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Chromosome

A chromosome is a package of DNA with part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells the most important of these proteins are the histones. These proteins, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity.[1][2] These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation.[3]

This article is about the DNA molecule. For the genetic algorithm, see Chromosome (genetic algorithm).

Chromosomes are normally visible under a light microscope only during the metaphase of cell division (where all chromosomes are aligned in the center of the cell in their condensed form).[4] Before this happens, each chromosome is duplicated (S phase), and both copies are joined by a centromere, resulting either in an X-shaped structure (pictured above), if the centromere is located equatorially, or a two-arm structure, if the centromere is located distally. The joined copies are now called sister chromatids. During metaphase, the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study.[5] In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.[6]


Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.


Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.

Etymology[edit]

The word chromosome (/ˈkroʊməˌsoʊm, -ˌzoʊm/[7][8]) comes from the Greek χρῶμα (chroma, "colour") and σῶμα (soma, "body"), describing their strong staining by particular dyes.[9] The term was coined by the German anatomist Heinrich Wilhelm Waldeyer,[10] referring to the term chromatin, which was introduced by Walther Flemming.


Some of the early karyological terms have become outdated.[11][12] For example, Chromatin (Flemming 1880) and Chromosom (Waldeyer 1888), both ascribe color to a non-colored state.[13]

which consists of DNA that is active, e.g., being expressed as protein.

Euchromatin

Heterochromatin

repetitive sequences

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; the condition was so-named because affected babies make high-pitched cries that sound like those of a cat. Affected individuals have wide-set eyes, a small head and jaw, moderate to severe mental health problems, and are very short.

Cri du chat

the most common trisomy, usually caused by an extra copy of chromosome 21 (trisomy 21). Characteristics include decreased muscle tone, stockier build, asymmetrical skull, slanting eyes and mild to moderate developmental disability.[53]

Down syndrome

or trisomy-18, the second most common trisomy.[54] Symptoms include motor retardation, developmental disability and numerous congenital anomalies causing serious health problems. Ninety percent of those affected die in infancy. They have characteristic clenched hands and overlapping fingers.

Edwards syndrome

also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15).

Isodicentric 15

which is very rare. It is also called the 11q terminal deletion disorder.[55] Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called Paris-Trousseau syndrome.

Jacobsen syndrome

(XXY). Men with Klinefelter syndrome are usually sterile and tend to be taller and have longer arms and legs than their peers. Boys with the syndrome are often shy and quiet and have a higher incidence of speech delay and dyslexia. Without testosterone treatment, some may develop gynecomastia during puberty.

Klinefelter syndrome

also called D-Syndrome or trisomy-13. Symptoms are somewhat similar to those of trisomy-18, without the characteristic folded hand.

Patau Syndrome

. 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.

Small supernumerary marker chromosome

(XXX). XXX girls tend to be tall and thin and have a higher incidence of dyslexia.

Triple-X syndrome

(X instead of XX or XY). In Turner syndrome, female sexual characteristics are present but underdeveloped. Females with Turner syndrome often have a short stature, low hairline, abnormal eye features and bone development and a "caved-in" appearance to the chest.

Turner syndrome

which is caused by partial deletion of the short arm of chromosome 4. It is characterized by growth retardation, delayed motor skills development, "Greek Helmet" facial features, and mild to profound mental health problems.

Wolf–Hirschhorn syndrome

. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.

XYY syndrome

Aneuploidy

Chromomere

Chromosome segregation

Cohesin

Condensin

DNA

Genetic deletion

Epigenetics

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

genetic algorithms

Genetic genealogy

Genealogical DNA test

Lampbrush chromosome

List of number of chromosomes of various organisms

(explains gene location nomenclature)

Locus

Maternal influence on sex determination

Microchromosome

Minichromosome

Non-disjunction

Secondary chromosome

Sex-determination system

XY sex-determination system

Polytene chromosome

Protamine

Neochromosome

Parasitic chromosome

from HOPES: Huntington's Outreach Project for Education at Stanford

An Introduction to DNA and Chromosomes

Chromosome Abnormalities at AtlasGeneticsOncology

On-line exhibition on chromosomes and genome (SIB)

from the University of Utah's Genetic Science Learning Center

What Can Our Chromosomes Tell Us?

from the University of Utah's Genetic Science Learning Center

Try making a karyotype yourself

Kimballs Chromosome pages

Chromosome News from Genome News Network

European network for Rare Chromosome Disorders on the Internet

Eurochromnet

Ensembl project, presenting chromosomes, their genes and syntenic loci graphically via the web

Ensembl.org

Archived 12 July 2007 at the Wayback Machine

Genographic Project

from the U.S. National Library of Medicine

Home reference on Chromosomes

and comparison to other species

Visualisation of human chromosomes

Support for people with rare chromosome disorders

Unique – The Rare Chromosome Disorder Support Group