Chromosome
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A chromosome is a
Chromosomes are normally visible under a
Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Usually, this will make the cell initiate apoptosis leading to its own death, but sometimes mutations in the cell hamper this process and thus cause progression of cancer.
Some use the term chromosome in a wider sense, to refer to the individualized portions of chromatin in cells, either visible or not under light microscopy. Others use the concept in a narrower sense, to refer to the individualized portions of chromatin during cell division, visible under light microscopy due to high condensation.
Etymology
The word chromosome (/ˈkroʊməˌsoʊm, -ˌzoʊm/[7][8]) comes from the Greek χρῶμα (chroma, "colour") and σῶμα (soma, "body"), describing their strong staining by particular dyes.[9] The term was coined by the German anatomist Heinrich Wilhelm Waldeyer,[10] referring to the term chromatin, which was introduced by Walther Flemming.
Some of the early
History of discovery
Otto Bütschli was the first scientist to recognize the structures now known as chromosomes.[14]
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'.[15]
Wilhelm Roux suggested that each chromosome carries a different genetic configuration, and Boveri was able to test and confirm this hypothesis. 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).[16]
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
The number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscope, he counted twenty-four pairs, which would mean forty-eight chromosomes. His error was copied by others and it was not until 1956 that the true number, forty-six, was determined by Indonesian-born cytogeneticist Joe Hin Tjio.[19]
Prokaryotes
The
Structure in sequences
Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the
DNA packaging
Prokaryotes do not possess nuclei. Instead, their DNA is organized into a structure called the nucleoid.[26][27] 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.[28] In archaea, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.[29][30]
Certain bacteria also contain
Bacterial chromosomes tend to be tethered to the
Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally
Eukaryotes
Each eukaryotic chromosome consists of a long linear DNA molecule associated with proteins, forming a compact complex of proteins and DNA called chromatin. Chromatin contains the vast majority of the DNA in an organism, but a small amount inherited maternally can be found in the mitochondria. It is present in most cells, with a few exceptions, for example, red blood cells.
Histones are responsible for the first and most basic unit of chromosome organization, the nucleosome.
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.
Interphase chromatin
The packaging of DNA into nucleosomes causes a 10 nanometer fibre which may further condense up to 30 nm fibres[32] Most of the euchromatin in interphase nuclei appears to be in the form of 30-nm fibers.[32] Chromatin structure is the more decondensed state, i.e. the 10-nm conformation allows transcription.[32]
During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished:
- 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
In the early stages of
The chromosome scaffold, which is made of proteins such as condensin, TOP2A and KIF4,[33] plays an important role in holding the chromatin into compact chromosomes. Loops of thirty-nanometer structure further condense with scaffold into higher order structures.[34]
This highly compact form makes the individual chromosomes visible, and they form the classic four-arm structure, a pair of sister
Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops.[36]
During mitosis,
Human chromosomes
Chromosomes in humans can be divided into two types:
Chromosome | Genes[38] | Total base pairs | % of bases | Sequenced base pairs[39] | % sequenced base pairs |
---|---|---|---|---|---|
1 |
2000 | 247,199,719 | 8.0 | 224,999,719 | 91.02% |
2 |
1300 | 242,751,149 | 7.9 | 237,712,649 | 97.92% |
3 |
1000 | 199,446,827 | 6.5 | 194,704,827 | 97.62% |
4 |
1000 | 191,263,063 | 6.2 | 187,297,063 | 97.93% |
5 |
900 | 180,837,866 | 5.9 | 177,702,766 | 98.27% |
6 |
1000 | 170,896,993 | 5.5 | 167,273,993 | 97.88% |
7 |
900 | 158,821,424 | 5.2 | 154,952,424 | 97.56% |
8 |
700 | 146,274,826 | 4.7 | 142,612,826 | 97.50% |
9 |
800 | 140,442,298 | 4.6 | 120,312,298 | 85.67% |
10 |
700 | 135,374,737 | 4.4 | 131,624,737 | 97.23% |
11 |
1300 | 134,452,384 | 4.4 | 131,130,853 | 97.53% |
12 |
1100 | 132,289,534 | 4.3 | 130,303,534 | 98.50% |
13 |
300 | 114,127,980 | 3.7 | 95,559,980 | 83.73% |
14 |
800 | 106,360,585 | 3.5 | 88,290,585 | 83.01% |
15 |
600 | 100,338,915 | 3.3 | 81,341,915 | 81.07% |
16 |
800 | 88,822,254 | 2.9 | 78,884,754 | 88.81% |
17 |
1200 | 78,654,742 | 2.6 | 77,800,220 | 98.91% |
18 |
200 | 76,117,153 | 2.5 | 74,656,155 | 98.08% |
19 |
1500 | 63,806,651 | 2.1 | 55,785,651 | 87.43% |
20 |
500 | 62,435,965 | 2.0 | 59,505,254 | 95.31% |
21 |
200 | 46,944,323 | 1.5 | 34,171,998 | 72.79% |
22 |
500 | 49,528,953 | 1.6 | 34,893,953 | 70.45% |
X (sex chromosome) | 800 | 154,913,754 | 5.0 | 151,058,754 | 97.51% |
Y (sex chromosome) | 200[40] | 57,741,652 | 1.9 | 25,121,652 | 43.51% |
Total | 21,000 | 3,079,843,747 | 100.0 | 2,857,698,560 | 92.79% |
Based on the micrographic characteristics of size, position of the
Group | Chromosomes | Features |
---|---|---|
A | 1–3 | Large, metacentric or submetacentric |
B | 4–5 | Large, submetacentric |
C | 6–12, X | Medium-sized, submetacentric |
D | 13–15 | Medium-sized, acrocentric, with satellite |
E | 16–18 | Small, metacentric or submetacentric |
F | 19–20 | Very small, metacentric |
G | 21–22, Y | Very small, acrocentric (and 21, 22 with satellite) |
Karyotype
In general, the karyotype is the characteristic chromosome complement of a eukaryote species.[43] The preparation and study of karyotypes is part of cytogenetics.
Although the
- 1. variation between the two sexes
- 2. variation between the germline and soma (between gametes and the rest of the body)
- 3. variation between members of a population, due to balanced genetic polymorphism
- 4. 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. Cells can be locked part-way through division (in metaphase) in vitro (in a reaction vial) with colchicine. 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.
Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males.
History and analysis techniques
Investigation into the human karyotype took many years to settle the most basic question: How many chromosomes does a normal
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 solution0.075 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.
Aberrations
Chromosomal aberrations are disruptions in the normal chromosomal content of a cell and are a major cause of genetic conditions in humans,
The gain or loss of DNA from chromosomes can lead to a variety of
- deletionof 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.
- trisomy 21). Characteristics include decreased muscle tone, stockier build, asymmetrical skull, slanting eyes and mild to moderate developmental disability.[53]
- Edwards syndrome, or trisomy-18, the second most common trisomy.[54]Symptoms include motor retardation, developmental disability and numerous congenital anomalies causing serious health problems. Ninety percent of those affected die in infancy. They have characteristic clenched hands and overlapping fingers.
- 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 11q terminal deletion disorder.[55] Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called Paris-Trousseau syndrome.
- 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.
- 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.
Sperm aneuploidy
Exposure of males to certain lifestyle, environmental and/or occupational hazards may increase the risk of aneuploid spermatozoa.[56] In particular, risk of aneuploidy is increased by tobacco smoking,[57][58] and occupational exposure to benzene,[59] insecticides,[60][61] and perfluorinated compounds.[62] Increased aneuploidy is often associated with increased DNA damage in spermatozoa.
Number in various organisms
In eukaryotes
The number of chromosomes in eukaryotes is highly variable (see table). In fact, chromosomes can fuse or break and thus evolve into novel karyotypes. Chromosomes can also be fused artificially. For example, the 16 chromosomes of yeast have been fused into one giant chromosome and the cells were still viable with only somewhat reduced growth rates.[63]
The tables below give the total number of chromosomes (including sex chromosomes) in a cell nucleus. For example, most
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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.
Some animal and plant species are
In prokaryotes
See also
- Aneuploidy
- Chromomere
- Chromosome segregation
- Cohesin
- Condensin
- DNA
- Genetic deletion
- Epigenetics
- 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
- Microchromosome
- Minichromosome
- Non-disjunction
- Secondary chromosome
- Sex-determination system
- Polytene chromosome
- Protamine
- Neochromosome
- Parasitic chromosome
Notes and references
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- ^ a b Schleyden, M. J. (1847). Microscopical researches into the accordance in the structure and growth of animals and plants. Printed for the Sydenham Society.
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- ^ "Chromosome". Merriam-Webster.com Dictionary.
- ^ Coxx, H. J. (1925). Biological Stains – A Handbook on the Nature and Uses of the Dyes Employed in the Biological Laboratory. Commission on Standardization of Biological Stains.
- ^ Waldeyer-Hartz (1888). "Über Karyokinese und ihre Beziehungen zu den Befruchtungsvorgängen". Archiv für Mikroskopische Anatomie und Entwicklungsmechanik. 32: 27.
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- ^ Battaglia, Emilio (2009). "Caryoneme alternative to chromosome and a new caryological nomenclature" (PDF). Caryologia – International Journal of Cytology, Cytosystematics. 62 (4): 1–80. Retrieved 6 November 2017.
- ^ Fokin SI (2013). "Otto Bütschli (1848–1920) Where we will genuflect?" (PDF). Protistology. 8 (1): 22–35.
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- ^ "Chromosome Mapping: Idiograms" Nature Education – 13 August 2013
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- ^ Vega.sanger.ad.uk, all data in this table was derived from this database, 11 November 2008.
- ^ "Ensembl genome browser 71: Homo sapiens – Chromosome summary – Chromosome 1: 1–1,000,000". apr2013.archive.ensembl.org. Retrieved 11 April 2016.
- ^ Sequenced percentages are based on fraction of euchromatin portion, as the Human Genome Project goals called for determination of only the euchromatic portion of the genome. Telomeres, centromeres, and other heterochromatic regions have been left undetermined, as have a small number of unclonable gaps. For more information on the Human Genome Project, see "Genome Sequencing". National Center for Biotechnology Information. Archived from the original on 1 April 2005.
- ^ "Chromosome Map". Genes and Disease. Bethesda, Maryland: National Center for Biotechnology Information. 1998.
- ^ The colors of each row match those of the karyogram (see Karyotype section)
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- ^ von Winiwarter H (1912). "Études sur la spermatogenèse humaine". Archives de Biologie. 27 (93): 147–9.
- ^ Painter TS (1922). "The spermatogenesis of man". Anat. Res. 23: 129.
- .
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- ^ "What is Trisomy 18?". Trisomy 18 Foundation. Archived from the original on 30 January 2017. Retrieved 4 February 2017.
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External links
- An Introduction to DNA and Chromosomes from HOPES: Huntington's Outreach Project for Education at Stanford
- Chromosome Abnormalities at AtlasGeneticsOncology
- On-line exhibition on chromosomes and genome (SIB)
- What Can Our Chromosomes Tell Us?, from the University of Utah's Genetic Science Learning Center
- Try making a karyotype yourself, from the University of Utah's Genetic Science Learning Center
- Kimballs Chromosome pages
- Chromosome News from Genome News Network
- Eurochromnet, European network for Rare Chromosome Disorders on the Internet
- Ensembl.org, loci graphically via the web
- Genographic Project Archived 12 July 2007 at the Wayback Machine
- Home reference on Chromosomes from the U.S. National Library of Medicine
- Visualisation of human chromosomes and comparison to other species
- Unique – The Rare Chromosome Disorder Support Group Support for people with rare chromosome disorders