Chromosome

The 24 human chromosome territories during prometaphase in fibroblast cells.

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.

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. Agriculturally important plants 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.^[4]

Historical note: In 1921, Theophilus Painter claimed, based on his observations, that human sex cells had 24 pairs of chromosomes, giving humans 48 chromosomes total. It wasn't until 1955 that the number of pairs was clearly shown to be 23.

[edit] Prokaryotes

Prokaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. Plasmids and plasmid-like small chromosomes are, like 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.

[edit] Karyotype

Main article: Karyotype

Figure 3: Karyotype of a human male

Karyotyping is a technique used to determine the (diploid) number of nuclear chromosomes of a eukaryotic organism, and may be used for determining sex and spotting chromosomal abnormalities. 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 karyotype (an ordered set of chromosomes, Fig. 3), also called karyogram.

Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males, and can be seen in the karyotype, Fig. 3.

[edit] 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, chromosome 21 is affected

Chromosomal aberrations are disruptions in the normal chromosomal content of a cell, and are a major cause of genetic disease 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 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. 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 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 syndrome, usually is caused by an extra copy of chromosome 21 (trisomy 21). Characteristics include decreased muscle tone, asymmetrical skull, slanting eyes and mild to moderate mental retardation.
• Edward's syndrome, which is the second most common trisomy after Down syndrome. 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 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 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.[1] 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 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 (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 [2].

[edit] The 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. All of the human chromosomes have been sequenced and a great deal is known about each of them.

• † Human Genome Project goals called for determination of only the euchromatic portion of the genome. Telomeres, centromeres, and other heterochromatic regions have been left undetermined, as have a small number of unclonable gaps.^[5]

[edit] 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

[edit] External links

• What Can Our Chromosomes Tell Us?,an accessible and comprehensive look at chromosomes, 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
• European Chromosome 11q 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]
• A Yahoo! group for Isodicentric 15

[edit] References

1. ^ Bogin, Barry, Edward Alcamo, Curtis Chubb, William J. Ehmann, Mark R. Feil, David R. Hershey, Mitchell Leslie, Karel F. Liem, William Thwaites, and Salvatore Tocci. Austin: Holt, Rinehart, and Winston, 1999. 146.
2. ^ Bogin, Barry, Edward Alcamo, Curtis Chubb, William J. Ehmann, Mark R. Feil, David R. Hershey, Mitchell Leslie, Karel F. Liem, William Thwaites, and Salvatore Tocci. Austin: Holt, Rinehart, and Winston, 1999. 146.
3. ^ Singapore Science Center ScienceNet, accessed January 30, 2007
4. ^ Sakamura, T. (1918), Kurze Mitteilung uber die Chromosomenzahlen und die Verwandtschaftsverhaltnisse der Triticum-Arten. Bot. Mag., 32: 151-154.
5. ^ http://www.ncbi.nlm.nih.gov/genome/seq/