype as the phenotypic appearance of the somatic chromosomes, in contrast to their genic contents.[5][6] The subsequent history of the concept can be followed in the works of Darlington[7] and White.[8][9] Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploid human cell contain? [10] In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism.[11] Painter in 1922 was not certain whether the diploid number of man was 46 or 48, at first favouring 46.[12] He revised his opinion later from 46 to 48, and he correctly insisted on man having an XX/XY system.[13] Considering their techniques, these results were quite remarkable. New techniques were needed to definitively solve the problem:
It took until the mid 1950s until it became generally accepted that the karyotype of man included only 46 chromosomes.[14][15] Rather interestingly, the great apes have 48 chromosomes. Diversity and evolution of karotypesAlthough the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karotypes, which are highly variable between species in chromosome number and in detailed organization despite being constructed out of the same macromolecules. In some cases there is even significant variation within species. This variation provides the basis for a range of studies in what might be called evolutionary cytology. Changes during developmentInstead of the usual gene repression, some organisms go in for large-scale elimination of heterochromatin, or other kinds of visible adjustment to the karyotype.
Number of chromosomes in a setA spectacular example of variability between closely related species is the muntjac, which was investigated by Kurt Benirschke and his colleague Doris Wurster. The diploid number of the Chinese muntjac, Muntiacus reevesi, was found to be 46, all telocentric. When they looked at the karyotype of the closely related Indian muntjac, Muntiacus muntjak, they were astonished to find it had female = 6, male = 7 chromosomes.[20]
The number of chromosomes in the karyotype between (relatively) unrelated species is hugely variable. The low record is held by the nematode Parascaris univalens, where the haploid n = 1; the high record would be somewhere amongst the ferns, with the Adder's Tongue Fern Ophioglossum ahead with an average of 1262 chromosomes.[22] Top score for animals might be the common hermit crab Eupagurus at a mere 127 chromosomes.[23] The existence of supernumerary or B chromosomes means that chromosome number can vary even within one interbreeding population; and aneuploids are another example, though in this case they would not be regarded as normal members of the population. Ploidy: the number of sets in a karyotype
Species treesThe detailed study of chromosome banding in insects with polytene chromosomes can reveal relationships between closely related species: the classic example is the study of chromosome banding in Hawaiian drosophilids by Hampton Carson. In about 6,500 square miles, the Hawaiian islands have the most diverse collection of drosophilid flies in the world, living from rainforests to subalpine meadows. These roughly 800 Hawaiian drosophilid species are usually assigned to two genera Drosophila and Scaptomyza in the family Drosophilidae. The polytene banding of the 'picture wing' group, the best-studied group of Hawaiian drosophilids, enabled Carson to work out the evolutionary tree long before genome analysis was practicable. In a sense, gene arrangements are visible in the banding patterns of each chromosome. Chromosome rearrangements, especially inversions, make it possible to see which species are closely related. The results are clear. The inversions, when plotted in tree form (and independent of all other information), show a clear "flow" of species from older to newer islands. There are also cases of colonization back to older islands, and skipping of islands, but these are much less frequent. Using K-Ar dating, the present islands date from 0.4 million years ago (mya) (Mauna Kea) to 10mya (Necker). The oldest member of the Hawaiian archipelago still above the sea is Kure Atoll, which can be dated to 30 mya. The archipelago itself (produced by the Pacific plate moving over a hot spot) has existed for far longer, at least into the Cretaceous. Previous islands now beneath the sea form the Emperor Seamount Chain. [34] All of the native Drosophila and Scaptomyza species in Hawaii have apparently descended from a single ancestral species that colonized the islands, probably 20 million years ago. The subsequent adaptive radiation was spurred by a lack of competition and a wide variety of niches. Although it would be possible for a single gravid female to colonise an island, it is more likely to have been a group from the same species. [35][36][37][38] There are other animals and plants on the Hawaiian archipelago which have undergone similar, if less spectacular, adaptive radiations. [39][40] OverviewAlthough much is known about karyotypes at the descriptive level, and it is clear that changes in karyotype organization has had effects on the evolutionary course of many species, it is quite unclear what the general significance might be.
Depiction of karyotypesTypes of bandingCytogenetics employs several techniques to visualize different aspects of chromosomes: [42]
Classic karyotype cytologyIn the "classic" (depicted) karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2).[43] Spectral karyotype (SKY technique)Spectral karyotyping is a molecular cytogenetic technique used to simultaneously visualize all the pairs of chromosomes in an organism in different colors. Fluorescently-labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophores. Because there are a limited number of spectrally-distinct fluorophores, a combinatorial labeling method is used to generate many different colors. Spectral differences generated by combinatorial labeling are captured and analyzed by using an interferometer attached to a fluorescence microscope. Image processing software then assigns a pseudo color to each spectrally different combination, allowing the visualization of the individually colored chromosomes.[44] This technique is used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough. Chromosome abnormalitiesChromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in translocations, inversions, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete; trisomies, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis and give rise to a genetic mosaic individual who has some normal and some abnormal cells. Chromosomal abnormalities that lead to disease in humans include:
Some disorders arise from loss of just a piece of one chromosome, including
Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; one well-documented example is the Philadelphia chromosome, a translocation mutation commonly associated with chronic myelogenous leukemia and less often with acute lymphoblastic leukemia. See also
References
Categories: Chromosomes | Genetics |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Karyotype". A list of authors is available in Wikipedia. |