This article is about the DNA molecule.
For the genetic algorithm, see Chromosome (genetic algorithm).
These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation.
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.
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.
Some of the early karyological terms have become outdated.
For example, Chromatin (Flemming 1880) and Chromosom (Waldeyer 1888), both ascribe color to a non-colored state.
History of discovery
In a series of experiments beginning in the mid-1880s, Theodor Boveri gave definitive contributions to elucidating that chromosomes are the vectors of heredity, with two notions that became known as ‘chromosome continuity’ and ‘chromosome individuality’.
Aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes.
Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter were all influenced by Boveri (Wilson, Stevens, and Painter actually worked with him).
In his famous textbook The Cell in Development and Heredity, Wilson linked together the independent work of Boveri and Sutton (both around 1902) by naming the chromosome theory of inheritance the Boveri–Sutton chromosome theory (the names are sometimes reversed).
Eventually, complete proof came from chromosome maps in Morgan's own lab.
The number of human chromosomes was published in 1923 by Theophilus Painter.
By inspection through the microscope, he counted 24 pairs, which would mean 48 chromosomes.
His error was copied by others and it was not until 1956 that the true number, 46, was determined by Indonesia-born cytogeneticist Joe Hin Tjio.
Main article: Nucleoid
The chromosomes of most bacteria, which some authors prefer to call genophores, can range in size from only 130,000 base pairs in the endosymbiotic bacteria Candidatus Hodgkinia cicadicola and Candidatus Tremblaya princeps, to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum.
Structure in sequences
Prokaryotic chromosomes have less sequence-based structure than eukaryotes.
Bacteria typically have a one-point (the origin of replication) from which replication starts, whereas some archaea contain multiple replication origins.
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.
In prokaryotes (see nucleoids) and viruses, the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by histone-like proteins.
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 the membranes (and the attached DNA).
Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled.
Main article: Chromatin
See also: Eukaryotic chromosome fine structure
Each eukaryotic chromosome consists of a long linear DNA molecule associated with proteins, forming a compact complex of proteins and DNA called chromatin.
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 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.
The packaging of DNA into nucleosomes causes a 10 nanometer fibre which may further condense up to 30 nm fibres Most of the euchromatin in interphase nuclei appears to be in the form of 30-nm fibers.
Chromatin structure is the more decondensed state, i.e. the 10-nm conformation allows transcription.
- Euchromatin, which consists of DNA that is active, e.g., being 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.
Metaphase chromatin and division
They cease to function as accessible genetic material (transcription stops) and become a compact transportable form.
The loops of 30-nm chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells.
The DNA is thus condense about 10,000 folds.
Loops of 30 nm structure further condense with scaffold into higher order structures.
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; q-g "grande"; alternatively it is sometimes said q is short for queue meaning tail in French).
This is the only natural context in which individual chromosomes are visible with an optical microscope.
Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops.
During mitosis, microtubules grow from centrosomes located at opposite ends of the cell and also attach to the centromere 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.
The microtubules then pull the chromatids apart toward the centrosomes, so that each daughter cell inherits one set of chromatids.
Once the cells have divided, the chromatids are uncoiled and DNA can again be transcribed.
In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus.
Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes.
The autosomes contain the rest of the genetic hereditary information.
All act in the same way during cell division.
Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell.
In addition to these, human cells have many hundreds of copies of the mitochondrial genome.
Number of genes is an estimate, as it is in part based on gene predictions.
Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions.
|Chromosome||Genes||Total base pairs||% of bases||Sequenced base pairs||% sequenced base pairs|
|X (sex chromosome)||800||154,913,754||5.0||151,058,754||97.51%|
|Y (sex chromosome)||200||57,741,652||1.9||25,121,652||43.51%|
Number in various organisms
Main article: List of organisms by chromosome count
These tables give the total number of chromosomes (including sex chromosomes) in a cell nucleus.
This gives 46 chromosomes in total.
Other organisms have more than two copies of their chromosome types, such as bread wheat, which is hexaploid and has six copies of seven different chromosome types – 42 chromosomes in total.
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 that are the same in all body cells.
However, asexual species can be either haploid or diploid.
Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes (23 pairs in humans with one set of 23 chromosomes from each parent), one set from the mother and one from the father.
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.
Plants important in agriculture 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 wheat types are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in the wild wheat.
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, as in eukaryotes, highly 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.
Main article: Karyotype
The preparation and study of karyotypes is part of cytogenetics.
There may be variation between species in chromosome number and in detailed organization.
In some cases, there is significant variation within species.
Often there is:
- 1. variation between the two sexes
- 2. variation between the germ-line and soma (between gametes and the rest of the body)
- 3. variation between members of a population, due to balanced genetic polymorphism
- 4. geographical variation between races
- 5. mosaics or otherwise abnormal individuals.
Also, variation in karyotype may occur during development from the fertilized egg.
The technique of determining the karyotype is usually called karyotyping.
These cells are then stained, photographed, and arranged into a karyogram, with the set of chromosomes arranged, autosomes in order of length, and sex chromosomes (here X/Y) at the end.
These are XX in females and XY in males.
History and analysis techniques
See also: Argument from authority § Use in science
Investigation into the human karyotype took many years to settle the most basic question: How many chromosomes does a normal diploid human cell contain?
Painter in 1922 was not certain whether the diploid number of man is 46 or 48, at first favouring 46.
He revised his opinion later from 46 to 48, and he correctly insisted on humans having an XX/XY system.
New techniques were needed to definitively solve the problem:
- Using cells in culture
- Arresting mitosis in metaphase by a solution of colchicine
- Pretreating cells in a hypotonic solution 0.075 M KCl, which swells them and spreads the chromosomes
- Squashing the preparation on the slide forcing the chromosomes into a single plane
- Cutting up a photomicrograph and arranging the result into an indisputable karyogram.
It took until 1954 before the human diploid number was confirmed as 46.
Considering the techniques of Winiwarter and Painter, their results were quite remarkable.
Chromosomal aberrations are disruptions in the normal chromosomal content of a cell and are a major cause of genetic conditions in humans, such as Down syndrome, although most aberrations have little to no effect.
Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of bearing a child with a chromosome disorder.
Abnormal numbers of chromosomes or chromosome sets, called aneuploidy, may be lethal or may give rise to genetic disorders.
Genetic counseling is offered for families that may carry a chromosome rearrangement.
The gain or loss of DNA from chromosomes 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; 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.
- Down syndrome, 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.
- Edwards syndrome, or trisomy-18, the second most common trisomy. 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.
- Isodicentric 15, also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15).
- Jacobsen syndrome, which is very rare. It is also called the terminal 11q deletion disorder. Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called Paris-Trousseau syndrome.
- Klinefelter 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.
- Patau Syndrome, also called D-Syndrome or trisomy-13. Symptoms are somewhat similar to those of trisomy-18, without the characteristic folded hand.
- 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.
- Triple-X syndrome (XXX). XXX girls tend to be tall and thin and have a higher incidence of dyslexia.
- Turner 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.
- Wolf–Hirschhorn 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.
- XYY syndrome. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.
Exposure of males to certain lifestyle, environmental and/or occupational hazards may increase the risk of aneuploid spermatozoa.
In particular, risk of aneuploidy is increased by tobacco smoking, and occupational exposure to benzene, insecticides, and perfluorinated compounds.
Increased aneuploidy is often associated with increased DNA damage in spermatozoa.
- Chromosome segregation
- Genetic deletion
- For information about chromosomes in genetic algorithms, see chromosome (genetic algorithm)
- Genetic genealogy
- Lampbrush chromosome
- List of number of chromosomes of various organisms
- Locus (explains gene location nomenclature)
- Maternal influence on sex determination
- Sex-determination system
- XY sex-determination system
- Polytene chromosome
- Parasitic chromosome
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Chromosome.