Why are chromosomes in pairs

Chromosomes

Chromosomes [from * chromosome -], structures in the cell nuclei of eukaryotic cells (eucyte, cell) that are responsible for the storage and transmission of genetic information serve (chromosome theory of inheritance). The term chromosome originally referred to the always present material in cell nuclei, which can be easily stained with basic dyes and which can usually only be visualized with a light microscope during nuclear divisions (mitosis, meiosis). In a purely morphological way, the chromosomes were also called earlier Core threads or. Core grinding designated. - The chromosomes of eukaryotes mainly contain DNA (deoxyribonucleic acids; 10–30%), structural and accompanying proteins (40–75%) - histones and non-histone proteins - as well as RNA (ribonucleic acids; 3–15%) . The amount of DNA in a chromosome is constant and species-specific, while that of RNA varies depending on the cell's transcription activity. The basic structure of the eucyte chromosomes is a DNA double helix (double strand) which, at the beginning of the nucleus division, is coiled up by histones and non-histone proteins in a complicated organization system made up of at least four packs on top of each other (chromatin organization). As a result of the multiple spiraling, the thin, intertwined chromosome threads condense into rod-shaped structures that are only about 1/10 000 (between 1–50 μm) of their original length. In the metaphase of mitosis and meiosis, the division of the chromosomes into two identical longitudinal units, the Chromatids, visible, with each chromatid representing a complete and functional DNA double strand. The chromatids are separated from each other in the anaphase and distributed to the daughter nuclei. During the next interphase, the identical replication of the DNA of the chromosomes (up to this point actually of the chromatids) then takes place, so that the two-part structure of the chromosomes can be seen again in the next mitosis or meiosis. The historically determined but often misleading term chromatids is, strictly speaking, identical sibling chromosomes that have emerged from DNA replication. - The two chromatids of a chromosome hang in the Centromere (see Fig. 1), the primary constriction of the chromosomes and at the same time the attachment point of the Kinetochore Protein Complexes (Attachment points of the spindle fibers during cell division; kinetochore), together, whereby the chromosomes are made up of four arms, which are normally pairs of equal length (Chromosome arms) exist (see Fig. 2). Depending on the position of the centromere, chromosomes can be differentiated in the metaphase using a light microscope (see Fig. 3): at metacentric The centromere lies roughly in the middle of the chromosomes acrocentric between the middle of the chromosome and the telomere (end of the chromosome, see below), at telocentric near the telomere, at subacrocentric or submetacentric approximated to one of the latter two positions. In the metaphase, depending on the position of the centromere, X- or V-shaped structures arise. If, in exceptional cases, two centromeres are formed on one chromosome, one speaks in contrast to the usual ones monocentric of dicentric Chromosomes. Many chromosomes also have a so-called. Secondary necking. These chromosomes were previously called SAT chromosomes referred to because, due to a negative Feulgen color reaction (Feulgen reaction), it was wrongly believed that no DNA would be present here (without DNA = sine acido thymonucleinico = SAT). The parts "pinched off" in this way become a bit misleading Satellites called (not to be confused with satellite DNA), the term SAT chromosomes is therefore still an abbreviation for Satellite chromosomes common. The area of ​​the secondary constriction itself is responsible for the formation of the nucleolus (core body), which is why it Nucleolus Organizer is called. The location of the nucleoli formation site is genetically determined. At the ends of the chromosomes are the so-called. Telomeres - DNA sequences that serve as a target for telomerases. They prevent the shortening of chromosomes during the replication process and the linking of chromosomes, and they also protect the ends from attack by nucleases. According to more recent research on mice (1999), the ends of the chromosomes of mammals are not, as previously assumed on the basis of studies on ciliates (such as mammals eukaryotes), as linear, but as loop-shaped structures with a complicated protective structure. - Three components - telomere, centromere, and a minimum amount of ordinary DNA - are enough to make one artificial chromosome that replicates, divides and can be passed on to daughter cells in human cells. - During the interphase in the metabolically active cell nucleus, all the DNA segments that have just been activated are largely de-spiralized; the chromosomes are not visible with a light microscope. Two forms of the Chromatins differentiated: less densely packed Euchromatin, whose DNA is currently being transcribed into RNA, and more densely packed, still stainable Heterochromatin, that is not transcribed. In the prophase of meiosis, the so-called chromosomes are particularly easy to stain. Chromomers in appearance, which are local, knot-like thickenings along the chromatids, which are caused by additional spiralization. They are larger in heterochromatic areas than in euchromatic areas. Chromomers are particularly good in the pachytene stage of meiosis as well as in Giant chromosomes and Lamp brush chromosomes recognizable where they emerge as a band pattern on the chromosomes. The arrangement of the chromomers, which can be distinguished by size and shape, is characteristic of each chromosome. The DNA content of chromosomes is particularly high in the chromomers, while the interchromomeric segments in between are particularly rich in histone and non-histone proteins. Not to be confused with the chromomers are the individual ones Band pattern, which can be displayed on chromosomes after pretreatment and staining with AT or GC base-specific dyes (base composition) and reflect the base distribution (base sequence) on the DNA. The so-called Q bands (Q bands) correspond to e.g. B. AT-rich sections (AT content), the C bands indicate highly repetitive DNA sequences (repetitive DNA) (they are mainly in areas that surround the centromere, and correspond to it constitutive heterochromatin). Furthermore, a distinction is made between G- (Giemsa-), G11-, T (telomere), NOR (nucleolus organizer), R (reverse) and replication bands, depending on the type of staining. - The number of chromosomes per nucleus that Chromosome set, is characteristic of every organism (cf. Tab. 2–5). on haploid and diploid chromosome numbers in the plant and animal kingdom). Each chromosome carries part of the genetic information. The diploid cells of the human body (diploidy) contain 46 chromosomes (see Fig. 4), 44 of them Autosomes (always present as a pair of homologous chromosomes, i.e. 22 pairs) and 2 Heterosomes (Sex chromosomes; female genotype = XX, male genotype = XY; X chromosome, Y chromosome). During gamete formation, the set of chromosomes is halved in the course of meiosis, so that human gametes have 22 autosomes and 1 sex chromosome, X or Y. The chromosomes can be divided into groups (humans: A – G) and into one according to their shape, length and band formation according to specific staining (chromosome staining) Karyotype be arranged (see Fig. 5, see Tab. 1). Closely related species often have similar chromosome numbers or multiples of a basic set - a phenomenon that is caused by so-called Polyploidization comes about. Polyploidization is mainly found in plants. Cultivated plants grown today often have the multiplied chromosome set of the original wild form (e.g. hexaploid wheat = Triticum;Wild type), but also in extreme locations, e.g. B. in the polar region (polar region), wild plants are partly polyploid (polyploidy). In animals, the multiplication of the chromosome set is limited to the cells of certain tissues, which leads to mosaicFormation leads (chromosome mosaics). - Those lying on the chromosomes Genes (Gen) represent the information-carrying units; depending on their size, they correspond to shorter or longer, microscopically non-resolvable sub-areas of the chromosomes and are arranged on them in a linear, defined sequence. The bulk of the DNA on the chromosomes of the eukaryotes, however, is non-information-bearing or non-coding; in humans, for example, only two to three percent consist of coding DNA (gene mosaic structure, processing, splicing). Genes on one chromosome together form one Coupling group. In meiosis, the genes belonging to a coupling group cannot be freely recombined to the same extent as genes located on different chromosomes, but are preferably distributed jointly to the daughter cells. Coupled genes are recombined in meiosis by crossing over(Coupling break) between two homologous chromosomes, d. H. same chromosomes from different parents. The frequency of exchange between two different genes located on homologous chromosomes serves as a measure of the relative distance between these genes on the linkage group. With the help of the exchange frequencies determined from a large number of crosses, the relative arrangement of genes on a chromosome can be determined. So were z. B. the distribution and relative arrangement of genes on the four chromosomes (in the haploid genome) of Drosophila melanogaster determined (chromosomes II). The result of such intersection analyzes is presented in a Chromosome map (Chromosome map I
Chromosome map II
Chromosome map III
Chromosome map IV
) or Gene map summarized, which not only shows the assignment of the genes to the respective chromosomes, but also their relative position on the chromosomes. The creation of gene maps is one of the classic methods of genetics and was excepted by Drosophila performed on many other organisms of the animal and plant kingdom, e.g. B. in the nematode Caenorhabditis elegans, the mouse and the crucifer Arabidopsis thaliana (Narrow wall). For ethical reasons (bioethics), an experimental crossbreeding analysis is prohibited in humans. Newer methods of assigning genes to individual human chromosomes are the in situ hybridization between radioactively labeled RNA and DNA (labeling) in chromosomes of isolated cells as well as the analysis of hybrid cells (mostly from mouse and human cells) that arise from cell fusions. (During the subsequent cell divisions, human chromosomes are preferably lost, whereby the loss of certain biochemical functions can be attributed to the loss of certain chromosomes.) The relative position of the genes (gene loci; gene location) is by analyzing artificially generated Chromosomal aberrations determined. For example, the frequency with which genes experience a common translocation can be determined, from which the relative distance between the genes in question can be estimated (the closer two genes are to one another, the more common translocation occurs). As part of the Genome project, which has already contributed to the complete sequencing of the genomes of several species, work is now intensifying on the creation of a map of the human genome. With the help of Coupling analysis (genetic mapping) a first localization of at least 3000 genetic markers on the chromosomes was carried out in the physical mapping their position was determined by the arrangement of chromosome fragments. In the Sequence analysis the exact nucleotide sequence of the DNA of each chromosome is now determined (in humans approx. 3 billion base pairs in the haploid chromosome set). - Prokaryotic cells (protocytes) contain a so-called nucleoid in a nucleoid-like area that is not surrounded by a membrane. Bacterial chromosome or Genophore, which differs fundamentally from the chromosomes of eukaryotic cells. It does not exist as a linear DNA double strand, but as a closed DNA ring, which is associated only to a small extent with accompanying proteins and RNA and is twisted to a so-called "super-twist" (supercoil) due to internal stresses in the ring shape. In the genetic sense, bacterial chromosomes - like the chromosomes of eukaryotes (for which the term chromosomes was originally introduced) - represent coupling groups. - Significant contributions to the study of chromosomes were made, among others. E. van Beneden, T. Boveri, C.B. Bridges, C.D. Darlington, J.L.H. Down, W. Flemming, O.W.H. Hertwig, W.F.B. Hofmeister, A. Klug, T.H. Morgan, C.W. by Naegeli, W. Roux, E.A. Strasburger, H.W.G. von Waldeyer-Hartz, A.F.L. Weismann, E.B. Wilson. - chromosome jumping, chromosome anomalies, chromosome banding technique, chromosome fusion, chromosome pairing, chromosome polymorphism, chromosome walking, cytogenetics, cytology, double-minute chromosomes, hereditary diseases (tab.), Genome project, genetic engineering, ergonomics, basic number, Harlequin chromosomes, color table rules , Morgan laws, ring chromosomes, scaffold, spindle apparatus; Chromosome map I
Chromosome map II
Chromosome map III
Chromosome map IV
, Replication of DNA II. Chromosomes I, chromosomes II.

G.St./S.Kl./M.B./P.E.

Lit .: Adolph, K.W .: Chromosomes & Chromatin. Boca Raton 1987. Adolph, K.W. (ed.): Molecular Biology of Chromosome Function. Berlin 1989. Bickmore, W.A., Craig, J .: Chromosome bands. Patterns in the Genome. Berlin 1997. Chadwick, D.J., Cardew, G.. (ed.): Telomeres and Telomerase. New York 1998. Murken, J.D., Wilmowsky, H. from: The human chromosomes. The story of their exploration. Munich 1973. Seyffert, W.. (Ed.): Textbook of Genetics. Stuttgart 1998. Traut, W .: Chromosomes. Classical and Molecular Cytogenetics. Berlin 1991. Wagner, R.P., Maguire, M.P., Stallings, R.L .: Chromosomes - a synthesis. New York 1993.




Fig.1: Structure of the chromosomes:
a chromonema, b chromomeres, c matrix, d centromere, e secondary constrictions, f satellite



Fig. 2: One of the 46 human chromosomes (mitosis status) in the microscopic image with the two clearly visible chromatids, each made up of densely folded chromatin fibers. The latter, in turn, results from the packaging of a DNA molecule that is wound around the histone proteins and forms a chain of nucleosomes.