Dictionary
a threadlike strand of DNA in the cell nucleus that carries the genes in a linear order "humans have 22 chromosome pairs plus two sex chromosomes"
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Wikipedia
For information about chromosomes in genetic algorithms, see Chromosome (genetic algorithm).'' . (2) C entromere.? The point where the two chromatids touch, and where the microtubules attach. (3) Short arm (4) Long arm.]]The DNA which carries genetic information in biological cells is normally packaged in the form of one or more large macromolecules called chromosomes.A chromosome (in Greek language Greek ''chroma'' = color and ''soma'' = body) is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory sequenceregulatory elements and other intervening genetic sequencenucleotide sequences. In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the cell nucleusnucleus, where it wraps around histones (structural proteins, Fig. 1), and where this composite material is called chromatin. During mitosis (cell division), the chromosomes are condensed and a spindle composed of microtubules is formed. Microtubules are self-assembled from dimers of alpha and beta tubulin. Microtubules attach to chromosomes at specialized structures, the kinetochores, one of which is present on each sister chromatid. The unfortunate term, "centromere" is sometimes used. There is no such structure. It is true that sister chromatids are attached to each other all along their lengths by proteins called cohesins. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region. This is the only natural context in which individual chromosomes are visible with an optical microscope. Each chromosome has two arms, the shorter one called p arm (from the French languageFrench ''petit'', small) and the longer one q arm (''q'' following ''p'' in the alphabet). Prokaryotes do not possess histones or nuclei. In its relaxed state, the DNA can be accessed for Transcription (genetics)transcription, regulation, and replication. Chromosomes were first observed by Karl Wilhelm von Nägeli in 1842 and their behavior later described in detail by Walther Flemming in 1882. In 1910, Thomas Hunt Morgan proved that chromosomes are the carriers of genes.
Chromosomes in plants, yeast and animals - Eukaryotes (cells with nuclei such as plants, yeast and animals) possess multiple linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere. The ends of the chromosomes are special structures called ''telomeres''. DNA replication begins at many different locations on the chromosome.
Chromosomes in bacteria - Bacterial chromosomes are often circular but sometimes linear. Some bacteria have one chromosome, while others have a few. Bacterial DNA also exists as plasmids. The distinction between plasmids and chromosomes is poorly defined, though size and necessity are generally taken into account. Bacterial chromosomes initiate replication and one origin of replication.When linear, 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 membranes (and the attached DNA).
Chromatin - Two types of chromatin can be distinguished:Euchromatin, which consists of DNA that is active, e.g., 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 Repeated sequence (DNA)repetitive sequences.*''Facultative heterochromatin'', which is sometimes expressed. with < b>centromere. (4) Condensed chromatin during prophase. (Two copies of the DNA molecule are now present) (5) Chromosome during metaphase.]]In the very early stages of mitosis, the chromatin strands become more and more condensed. They cease to function as accessible genetic material and become a compact transport form. Eventually, the two matching chromatids (condensed chromatin strands) become visible as a chromosome, linked at the centromere. Long microtubules are attached at the centromere and two opposite ends of the cell. During mitosis, the microtubules 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 highly structured (Fig. 2). For example, genes with similar functions are often kept close together in the nucleus, even if they are far apart on the chromosome. The short arm of a chromosome can be extended by a satellite chromosome that contains codes for ribosomeribosomal RNA.
Chromosomes in different species - Normal members of a particular species all have the same number of chromosomes (see the table).asexual reproductionAsexually reproducing species have one set of chromosomes, which is the same in all body cells.sexual reproductionSexually reproducing species have somatic cells (body cells), which are diploid 2n (they have two sets of chromosomes, one from the mother, one from the father) or polyploid Xn (more than two sets of chromosomes), and gametes (reproductive cells) which are haploid n (they have only 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 (Chromosomal crossovercrossover), 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. -
Karyotype - To determine the (diploid) number of chromosomes of an organism, cells can be locked in metaphase in vitro (in a reaction vial) with colchicine. These cells are then stained (the name chromosome was given because of their ability to be stained), photographed and arranged into a karyotype (an ordered set of chromosomes, Fig. 3), also called ''karyogram''. Like many sexually reproducing species, humans have special XY sex-determination systemgonosomes (sex chromosomes, in contrast to autosomes for body functions). These are XX in females and XY in males. In females, one of the two X chromosomes is inactive and can be seen under a microscope as Barr bodyBarr bodies.
Human - * Human Genome Project goals called for determination of only the euchromatineuchromatic portion of the genome. Telomeres, centromeres, and other heterochromatinheterochromatic regions have been left undetermined, as have a small number of unclonable gaps. ncbi.nlm.nih.gov
Chromosomal aberrations - Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although it may lead to a higher chance of having 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 chromosome material can lead to a variety of genetic disorders. Examples include:Cri du chat, which is caused by the Genetic deletiondeletion of part of the short arm of chromosome 5. The name cri du chat means "cat's cry"; this condition was called this because affected babies make high-pitched cries that sound like a cat. They 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 syndrome (extra chromosome 21). This is also known as mongolism or trisomy 21. Characteristics are decreased muscle tone, asymmetrical skull, slanting eyes and mild to moderate mental retardation.Edward's syndrome is the second most common trisomy after Down's Syndrome. It is a trisomy of chromosome 18. Symptoms include mental and motor retardation as well as numerous congenital anomalies causing serious health problems. Ninety percent die in infancy; however, those who live past their first birthday usually are quite healthy thereafter. They have a characteristic hand appearance with clenched hands and overlapping fingers.Patau Syndrome, also called D-Syndrome or trisomy-13. Symptoms somewhat similar to those of trisomy-18, but they do not have the characteristic hand shape.Jacobsen syndrome, also called the terminal 11q deletion disorder. A very rare disorder. More information at http://www.11q.org. They have normal IQ 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. They tend to have longer arms and legs and tend to be taller than their peers. They are often shy, quiet boys, and have a higher incidence of speech delay and dyslexia. During puberty, some of them grow breasts and get a curvy figure.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 brothers. They are more likely to be hyperactive, enjoying active games. Despite what was previously believed, XYY boys are no more likely than other boys to be violent.Triple-X syndrome (XXX). XXX girls tend to be tall and thin and are often shy. 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 are both caused by a supernumerary marker chromosome, as is Pallister Killian Syndrome.You can find a detailed graphical display of all human chromosomes and the diseases annotated at the correct spot at ornl.gov.
See also - XY sex-determination system* X chromosome** Lyon hypothesis* Y chromosome** Y-chromosomal Adam** Y-chromosomal Aaron Genetic genealogy* Genealogical DNA testGenetic deletionList of number of chromosomes of various organisms
External links - gslc.genetics.utah.edu - What Can Our Chromosomes Tell Us?,an accessible and comprehensive look at chromosomes, from the University of Utah's Genetic Science Learning Centergslc.genetics.utah.edu - Try making a karyotype yourself, from the University of Utah's Genetic Science Learning Centerusers.rcn.com - Kimballs Chromosome pagesgenomenewsnetwork.org - Chromosome News from Genome News Network11q.org - European Chromosome 11q Networkchromosomehelpstation.com - Eurochromnet, European network for Rare Chromosome Disorders on the Internethttp://www.ensembl.org Ensembl project, presenting chromosomes, their genes and syntenysyntenic loci graphically via the webLink FAde !Category:Geneticsbg:Хромо зомиbn:ক্রোমো োমca:Cromosomacs:Chromoz ómcy:Cromosomda:Kromosomde:Ch romosomes:Cromosomafa:رنگی نتنfr:Chromosomegl:Cromo somako:염색체id:Kromosomit: Cromosomahe:כרומוזוםlt :Chromosomahu:Kromoszómanl:Ch romosoomja:染色体pl:Chromos ompt:Cromossomoru:Хромос омаsk:Chromozómfi:Kromosom isv:Kromosomtr:Kromozomzh:染 體
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