Karyotype
Encyclopedia
A karyotype is the number and appearance
of chromosome
s in the nucleus
of an eukaryotic cell
. The term is also used for the complete set of chromosomes in a species
, or an individual organism.p28
Karyotypes describe the number of chromosomes, and what they look like under a light microscope
. Attention is paid to their length, the position of the centromere
s, banding pattern, any differences between the sex chromosomes, and any other physical characteristics. The preparation and study of karyotypes is part of cytogenetics
.
The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.
The basic number of chromosomes in the somatic
cells of an individual or a species is called the somatic number and is designated 2n. Thus, in humans 2n = 46. In the germ-line (the sex cells) the chromosome number is n (humans: n = 23).p28
So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid
cells have multiple copies of chromosomes and haploid
cells have single copies.
The study of karyotypes is important for cell biology
and genetics
, and the results may be used in evolutionary biology and medicine
. Karyotypes can be used for many purposes; such as, to study chromosomal aberrations, cellular
function, taxonomic relationships, and to gather information about past evolutionary events.
in 1842. Their behavior in animal (salamander
) cells was described by Walther Flemming
, the discoverer of mitosis
, in 1882. The name was coined by another German anatomist, von Waldeyer
in 1888.
The next stage took place after the development of genetics in the early 20th century, when it was appreciated that the set of chromosomes (the karyotype) was the carrier of the genes. Levitsky seems to have been the first to define the karyotype as the phenotypic appearance of the somatic
chromosomes, in contrast to their genic
contents. The subsequent history of the concept can be followed in the works of Darlington and White
.
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 humans was 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. Considering their techniques, these results were quite remarkable.
New techniques were needed to definitively solve the problem:
It took until the mid 1950s until it became generally accepted that the karyotype of humans included only 46 chromosomes. Rather interestingly, the great apes
have 48 chromosomes. Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number.
, such as Giemsa, is applied after cells
have been arrested during cell division
by a solution of colchicine
. For humans, white blood cells are used most frequently because they are easily induced to divide and grow in tissue culture
. Sometimes observations may be made on non-dividing (interphase
) cells. The sex of an unborn fetus
can be determined by observation of interphase cells (see amniotic centesis
and Barr body
).
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.
Variation is often found:
s and are denoted 46,XX; males have both an X and a Y chromosome
denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
and transcription
of DNA
is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are highly variable. There is variation between species in chromosome number, and in detailed organization, despite their construction from the same macromolecules
. This variation provides the basis for a range of studies in evolutionary cytology
.
In some cases there is even significant variation within species. In a review, Godfrey and Masters conclude:
Although much is known about karyotypes at the descriptive level, and it is clear that changes in karyotype organization has had effects on the evolutionary course of many species, it is quite unclear what the general significance might be.
, or other kinds of visible adjustment to the karyotype.
, which was investigated by Kurt Benirschke
and his colleague Doris Wurster. The diploid number of the Chinese muntjac, Muntiacus reevesi, was found to be 46, all telocentric. When they looked at the karyotype of the closely related Indian muntjac, Muntiacus muntjak, they were astonished to find it had female = 6, male = 7 chromosomes.
The number of chromosomes in the karyotype between (relatively) unrelated species is hugely variable. The low record is held by the nematode
Parascaris univalens, where the haploid n = 1; the high record would be somewhere amongst the fern
s, with the Adder's Tongue Fern Ophioglossum
ahead with an average of 1262 chromosomes. Top score for animals might be the shortnose sturgeon
Acipenser brevirostrum at a mere 372 chromosomes. The existence of supernumerary or B chromosomes means that chromosome number can vary even within one interbreeding population; and aneuploids are another example, though in this case they would not be regarded as normal members of the population.
, 14
, 15
, 21
and 22
).
is the number of complete sets of chromosomes in a cell.
is the condition in which the chromosome number in the cells is not the typical number for the species. This would give rise to a chromosome abnormality such as an extra chromosome or one or more chromosomes lost. Abnormalities in chromosome number usually cause a defect in development. Down syndrome
and Turner syndrome
are examples of this.
Aneuploidy may also occur within a group of closely related species. Classic examples in plants are the genus Crepis
, where the gametic (= haploid)
numbers form the series x = 3, 4, 5, 6, and 7; and Crocus
, where every number from x = 3 to x = 15 is represented by at least one species. Evidence of various kinds shows that that trends of evolution have gone in different directions in different groups. Closer to home, the great apes
have 24x2 chromosomes whereas humans have 23x2. Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number.
for chromosome fusions or dissociations. When this happens, the chromosome number is variable from one individual to another. Well-researched examples are the ladybird beetle Chilocorus stigma, some mantids of the genus Ameles
, the European shrew Sorex araneus. There is some evidence from the case of the mollusc Thais lapillus (the dog whelk
) on the Brittany
coast, that the two chromosome morphs are adapted
to different habitats.
s can reveal relationships between closely related species: the classic example is the study of chromosome banding in Hawaiian drosophilids
by Hampton Carson
.
In about 6500 sq mi (16,834.9 km²), the Hawaiian Islands
have the most diverse collection of drosophilid flies in the world, living from rainforests to subalpine meadows
. These roughly 800 Hawaiian drosophilid species are usually assigned to two genera, Drosophila
and Scaptomyza
, in the family Drosophilidae
.
The polytene banding of the 'picture wing' group, the best-studied group of Hawaiian drosophilids, enabled Carson to work out the evolutionary tree long before genome analysis was practicable. In a sense, gene arrangements are visible in the banding patterns of each chromosome. Chromosome rearrangements, especially inversions, make it possible to see which species are closely related.
The results are clear. The inversions, when plotted in tree form (and independent of all other information), show a clear "flow" of species from older to newer islands. There are also cases of colonization back to older islands, and skipping of islands, but these are much less frequent. Using K-Ar
dating, the present islands date from 0.4 million years ago (mya) (Mauna Kea
) to 10mya (Necker). The oldest member of the Hawaiian archipelago still above the sea is Kure Atoll
, which can be dated to 30 mya. The archipelago itself (produced by the Pacific plate
moving over a hot spot) has existed for far longer, at least into the Cretaceous
. Previous islands now beneath the sea (guyot
s) form the Emperor Seamount Chain
.
All of the native Drosophila and Scaptomyza species in Hawaii have apparently descended from a single ancestral species that colonized the islands, probably 20 million years ago. The subsequent adaptive radiation
was spurred by a lack of competition
and a wide variety of niches. Although it would be possible for a single gravid female to colonise an island, it is more likely to have been a group from the same species.
There are other animals and plants on the Hawaiian archipelago which have undergone similar, if less spectacular, adaptive radiations.
employs several techniques to visualize different aspects of chromosomes:
In the "classic" (depicted) karyotype, a dye
, often Giemsa (G-banding), less frequently Quinacrine
, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate
groups of DNA
. Quinacrine binds to the adenine
-thymine
-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern.
Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat
syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2).
Spectral karyotyping is a molecular cytogenetic technique used to simultaneously visualize all the pairs of chromosome
s in an organism in different colors. Fluorescently labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophore
s. Because there are a limited number of spectrally-distinct fluorophores, a combinatorial labeling method is used to generate many different colors. Spectral differences generated by combinatorial labeling are captured and analyzed by using an interferometer attached to a fluorescence microscope. Image processing software then assigns a pseudo color to each spectrally different combination, allowing the visualization of the individually colored chromosomes.
This technique is used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough.
.
, translocations
, inversions
, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy
, often occur as a result of nondisjunction
during meiosis
in the formation of a gamete
; trisomies
, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination
. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis
and give rise to a genetic mosaic individual who has some normal and some abnormal cells.
Chromosomal abnormalities that lead to disease in humans include
Some disorders arise from loss of just a piece of one chromosome, including
Chromosomal abnormalities can also occur in cancer
ous cells of an otherwise genetically normal individual; one well-documented example is the Philadelphia chromosome
, a translocation mutation commonly associated with chronic myelogenous leukemia
and less often with acute lymphoblastic leukemia
.
Visual appearance
The visual appearance of objects is given by the way in which they reflect and transmit light. The color of objects is determined by the parts of the spectrum of light that are reflected or transmitted without being absorbed...
of chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s in the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
of an eukaryotic cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. The term is also used for the complete set of chromosomes in a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, or an individual organism.p28
Karyotypes describe the number of chromosomes, and what they look like under a light microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
. Attention is paid to their length, the position of the centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
s, banding pattern, any differences between the sex chromosomes, and any other physical characteristics. The preparation and study of karyotypes is part of cytogenetics
Cytogenetics
Cytogenetics is a branch of genetics that is concerned with the study of the structure and function of the cell, especially the chromosomes. It includes routine analysis of G-Banded chromosomes, other cytogenetic banding techniques, as well as molecular cytogenetics such as fluorescent in situ...
.
The basic number of chromosomes in the somatic
Somatic
The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes...
cells of an individual or a species is called the somatic number and is designated 2n. Thus, in humans 2n = 46. In the germ-line (the sex cells) the chromosome number is n (humans: n = 23).p28
So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid
Polyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
cells have multiple copies of chromosomes and haploid
Ploidy
Ploidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
cells have single copies.
The study of karyotypes is important for cell biology
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
and genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, and the results may be used in evolutionary biology and medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
. Karyotypes can be used for many purposes; such as, to study chromosomal aberrations, cellular
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
function, taxonomic relationships, and to gather information about past evolutionary events.
History of karyotype studies
Chromosomes were first observed in plant cells by Karl Wilhelm von NägeliKarl Wilhelm von Nägeli
Karl Wilhelm von Nägeli was a Swiss botanist. He studied cell division and pollination, but became known as the man who discouraged Gregor Mendel from further work on genetics.-Birth and education:...
in 1842. Their behavior in animal (salamander
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...
) cells was described by Walther Flemming
Walther Flemming
Walther Flemming was a German biologist and a founder of cytogenetics.He was born in Sachsenberg near Schwerin as the fifth child and only son of the psychiatrist Carl Friedrich Flemming and his second wife, Auguste Winter...
, the discoverer of mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, in 1882. The name was coined by another German anatomist, von Waldeyer
Heinrich Wilhelm Gottfried von Waldeyer-Hartz
Heinrich Wilhelm Gottfried von Waldeyer-Hartz was a German anatomist, famous for consolidating the neuron theory of organization of the nervous system and for naming the chromosome...
in 1888.
The next stage took place after the development of genetics in the early 20th century, when it was appreciated that the set of chromosomes (the karyotype) was the carrier of the genes. Levitsky seems to have been the first to define the karyotype as the phenotypic appearance of the somatic
Somatic
The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes...
chromosomes, in contrast to their genic
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
contents. The subsequent history of the concept can be followed in the works of Darlington and White
Michael JD White
*White M.J.D. and Webb G.C. Blattodea, Mantodea, Isoptera, Grylloblattodea, Phasmatodea, Dermaptera, and Embioptera. Borntraeger. ISBN 3443260055...
.
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
Theophilus Painter
Theophilus Shickel Painter was an American zoologist known for his work in identifying genes in fruit flies...
in 1922 was not certain whether the diploid number of humans was 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
XY sex-determination system
The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...
system. Considering their techniques, these results were quite remarkable.
New techniques were needed to definitively solve the problem:
- Using cells in culture
- Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes
- Arresting mitosisMitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
in metaphaseMetaphaseMetaphase, from the ancient Greek μετά and φάσις , is a stage of mitosis in the eukaryotic cell cycle in which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells...
by a solution of colchicineColchicineColchicine is a medication used for gout. It is a toxic natural product and secondary metabolite, originally extracted from plants of the genus Colchicum... - 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 the mid 1950s until it became generally accepted that the karyotype of humans included only 46 chromosomes. Rather interestingly, the great apes
Great Apes
Great Apes may refer to*Great apes, species in the biological family Hominidae, including humans, chimpanzees, gorillas, and orangutans*Great Apes , a 1997 novel by Will Self...
have 48 chromosomes. Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number.
Staining
The study of karyotypes is made possible by staining. Usually, a suitable dyeDye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....
, such as Giemsa, is applied after cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
have been arrested during cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
by a solution of colchicine
Colchicine
Colchicine is a medication used for gout. It is a toxic natural product and secondary metabolite, originally extracted from plants of the genus Colchicum...
. For humans, white blood cells are used most frequently because they are easily induced to divide and grow in tissue culture
Tissue culture
Tissue culture is the growth of tissues or cells separate from the organism. This is typically facilitated via use of a liquid, semi-solid, or solid growth medium, such as broth or agar...
. Sometimes observations may be made on non-dividing (interphase
Interphase
Interphase is the phase of the cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for cell division. In preparation for cell division, it increases its size and makes a copy of its DNA...
) cells. The sex of an unborn fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
can be determined by observation of interphase cells (see amniotic centesis
Amniocentesis
Amniocentesis is a medical procedure used in prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for...
and Barr body
Barr body
A Barr body is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, in those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z...
).
Observations
Six different characteristics of karyotypes are usually observed and compared:- Differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family: Lotus tenuis and Vicia fabaVicia fabaThis article refers to the Broad Bean plant. For Broadbean the company, see Broadbean, Inc.Vicia faba, the Broad Bean, Fava Bean, Field Bean, Bell Bean or Tic Bean, is a species of bean native to north Africa and southwest Asia, and extensively cultivated elsewhere. A variety is provisionally...
(legumes), both have six pairs of chromosomes (n=6) yet V. faba chromosomes are many times larger. This feature probably reflects different amounts of DNA duplication. - Differences in the position of centromeres. This is brought about by translocations.
- Differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths.
- Differences in basic number of chromosomes may occur due to successive unequal translocations which finally remove all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis). Humans have one pair fewer chromosomes than the great apes, but the genes have been mostly translocated (added) to other chromosomes.
- Differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread.
- Differences in degree and distribution of heterochromaticHeterochromatinHeterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...
regions. Heterochromatin stains darker than euchromatinEuchromatinEuchromatin is a lightly packed form of chromatin that is rich in gene concentration, and is often under active transcription. Unlike heterochromatin, it is found in both cells with nuclei and cells without nuclei...
, indicating tighter packing, and mainly consists of genetically inactive repetitive DNA sequences.
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.
Variation is often found:
- Between the sexes
- Between the germ-line and somaSoma (biology)The soma , or perikaryon , or cyton, is the bulbous end of a neuron, containing the cell nucleus. The word "soma" comes from the Greek σῶμα, meaning "body"; the soma of a neuron is often called the "cell body"...
(between gametes and the rest of the body) - Between members of a population (chromosome polymorphismPolymorphism (biology)Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
) - Geographical variationAllopatric speciationAllopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
between races - MosaicsMosaic (genetics)In genetic medicine, a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one individual who has developed from a single fertilized egg...
or otherwise abnormal individuals.
The human karyotype
Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for females contain two X chromosomeX chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
s and are denoted 46,XX; males have both an X and a Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
Diversity and evolution of karyotypes
Although the replicationDNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...
and transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are highly variable. There is variation between species in chromosome number, and in detailed organization, despite their construction from the same macromolecules
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
. This variation provides the basis for a range of studies in evolutionary cytology
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
.
In some cases there is even significant variation within species. In a review, Godfrey and Masters conclude:
- "In our view, it is unlikely that one process or the other can independently account for the wide range of karyotype structures that are observed... But, used in conjunction with other phylogenetic data, karyotypic fissioning may help to explain dramatic differences in diploid numbers between closely related species, which were previously inexplicable.
Although much is known about karyotypes at the descriptive level, and it is clear that changes in karyotype organization has had effects on the evolutionary course of many species, it is quite unclear what the general significance might be.
- "We have a very poor understanding of the causes of karyotype evolution, despite many careful investigations... the general significance of karyotype evolution is obscure." Maynard Smith.
Changes during development
Instead of the usual gene repression, some organisms go in for large-scale elimination of heterochromatinHeterochromatin
Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...
, or other kinds of visible adjustment to the karyotype.
- Chromosome elimination. In some species, as in many sciarid fliesSciaridaeSciaridae is a family of flies, commonly known as dark-winged fungus gnats. Commonly found in moist environments, they are known to be a pest of mushroom farms and are commonly found in household plant pots. This is one of the least studied of the large Diptera families, probably due to the small...
, entire chromosomes are eliminated during development.
- Chromatin diminution (founding father: Theodor BoveriTheodor Boveri-External links:* Fritz Baltzer. . excerpt from . University of California Press, Berkeley; pp. 85–97....
). In this process, found in some copepods and roundworms such as Ascaris suum, portions of the chromosomes are cast away in particular cells. This process is a carefully organised genome rearrangement where new telomeres are constructed and certain heterochromatin regions are lost. In A. suum, all the somatic cell precursors undergo chromatin diminution.
- X-inactivationX-inactivationX-inactivation is a process by which one of the two copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by packaging into transcriptionally inactive heterochromatin...
. The inactivation of one X chromosome takes place during the early development of mammals (see Barr bodyBarr bodyA Barr body is the inactive X chromosome in a female somatic cell, rendered inactive in a process called lyonization, in those species in which sex is determined by the presence of the Y or W chromosome rather than the diploidy of the X or Z...
and dosage compensationDosage compensationDosage compensation, also known as Ohno's Hypothesis, is a hypothetical genetic regulatory mechanism which operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female. In 2006,...
). In placental mammals, the inactivation is random as between the two Xs; thus the mammalian female is a mosaic in respect of her X chromosomes. In marsupials it is always the paternal X which is inactivated. In human females some 15% of somatic cells escape inactivation.
Number of chromosomes in a set
A spectacular example of variability between closely related species is the muntjacMuntjac
Muntjac, also known as Barking Deer and Mastreani Deer, are small deer of the genus Muntiacus. Muntjac are the oldest known deer, appearing 15–35 million years ago, with remains found in Miocene deposits in France, Germany and Poland....
, which was investigated by Kurt Benirschke
Kurt Benirschke
Kurt Benirschke is a German-born American pathologist, geneticist and expert on the placenta and reproduction in humans and myriad mammalian species....
and his colleague Doris Wurster. The diploid number of the Chinese muntjac, Muntiacus reevesi, was found to be 46, all telocentric. When they looked at the karyotype of the closely related Indian muntjac, Muntiacus muntjak, they were astonished to find it had female = 6, male = 7 chromosomes.
- "They simply could not believe what they saw... They kept quiet for two or three years because they thought something was wrong with their tissue culture... But when they obtained a couple more specimens they confirmed [their findings]" Hsu p73-4
The number of chromosomes in the karyotype between (relatively) unrelated species is hugely variable. The low record is held by the nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
Parascaris univalens, where the haploid n = 1; the high record would be somewhere amongst the fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s, with the Adder's Tongue Fern Ophioglossum
Ophioglossum
Ophioglossum is a genus of about 25-30 species of Ophioglossales in the family Ophioglossaceae, with a cosmopolitan but primarily tropical and subtropical distribution. The name Ophioglossum comes from the Greek, and means "snake-tongue".Adders-tongues are so-called because the spore-bearing stalk...
ahead with an average of 1262 chromosomes. Top score for animals might be the shortnose sturgeon
Shortnose sturgeon
The shortnose sturgeon is a small North American sturgeon which can be found in 16 to 19 large river and estuary systems along the Atlantic seaboard from the Saint John River in New Brunswick, Canada, to the St. Johns River in Florida, United States.National Marine Fisheries Service. 1998...
Acipenser brevirostrum at a mere 372 chromosomes. The existence of supernumerary or B chromosomes means that chromosome number can vary even within one interbreeding population; and aneuploids are another example, though in this case they would not be regarded as normal members of the population.
Fundamental number
The fundamental number, FN, of a karyotype is the number of visible major chromosomal arms per set of chromosomes. Thus, FN ≤ 2n, the difference depending on the number of chromosomes considered single-armed (acrocentric or telocentric) present. Humans have FN = 82, due to the presence of five acrocentric chromosome pairs (13Chromosome 13 (human)
Chromosome 13 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 13 spans about 114 million base pairs and represents between 3.5 and 4 % of the total DNA in cells....
, 14
Chromosome 14 (human)
rightChromosome14 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 14 spans about 109 million base pairs and represents between 3 and 3.5% of the total DNA in cells....
, 15
Chromosome 15 (human)
right|frame|Human chromosome 15Chromosome 15 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 15 spans about 106 million base pairs and represents between 3% and 3.5% of the total DNA in cells.Identifying genes on each chromosome is an...
, 21
Chromosome 21 (human)
Chromosome 21 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. The trisomy of the 21st chromosome causes Down Syndrome...
and 22
Chromosome 22 (human)
Chromosome 22 is one of the 23 pairs of chromosomes in human cells. Humans normally have two copies of Chromosome 22 in each cell. Chromosome 22 is the second smallest human chromosome, spanning about 49 million DNA base pairs and representing between 1.5 and 2 % of the total DNA in cells.In 1999,...
).
Ploidy
PloidyPloidy
Ploidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
is the number of complete sets of chromosomes in a cell.
- PolyploidyPolyploidyPolyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
, where there are more than two sets of homologous chromosomes in the cells, occurs mainly in plants. It has been of major significance in plant evolution according to StebbinsG. Ledyard StebbinsGeorge Ledyard Stebbins, Jr. was an American botanist and geneticist who is widely regarded as one of the leading evolutionary biologists of the 20th century. Stebbins received his Ph.D. in botany from Harvard University in 1931. He went on to the University of California, Berkeley, where his work...
. The proportion of flowering plants which are polyploid was estimated by Stebbins to be 30-35%, but in grasses the average is much higher, about 70%. Polyploidy in lower plants (fernFernA fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
s, horsetails and psilotales) is also common, and some species of ferns have reached levels of polyploidy far in excess of the highest levels known in flowering plants.
Polyploidy in animals is much less common, but it has been significant in some groups.
Polyploid series in related species which consist entirely of multiples of a single basic number are known as euploid.
- Haplo-diploidyHaplo-diploid sex-determination systemThe haplodiploid sex-determination system determines the sex of the offspring of many hymenopterans , spider mites, coleopterans and rotifers. In this system, sex is determined by the number of sets of chromosomes an individual receives...
, where one sex is diploid, and the other haploid. It is a common arrangement in the HymenopteraHymenopteraHymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees and ants. There are over 130,000 recognized species, with many more remaining to be described. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and...
, and in some other groups.
- Endopolyploidy occurs when in adult differentiatedCellular differentiationIn developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
tissues the cells have ceased to divide by mitosisMitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
, but the nucleiCell nucleusIn cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
contain more than the original somaticSomatic cellA somatic cell is any biological cell forming the body of an organism; that is, in a multicellular organism, any cell other than a gamete, germ cell, gametocyte or undifferentiated stem cell...
number of chromosomes. In the endocycle (endomitosis or endoreduplicationEndoreduplicationEndoreplication is replication of the nuclear genome in the absence of cell division, which leads to elevated nuclear gene content and polyploidy...
) chromosomes in a 'resting' nucleus undergo reduplicationReduplicationReduplication in linguistics is a morphological process in which the root or stem of a word is repeated exactly or with a slight change....
, the daughter chromosomes separating from each other inside an intact nuclear membrane.
In many instances, endopolyploid nuclei contain tens of thousands of chromosomes (which cannot be exactly counted). The cells do not always contain exact multiples (powers of two), which is why the simple definition 'an increase in the number of chromosome sets caused by replication without cell division' is not quite accurate.
This process (especially studied in insects and some higher plants such as maize) may be a developmental strategy for increasing the productivity of tissues which are highly active in biosynthesis.
The phenomenon occurs sporadically throughout the eukaryoteEukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
kingdom from protozoaProtozoaProtozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
to man; it is diverse and complex, and serves differentiation and morphogenesisMorphogenesisMorphogenesis , is the biological process that causes an organism to develop its shape...
in many ways.
- See palaeopolyploidyPaleopolyploidyPaleopolyploidy refers to ancient genome duplications which occurred at least several million years ago . The genome doubling event could either be an autopolyploidy or an allopolyploidy. Due to functional redundancy, genes are rapidly silenced and/or lost from the duplicated genomes...
for the investigation of ancient karyotype duplications.
Aneuploidy
AneuploidyAneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...
is the condition in which the chromosome number in the cells is not the typical number for the species. This would give rise to a chromosome abnormality such as an extra chromosome or one or more chromosomes lost. Abnormalities in chromosome number usually cause a defect in development. Down syndrome
Down syndrome
Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
and Turner syndrome
Turner syndrome
Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...
are examples of this.
Aneuploidy may also occur within a group of closely related species. Classic examples in plants are the genus Crepis
Crepis
Crepis, commonly known in some parts of the world as hawksbeard or hawk's-beard , is a genus of about 200 annual and perennial flowering plants of the family Asteraceae superficially resembling the dandelion, the most conspicuous difference being that Crepis usually has branching...
, where the gametic (= haploid)
numbers form the series x = 3, 4, 5, 6, and 7; and Crocus
Crocus
Crocus is a genus in the iris family comprising about 80 species of perennials growing from corms. Many are cultivated for their flowers appearing in autumn, winter, or spring...
, where every number from x = 3 to x = 15 is represented by at least one species. Evidence of various kinds shows that that trends of evolution have gone in different directions in different groups. Closer to home, the great apes
Great Apes
Great Apes may refer to*Great apes, species in the biological family Hominidae, including humans, chimpanzees, gorillas, and orangutans*Great Apes , a 1997 novel by Will Self...
have 24x2 chromosomes whereas humans have 23x2. Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number.
Chromosomal polymorphism
Some animal species are polymorphicPolymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
for chromosome fusions or dissociations. When this happens, the chromosome number is variable from one individual to another. Well-researched examples are the ladybird beetle Chilocorus stigma, some mantids of the genus Ameles
Ameles
Ameles is a wide-ranging genus of praying mantises represented in Africa, Asia, and Europe by species such as:*A. aegyptiaca*A. africana*A. arabica*A. assoi*A. crassinervis*A. cyprica*A. decolor*A. dumonti...
, the European shrew Sorex araneus. There is some evidence from the case of the mollusc Thais lapillus (the dog whelk
Dog Whelk
The dog whelk, dogwhelk, or Atlantic dogwinkle, scientific name Nucella lapillus, is a species of predatory sea snail, a carnivorous marine gastropod mollusc in the family Muricidae, the rock snails....
) on the Brittany
Brittany
Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...
coast, that the two chromosome morphs are adapted
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
to different habitats.
Species trees
The detailed study of chromosome banding in insects with polytene chromosomePolytene chromosome
To increase cell volume, some specialized cells undergo repeated rounds of DNA replication without cell division , forming a giant polytene chromosome...
s can reveal relationships between closely related species: the classic example is the study of chromosome banding in Hawaiian drosophilids
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
by Hampton Carson
Hampton Carson
Hampton Lawrence Carson was an eminent American biologist best known for his work on the chromosomes of new species of the fruit fly Drosophila and his contributions to our understanding of their evolution....
.
In about 6500 sq mi (16,834.9 km²), the Hawaiian Islands
Hawaiian Islands
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
have the most diverse collection of drosophilid flies in the world, living from rainforests to subalpine meadows
Hawaiian tropical high shrublands
The Hawaiian tropical high shrublands are a tropical savanna ecoregion in the Hawaiian Islands. They cover an area of on the upper slopes of the volcanoes Mauna Kea, Mauna Loa, Hualālai, and Haleakalā. It includes open shrublands, grasslands, and deserts...
. These roughly 800 Hawaiian drosophilid species are usually assigned to two genera, Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
and Scaptomyza
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, in the family Drosophilidae
Drosophilidae
Drosophilidae is a diverse, cosmopolitan family of flies, which includes fruit flies. Another family of flies called Tephritidae also includes fruit flies. The best known species of Drosophilidae is Drosophila melanogaster, within the genus Drosophila, and this species Is used extensively for...
.
The polytene banding of the 'picture wing' group, the best-studied group of Hawaiian drosophilids, enabled Carson to work out the evolutionary tree long before genome analysis was practicable. In a sense, gene arrangements are visible in the banding patterns of each chromosome. Chromosome rearrangements, especially inversions, make it possible to see which species are closely related.
The results are clear. The inversions, when plotted in tree form (and independent of all other information), show a clear "flow" of species from older to newer islands. There are also cases of colonization back to older islands, and skipping of islands, but these are much less frequent. Using K-Ar
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
dating, the present islands date from 0.4 million years ago (mya) (Mauna Kea
Mauna Kea
Mauna Kea is a volcano on the island of Hawaii. Standing above sea level, its peak is the highest point in the state of Hawaii. However, much of the mountain is under water; when measured from its oceanic base, Mauna Kea is over tall—significantly taller than Mount Everest...
) to 10mya (Necker). The oldest member of the Hawaiian archipelago still above the sea is Kure Atoll
Kure Atoll
Kure Atoll or Ocean Island is an atoll in the Pacific Ocean beyond Midway Atoll in the Northwestern Hawaiian Islands at . The only land of significant size is called Green Island and is habitat for hundreds of thousands of seabirds...
, which can be dated to 30 mya. The archipelago itself (produced by the Pacific plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
moving over a hot spot) has existed for far longer, at least into the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
. Previous islands now beneath the sea (guyot
Guyot
A guyot , also known as a tablemount, is an isolated underwater volcanic mountain , with a flat top over 200 meters below the surface of the sea. The diameters of these flat summits can exceed ....
s) form the Emperor Seamount Chain
Hawaiian-Emperor seamount chain
The Hawaiian–Emperor seamount chain is composed of the Hawaiian ridge, consisting of the islands of the Hawaiian chain northwest to Kure Atoll, and the Emperor Seamounts, a vast underwater mountain region of islands and intervening seamounts, atolls, shallows, banks and reefs along a line trending...
.
All of the native Drosophila and Scaptomyza species in Hawaii have apparently descended from a single ancestral species that colonized the islands, probably 20 million years ago. The subsequent adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
was spurred by a lack of competition
Competition (biology)
Competition is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both is required. Competition both within and between species is an important topic in ecology, especially community ecology...
and a wide variety of niches. Although it would be possible for a single gravid female to colonise an island, it is more likely to have been a group from the same species.
There are other animals and plants on the Hawaiian archipelago which have undergone similar, if less spectacular, adaptive radiations.
Types of banding
CytogeneticsCytogenetics
Cytogenetics is a branch of genetics that is concerned with the study of the structure and function of the cell, especially the chromosomes. It includes routine analysis of G-Banded chromosomes, other cytogenetic banding techniques, as well as molecular cytogenetics such as fluorescent in situ...
employs several techniques to visualize different aspects of chromosomes:
- G-banding is obtained with Giemsa stain following digestion of chromosomes with trypsinTrypsinTrypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine, except when...
. It yields a series of lightly and darkly stained bands - the dark regions tend to be heterochromatic, late-replicating and AT rich. The light regions tend to be euchromatic, early-replicating and GC rich. This method will normally produce 300-400 bands in a normal, human genome. - R-banding is the reverse of G-banding (the R stands for "reverse"). The dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions).
- C-banding: Giemsa binds to constitutive heterochromatinConstitutive heterochromatinConstitutive heterochromatin domains are sections of DNA that occur throughout the chromosomes of eukaryotes, but particularly at the centromeres and telomeres. They often consist of very highly condensed, repetitive DNA and are largely transcriptionally silent...
, so it stains centromereCentromereA centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
s. - Q-banding is a fluorescent pattern obtained using quinacrineQuinacrineQuinacrine is a drug with a number of different medical applications. It is related to mefloquine.-Uses:Its main effects are as an antiprotozoal, antirheumatic and an intrapleural sclerosing agent....
for staining. The pattern of bands is very similar to that seen in G-banding. - T-banding: visualize telomereTelomereA telomere is a region of repetitive DNA sequences at the end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. Its name is derived from the Greek nouns telos "end" and merοs "part"...
s. - Silver staining: Silver nitrateSilver nitrateSilver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...
stains the nucleolar organization region-associated protein. This yields a dark region where the silver is deposited, denoting the activity of rRNA genes within the NOR.
Classic karyotype cytogenetics
Dye
A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....
, often Giemsa (G-banding), less frequently Quinacrine
Quinacrine
Quinacrine is a drug with a number of different medical applications. It is related to mefloquine.-Uses:Its main effects are as an antiprotozoal, antirheumatic and an intrapleural sclerosing agent....
, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
groups of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. Quinacrine binds to the adenine
Adenine
Adenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and protein synthesis, as a chemical component of DNA...
-thymine
Thymine
Thymine is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. As the name suggests, thymine may be derived by methylation of uracil at...
-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern.
Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat
Cri du chat
Cri du chat syndrome, also known as chromosome 5p deletion syndrome, 5p minus syndrome or Lejeune’s syndrome, is a rare genetic disorder due to a missing part of chromosome 5. Its name is a French term referring to the characteristic cat-like cry of affected children. It was first described by...
syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2).
Spectral karyotype (SKY technique)
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s in an organism in different colors. Fluorescently labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophore
Fluorophore
A fluorophore, in analogy to a chromophore, is a component of a molecule which causes a molecule to be fluorescent. It is a functional group in a molecule which will absorb energy of a specific wavelength and re-emit energy at a different wavelength...
s. Because there are a limited number of spectrally-distinct fluorophores, a combinatorial labeling method is used to generate many different colors. Spectral differences generated by combinatorial labeling are captured and analyzed by using an interferometer attached to a fluorescence microscope. Image processing software then assigns a pseudo color to each spectrally different combination, allowing the visualization of the individually colored chromosomes.
This technique is used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough.
Digital karyotyping
Digital karyotyping is a technique used to quantify the DNA copy number on a genomic scale. Short sequences of DNA from specific loci all over the genome are isolated and enumerated. This method is also known as virtual karyotypingVirtual Karyotype
Virtual karyotype detects genomic copy number variations at a higher resolution level than conventional karyotyping or chromosome-based comparative genomic hybridization .-Background:...
.
Chromosome abnormalities
Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in derivative chromosomeDerivative chromosome
A derivative chromosome is a structurally rearranged chromosome generated either by a rearrangement involving two or more chromosomes or by multiple aberrations within a single chromosome...
, translocations
Chromosomal translocation
In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on...
, inversions
Chromosomal inversion
An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.Paracentric inversions do not include the...
, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy
Aneuploidy
Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...
, often occur as a result of nondisjunction
Nondisjunction
Nondisjunction is the failure of chromosome pairs to separate properly during meiosis stage 1 or stage 2. This could arise from a failure of homologous chromosomes to separate in meiosis I, or the failure of sister chromatids to separate during meiosis II or mitosis. The result of this error is a...
during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
in the formation of a gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
; trisomies
Trisomy
A trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...
, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination
Homologous recombination
Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of DNA, known as double-strand breaks...
. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
and give rise to a genetic mosaic individual who has some normal and some abnormal cells.
Chromosomal abnormalities that lead to disease in humans include
- Turner syndromeTurner syndromeTurner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...
results from a single X chromosome (45, X or 45, X0). - Klinefelter syndrome, the most common male chromosomal disease, otherwise known as 47, XXY is caused by an extra X chromosome.
- Edwards syndromeEdwards syndromeTrisomy 18 is a genetic disorder caused by the presence of all or part of an extra 18th chromosome. It is named after John H. Edwards, who first described the syndrome in 1960...
is caused by trisomyTrisomyA trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...
(three copies) of chromosome 18. - Down syndromeDown syndromeDown syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...
, a common chromosomal disease, is caused by trisomy of chromosome 21. - Patau syndromePatau syndromePatau syndrome, also known as trisomy 13 and trisomy D, is a chromosomal abnormality, a syndrome in which a patient has an additional chromosome 13 due to a nondisjunction of chromosomes during meiosis. Some are caused by Robertsonian translocations...
is caused by trisomy of chromosome 13. - Also documented are trisomy 8, trisomy 9 and trisomy 16, although they generally do not survive to birth.
Some disorders arise from loss of just a piece of one chromosome, including
- Cri du chatCri du chatCri du chat syndrome, also known as chromosome 5p deletion syndrome, 5p minus syndrome or Lejeune’s syndrome, is a rare genetic disorder due to a missing part of chromosome 5. Its name is a French term referring to the characteristic cat-like cry of affected children. It was first described by...
(cry of the cat), from a truncated short arm on chromosome 5. The name comes from the babies' distinctive cry, caused by abnormal formation of the larynx. - 1p36 Deletion syndrome1p36 Deletion Syndrome1p36 deletion syndrome is a congenital genetic disorder characterized by moderate to severe intellectual disability, delayed growth, hypotonia, seizures, limited speech ability, malformations, hearing and vision impairment, and distinct facial features...
, from the loss of part of the short arm of chromosome 1. - Angelman syndromeAngelman syndromeAngelman syndrome is a neuro-genetic disorder characterized by intellectual and developmental delay, sleep disturbance, seizures, jerky movements , frequent laughter or smiling, and usually a happy demeanor....
– 50% of cases have a segment of the long arm of chromosome 15 missing; a deletion of the maternal genes, example of imprintingImprintingImprinting may refer to:* Genomic imprinting , a mechanism of regulating gene expression* Imprinting , in psychology and ethology* Molecular imprinting, in polymer chemistry...
disorder. - Prader-Willi syndromePrader-Willi syndromePrader–Willi syndrome is a rare genetic disorder in which seven genes on chromosome 15 are deleted or unexpressed on the paternal chromosome...
– 50% of cases have a segment of the long arm of chromosome 15 missing; a deletion of the paternal genes, example of imprintingImprintingImprinting may refer to:* Genomic imprinting , a mechanism of regulating gene expression* Imprinting , in psychology and ethology* Molecular imprinting, in polymer chemistry...
disorder.
Chromosomal abnormalities can also occur in cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
ous cells of an otherwise genetically normal individual; one well-documented example is the Philadelphia chromosome
Philadelphia chromosome
Philadelphia chromosome or Philadelphia translocation is a specific chromosomal abnormality that is associated with chronic myelogenous leukemia . It is the result of a reciprocal translocation between chromosome 9 and 22, and is specifically designated t...
, a translocation mutation commonly associated with chronic myelogenous leukemia
Chronic myelogenous leukemia
Chronic myelogenous leukemia , also known as chronic granulocytic leukemia , is a cancer of the white blood cells. It is a form of leukemia characterized by the increased and unregulated growth of predominantly myeloid cells in the bone marrow and the accumulation of these cells in the blood...
and less often with acute lymphoblastic leukemia
Acute lymphoblastic leukemia
Acute lymphoblastic leukemia is a form of leukemia, or cancer of the white blood cells characterized by excess lymphoblasts.Malignant, immature white blood cells continuously multiply and are overproduced in the bone marrow. ALL causes damage and death by crowding out normal cells in the bone...
.
External links
- Making a karyotype, an online activity from the University of Utah's Genetic Science Learning Center.
- Karyotyping activity with case histories from the University of Arizona's Biology Project.
- Printable karyotype project from Biology Corner, a resource site for biology and science teachers.
- Chromosome Staining and Banding Techniques