Chromosome

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For information about chromosomes in genetic algorithms, see chromosome (genetic algorithm).

Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division.

A chromosome is  a single large macromolecule of DNA, and  is the basic 'unit' of DNA in a cell. it  is a very long, continuous piece of DNA (a single DNA molecule), which contains many genes, regulatory elements and  other intervening nucleotide sequences. In some circumstances the definition of "chromosome" also includes the DNA bound proteins which package and  manage the DNA. The word chromosome comes from the Greek χρώμα (chroma, color) and  σώμα (soma, body) due to its strong staining.

Chromosomes vary extensively between different organisms. The DNA molecule may be circular or  linear, and  can contain anything from tens of kilobase pairs to hundreds of megabase pairs. Typically eukaryotic cells have  large linear chromosomes and  prokaryotic cells have  smaller circular chromosomes, although there are  many exceptions to this rule. In addition cells may contain more  than one type of chromosome, for example mitochondria in most eukaryotes have  their own small genome in addition to the nuclear chromosomes.

In eukaryotes chromosomes are  packaged by proteins (particularly histones) into chromatin to fit the massive molecules into the nucleus. The structure of chromatin varies through the cell cycle, and  is responsible for the compaction of DNA into the classic four armed structure during mitosis and  meiosis. Prokaryotes do not form chromatin, the cells lack proteins required and  the circular configuration of the molecule prevents this.

"Chromosome" is  loosely defined - in eukaryotes a small circular DNA molecule may be called either a plasmid or  a chromosome, and  in viruses the DNA is  generally called a genome, despite being a single molecule.

[edit] History

Chromosomes were first observed in plant cells by Swiss botanist Karl Wilhelm von Nägeli in 1842, and  independently in Ascaris worms by Belgian scientist Edouard Van Beneden (1846-1910). The use of basophilic aniline dyes was a fundamentally new technique for effectively staining the chromatin material in the nucleus. Their behavior in animal (salamander) cells was later described in detail by German anatomist Walther Flemming, the discoverer of mitosis, in 1882. The name was invented later by another German anatomist, Heinrich von Waldeyer.

[edit] Chromosomes in eukaryotes

Eukaryotes (cells with nuclei such as plants, yeast, and  animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or  two arms projecting from the centromere, although under most circumstances these arms are  not visible as such. In addition most eukaryotes have  a small circular mitochondrial genome, and  some eukaryotes may have  additional small circular or  linear cytoplasmic chromosomes.

In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it  is wrapped around histones (structural proteins), forming a composite material called chromatin.

[edit] Chromatin

Main article: chromatin

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Fig. 2: The major structures in DNA compaction; DNA, the nucleosome, the 10nm "beads-on-a-string" fibre, the 30nm fibre and  the metaphase chromosome.

Chromatin is  the complex of DNA and  protein found in the eukaryotic nucleus which packages chromosomes. The structure of chromatin varies significantly between different stages of the cell cycle, according to the requirements of the DNA.

[edit] Interphase chromatin

During interphase (the period of the cell cycle where the cell is  not dividing) 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 repetitive sequences.
  • Facultative heterochromatin, which is  sometimes expressed.

Individual chromosomes cannot be distinguished at this stage - they appear in the nucleus as a homogeneous tangled mix of DNA and  protein.

[ and  division">edit] Metaphase chromatin and  division

See also: mitosis and meiosis

In the early stages of mitosis or  meiosis (cell division), the chromatin strands become more  and more  condensed. They cease to function as accessible genetic material (transcription stops) and  become a compact transportable form. This compact form makes the individual chromosomes visible, and  they form the classic four arm structure, a pair of sister chromatids attached at to each other at the centromere. The shorter arms are  called p arms (from the French petit, small) and  the longer arms are  called q arms (q follows p in the Latin alphabet). This is  the only natural context in which individual chromosomes are  visible with an  optical microscope.

During divisions long microtubules attach to the centromere and  the two opposite ends of the cell. The microtubules then 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 which enables these giant DNA structures to be contained within a cell nucleus (Fig. 2).

The self assembled microtubules form the spindle, which attaches to chromosomes at specialized structures called kinetochores, one of which is  present on each sister chromatid. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region.

[edit] Chromosomes in prokaryotes

Prokaryotes (eg. Bacteria) typically have  a single circular chromosome, but many variations do exist. Bacterial DNA also exists as plasmids, essentially miniature chromosomes, which are  small circular pieces of DNA that are  readily transmitted between bacteria. The distinction between plasmids and  chromosomes is  poorly defined, though size and  necessity are  generally taken into account.

[edit] Structure in sequences

Prokaryotes chromosomes have  less sequence based structure than eukaryotes. They do, however, have  a single point, the origin of replication, from which replication starts.

The genes in prokaryotes are  often organised in operons, and  do not contain introns, unlike eukaryotes.

[edit] Location in the cell

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).

[edit] DNA packaging

Prokaryotes do not possess histones or  nuclei, and  so do not possess chromatin like eukaryotes. There is, however, thought to be some structural organisation to help condense the large molecule into the small prokaryotic cell.

In prokaryotes chromosomes and  plasmids are  generally supercoiled. The DNA must first be released into its relaxed state for access for transcription, regulation, and  replication.

[edit] Number of chromosomes in various organisms

[edit] Eukaryotes

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^[1].

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

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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.

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. ^[2]

[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
• Random Orientation of Chromosomes Animation
• 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. ^ Sakamura, T. (1918), Kurze Mitteilung uber die Chromosomenzahlen und die Verwandtschaftsverhaltnisse der Triticum-Arten. Bot. Mag., 32: 151-154.
2. ^ http://www.ncbi.nlm.nih.gov/genome/seq/