Chromosome

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A chromosome and its
base pairs
encode genes, which provide functions. A human DNA can have up to 500 million base pairs with thousands of genes.
Diagram of a replicated and condensed metaphase eukaryotic chromosome:
  1. Chromatid
  2. Centromere
  3. Short arm
  4. Long arm

A chromosome is a

eukaryotic cells the most important of these proteins are the histones. These proteins, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity.[1][2] These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation.[3]

Chromosomes are normally visible under a

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

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

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

Etymology

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

Some of the early

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

History of discovery

Walter Sutton (left) and Theodor Boveri (right) independently developed the chromosome theory of inheritance in 1902.

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

T.H. Morgan, all of a rather dogmatic turn of mind. Eventually, complete proof came from chromosome maps in Morgan's own lab.[18]

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

Candidatus Tremblaya princeps,[22] to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum.[23] Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.[24]

Structure in sequences

Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the

operons, and do not usually contain introns
, unlike eukaryotes.

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

nucleoids) and viruses,[31] the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by histone-like
proteins.

Bacterial chromosomes tend to be tethered to the

plasma membrane
of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of the membranes (and the attached DNA).

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally

transcription, regulation, and replication
.

Eukaryotes

Organization of DNA in a eukaryotic cell

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.

Eukaryotes (cells with nuclei such as those found in plants, fungi, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic
chromosomes.

The major structures in DNA compaction: DNA, the nucleosome, the 10 nm "beads-on-a-string" fibre, the 30 nm fibre and the metaphase chromosome.

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]

Heterochromatin vs. euchromatin

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

Human chromosomes during metaphase
Stages of early mitosis in a vertebrate cell with micrographs of chromatids

In the early stages of

transcription stops) and become a compact transportable form. The loops of thirty-nanometer chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells. The DNA is thus condensed about ten-thousand-fold.[32]

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

q arms (q follows p in the Latin alphabet; q-g "grande"; alternatively it is sometimes said q is short for queue meaning tail in French[35]). This is the only natural context in which individual chromosomes are visible with an optical microscope
.

Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops.[36]

During mitosis,

kinetochores, one of which is present on each sister chromatid
. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region. The microtubules then pull the chromatids apart toward the centrosomes, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and DNA can again be transcribed. In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus.

Human chromosomes

Chromosomes in humans can be divided into two types:

Vertebrate Genome Annotation (VEGA) database.[37] Number of genes is an estimate, as it is in part based on gene predictions. Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin
regions.

Chromosome Genes[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

chromosomal satellite, the human chromosomes are classified into the following groups:[41][42]

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

Karyogram of a human male
diploid karyotype. It shows dark and white regions on G banding. Each row is vertically aligned at centromere level. It shows 22 homologous chromosomes, both the female (XX) and male (XY) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (at bottom left).

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

eukaryotes
, the same cannot be said for their karyotypes, which are often highly variable. There may be variation between species in chromosome number and in detailed organization. In some cases, there is significant variation within species. Often there is:

1. variation between the two sexes
2. variation between the 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

oogonia, concluding an XX/XO sex determination mechanism.[44] Painter in 1922 was not certain whether the diploid number of man is 46 or 48, at first favouring 46.[45] He revised his opinion later from 46 to 48, and he correctly insisted on humans having an XX/XY system.[46]

New techniques were needed to definitively solve the problem:

  1. Using cells in culture
  2. Arresting mitosis in metaphase by a solution of colchicine
  3. Pretreating cells in a
    hypotonic solution
    0.075 M KCl, which swells them and spreads the chromosomes
  4. Squashing the preparation on the slide forcing the chromosomes into a single plane
  5. 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.

chromosome 2
.

Aberrations

In Down syndrome, there are three copies of chromosome 21.

Chromosomal aberrations are disruptions in the normal chromosomal content of a cell and are a major cause of genetic conditions in humans,

chromosomal inversions, although they may lead to a higher chance of bearing a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets, called aneuploidy, may be lethal or may give rise to genetic disorders.[51] Genetic counseling
is offered for families that may carry a chromosome rearrangement.

The gain or loss of DNA from chromosomes can lead to a variety of

Human examples include:

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

sex chromosomes. This gives 46 chromosomes in total. Other organisms have more than two copies of their chromosome types, such as bread wheat
, which is hexaploid and has six copies of seven different chromosome types – 42 chromosomes in total.

Chromosome numbers in some plants
Plant species #
Arabidopsis thaliana (diploid)[64] 10
Rye (diploid)[65] 14
Einkorn wheat (diploid)[66] 14
Maize (diploid or palaeotetraploid)[67] 20
Durum wheat (tetraploid)[66]
28
Bread wheat (hexaploid)[66]
42
Cultivated tobacco (tetraploid)[68]
48
Adder's tongue fern (polyploid)[69] approx. 1,200
Chromosome numbers (2n) in some animals
Species #
Indian muntjac
7
Common fruit fly 8
Pill millipede (Arthrosphaera fumosa)[70] 30
Earthworm (Octodrilus complanatus)[71] 36
Tibetan fox 36
Domestic cat[72]
38
Domestic pig
38
Laboratory mouse[73][74] 40
Laboratory rat[74] 42
Rabbit (Oryctolagus cuniculus)[75] 44
Syrian hamster[73]
44
Guppy (poecilia reticulata)[76] 46
Human[77] 46
Hares[78][79] 48
Gorillas[77]
48
Chimpanzees 48
Domestic sheep
54
Garden snail[80]
54
Silkworm[81] 56
Elephant[82]
56
Cow 60
Donkey 62
Guinea pig[83] 64
Horse 64
Dog[84] 78
Hedgehog 90
Goldfish[85] 100–104
Kingfisher[86] 132
Chromosome numbers in other organisms
Species Large
chromosomes
Intermediate
chromosomes
Microchromosomes
Trypanosoma brucei 11 6 ≈100
Domestic pigeon
(Columba livia domestica)[87]
18 59–63
Chicken[88] 8 2 sex chromosomes 60

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.

The 23 human chromosome territories during prometaphase in fibroblast cells

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.

fertilization
), a new diploid organism is formed.

Some animal and plant species are

polyploid [Xn]: They have more than two sets of homologous chromosomes. Plants important in agriculture such as tobacco or wheat are often polyploid, compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some cultivars as well as the wild progenitors. The more-common pasta and bread wheat types are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in the wild wheat.[89]

In prokaryotes

Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present.[92]
Plasmids and plasmid-like small chromosomes are, as in eukaryotes, highly variable in copy number. The number of plasmids in the cell is almost entirely determined by the rate of division of the plasmid – fast division causes high copy number.

See also

Notes and references

  1. PMID 28053344
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  2. .
  3. .
  4. .
  5. ^ 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.
  6. PMID 26895139
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  7. ^ "Chromosome". Merriam-Webster.com Dictionary.
  8. ^ 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.
  9. ^ Waldeyer-Hartz (1888). "Über Karyokinese und ihre Beziehungen zu den Befruchtungsvorgängen". Archiv für Mikroskopische Anatomie und Entwicklungsmechanik. 32: 27.
  10. S2CID 83748967
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  11. .
  12. ^ 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.
  13. ^ Fokin SI (2013). "Otto Bütschli (1848–1920) Where we will genuflect?" (PDF). Protistology. 8 (1): 22–35.
  14. S2CID 15479331
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  15. .
  16. ^ Wilson, E.B. (1925). The Cell in Development and Heredity, Ed. 3. Macmillan, New York. p. 923.
  17. .
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  25. .
  26. .
  27. .
  28. .
  29. .
  30. ^ .
  31. .
  32. .
  33. ^ "Chromosome Mapping: Idiograms" Nature Education – 13 August 2013
  34. PMID 24200812
    .
  35. ^ Vega.sanger.ad.uk, all data in this table was derived from this database, 11 November 2008.
  36. ^ "Ensembl genome browser 71: Homo sapiens – Chromosome summary – Chromosome 1: 1–1,000,000". apr2013.archive.ensembl.org. Retrieved 11 April 2016.
  37. ^ 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.
  38. ^ "Chromosome Map". Genes and Disease. Bethesda, Maryland: National Center for Biotechnology Information. 1998.
  39. ^ The colors of each row match those of the karyogram (see Karyotype section)
  40. S2CID 90739754
    .
  41. .
  42. ^ von Winiwarter H (1912). "Études sur la spermatogenèse humaine". Archives de Biologie. 27 (93): 147–9.
  43. ^ Painter TS (1922). "The spermatogenesis of man". Anat. Res. 23: 129.
  44. .
  45. .
  46. .
  47. p. 10: "It's amazing that he [Painter] even came close!"
  48. ^ "Structural Chromosome Aberration – an overview". ScienceDirect Topics. Retrieved 27 April 2022.
  49. S2CID 205495880
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  50. ^ "Genetic Disorders". medlineplus.gov. Retrieved 27 April 2022.
  51. .
  52. ^ "What is Trisomy 18?". Trisomy 18 Foundation. Archived from the original on 30 January 2017. Retrieved 4 February 2017.
  53. ^ "Terminal deletion". European Chromosome 11 Network. Retrieved 20 February 2023.
  54. PMID 23720770
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  61. .
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  63. .
  64. ^ .
  65. .
  66. .
  67. .
  68. .
  69. .
  70. .
  71. ^ .
  72. ^ .
  73. .
  74. ^ "The Genetics of the Popular Aquarium Pet – Guppy Fish". Archived from the original on 31 May 2023. Retrieved 6 December 2009.
  75. ^
    S2CID 1098866
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  76. .
  77. .
  78. .
  79. .
  80. .
  81. .
  82. .
  83. .
  84. .
  85. .
  86. .
  87. ^ Charlebois R.L. (ed) 1999. Organization of the prokaryote genome. ASM Press, Washington DC.
  88. PMID 10732993
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  89. .

External links