AskDefine | Define chromosome

Dictionary Definition

chromosome n : a threadlike body in the cell nucleus that carries the genes in a linear order

User Contributed Dictionary



19th century: from Chromosom, ultimately from χρώμα + σῶμα (because they are stained under the microscope).


  • /ˈkrəʊməˌsəʊm/ (UK) or /ˈkroʊməˌsoʊm/ (US)
  • /"kr@Um@%s@Um/ (UK) or /"kroUm@%soUm/ (US)


  1. In the context of "biology|cytology": A structure in the cell nucleus that contains DNA, histone protein, and other structural proteins.


Extensive Definition

Chromosomes are organized structures of DNA and proteins that are found in cells. A chromosome is a singular piece of DNA, which contains many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek (chroma, color) and (soma, body) due to their property of being stained very strongly by some dyes.
In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes may exist as either duplicated or unduplicated—unduplicated chromosomes are single linear strands, while duplicated chromosomes (copied during S phase) contain two copies joined by a centromere. Compaction of the duplicated chromosomes during mitosis and meiosis results in the classic four-arm structure (pictured to the right).


Visual discovery of chromosomes. Textbooks have often said that chromosomes were first observed in plant cells by a Swiss botanist named Karl Wilhelm von Nägeli in 1842. However, this opinion has been challenged, perhaps decisively, by Henry Harris, who has freshly reviewed the primary literature. In his opinion the claim of Nägeli to have seen spore mother cells divide is mistaken, as are some of his interpretations. Harris considers other candidates, especially Wilhelm Hofmeister, whose publications in 1848-9 include plates which definitely show mitotic events. Hofmeister was also the choice of Cyril Darlington.
The work of other cytologists such as Walther Flemming, Eduard Strasburger, Otto Bütschli, Oskar Hertwig and Carl Rabl should definitely be acknowledged. The use of basophilic aniline dyes was a new technique for effectively staining the chromatin material in the nucleus. Their behavior in animal (salamander) cells was later described in detail by Walther Flemming, who in 1882 "provided a superb summary of the state of the field". The name chromosome was invented in 1888 by Heinrich von Waldeyer. However, van Beneden's monograph of 1883 on the fertilised eggs of the parasitic roundworm Ascaris megalocephala was the outstanding work of this period. His conclusions are classic:
Van Beneden's master work was closely followed by that of Carl Rabl, who reached similar conclusions. This more or less concludes the first period, in which chromosomes were visually sighted, and the morphological stages of mitosis were described. Coleman also gives a useful review of these discoveries.
The continuity of chromosomes
The individuality of chromosomes.

Chromosomes in eukaryotes




Interphase chromatin

    • Facultative heterochromatin, which is sometimes expressed.

Metaphase chromatin and division


Chromosomes in prokaryotes


Structure in sequences

Prokaryotes chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a single point (the origin of replication) from which replication starts, while some archaea contain multiple replication origins. The genes in prokaryotes are often organised in operons, and do not contain introns, unlike eukaryotes.

DNA packaging

Prokaryotes do not possess nuclei, instead their DNA is organized into a structure called the nucleoid. The nucleoid is a distinct structure and occupies a defined region of the bacterial cell. This structure is however dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome. In archaea, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.

Number of chromosomes in various organisms


These tables give the total number of chromosomes (including sex chromosomes) in a cell nucleus. For example, human cells are diploid and have 22 different types of autosomes, each present as two copies, and two sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosomes, such as Bread wheat which is hexaploid and has six copies of 6 different chromosomes - 42 chromosomes in total.
| | |- | colspan="2" | 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.


Prokaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. For example, Buchnera, a symbiont of aphids has multiple copies of its chromosome, ranging from 10–400 copies per cell. However, in some large bacteria, such as Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present. 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.


In general, the karyotype is the characteristic chromosome complement of a eukaryote species. The preparation and study of karyotypes is part of cytogenetics.

Historical note

Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploid human cell contain? In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism. Painter in 1922 was not certain whether the diploid number of man was 46 or 48, at first favouring 46. He revised his opinion later from 46 to 48, and he correctly insisted on man having an XX/XY system. Considering their techniques, these results were quite remarkable.
New techniques were needed to definitively solve the problem:
1. Using cells in culture
3. Arresting mitosis in metaphase by a solution of colchicine
It took until the mid 1950s until it became generally accepted that the human karyotype included only 46 chromosomes. Rather interestingly, chimpanzees (our closest living relatives) have 48 chromosomes.

Chromosomal aberrations

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's syndrome, usually is 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 mental retardation.
  • Edwards 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. 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. 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.
  • 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
Most mutations are neutral - have little or no effect

Human chromosomes


External links


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Synonyms, Antonyms and Related Words

Altmann theory, DNA, De Vries theory, Galtonian theory, Mendelianism, Mendelism, RNA, Verworn theory, W chromosome, Weismann theory, Weismannism, Wiesner theory, X chromosome, Y chromosome, Z chromosome, accessory chromosome, allele, allelomorph, allosome, autosome, birth, centromere, character, chromatid, chromatin, chromosome complement, chromosome number, determinant, determiner, diathesis, diploid number, endowment, euchromosome, eugenics, factor, gene, gene-string, genesiology, genetic code, genetics, genome, haploid number, hereditability, heredity, heritability, heritage, heterochromosome, homologous chromosomes, idiochromosome, inborn capacity, inheritability, inheritance, matrocliny, monosome, patrocliny, pharmacogenetics, polyploidy, recessive character, replication, sex chromosome, univalent chromosome
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