A chromosome (in Greek chroma = colour and soma = body) is, minimally, a very long, continuous piece of DNA, which contains many genes, regulatory elements and other intervening nucleotide sequences. In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, 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 called metaphasic chromosomes. This is the only natural context in which individual chromosomes are visible with an optical microscope. Prokaryotes do not possess histones or nuclei. In its relaxed state, the DNA can be accessed for transcription, regulation, and replication. Chromosomes were first observed by Karl Wilhelm von NÃ¤geli 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.
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.
In the 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 ribosomal RNA.
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 gonosomes (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 bodies.
* 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. 
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 an 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 syndrome, which is caused by the deletion of part of the short arm of chromosome 5. Victims make high-pitched cries that sound like a cat. They have wide-set eyes, a small head and jaw and are mentally retarded.
- Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4. It is characterized by severe growth retardation and mental defect.
- Down syndrome (extra chromosome 21). This is also known as mongolism or trisomy 21. Symptoms are decreased muscle tone, asymmetrical skull, slanting eyes and 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.
- Klinefelters 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. Other common symptoms are fatigue, apathy, lack of emotion, and an increased tendency to develop psychiatric disorders.
- 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
- Triple-X syndrome
You can find a detailed graphical display of all human chromosomes and the diseases annotated at the correct spot at .
- Readers may use this email link to report errors and/or omissions they have discovered, or to add additional material or comments regarding this article "Chromosome"
- Wiki Staff should discuss this article in the Wiki Staff Forum
- Susan's Place Transgender Resources Forums
- Susan's Place Transgender Chat
Browse: Gender | Cross-dressing | Intersexuality | Transgender topics | Transsexualism | Hormone Therapy | Surgery | Standards of Care | Legal Information | Psychology | Transitioning | Family & Friends | People | Books | Abbreviations | Browse All TopicsRead the FAQ | Return to the Main Page
Want to help us? Write New Articles and/or Expand Current Articles
*Some information provided in whole or in part by http://en.wikipedia.org/