Chromosome

From Wikipedia for FEVERv2
Jump to navigation Jump to search

This article is about the DNA molecule. Chromosome_sentence_0

For the genetic algorithm, see Chromosome (genetic algorithm). Chromosome_sentence_1

A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Chromosome_sentence_2

Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity. Chromosome_sentence_3

These chromosomes display a complex three-dimensional structure, which plays a significant role in transcriptional regulation. Chromosome_sentence_4

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). Chromosome_sentence_5

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. Chromosome_sentence_6

The joined copies are now called sister chromatids. Chromosome_sentence_7

During metaphase the X-shaped structure is called a metaphase chromosome, which is highly condensed and thus easiest to distinguish and study. Chromosome_sentence_8

In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation. Chromosome_sentence_9

Chromosomal recombination during meiosis and subsequent sexual reproduction play a significant role in genetic diversity. Chromosome_sentence_10

If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. Chromosome_sentence_11

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. Chromosome_sentence_12

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. Chromosome_sentence_13

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. Chromosome_sentence_14

Etymology Chromosome_section_0

The word chromosome (/ˈkroʊməˌsoʊm, -ˌzoʊm/) comes from the Greek χρῶμα (chroma, "colour") and σῶμα (soma, "body"), describing their strong staining by particular dyes. Chromosome_sentence_15

The term was coined by the German anatomist Heinrich Wilhelm Waldeyer, referring to the term chromatin, which was introduced by Walther Flemming, the discoverer of cell division. Chromosome_sentence_16

Some of the early karyological terms have become outdated. Chromosome_sentence_17

For example, Chromatin (Flemming 1880) and Chromosom (Waldeyer 1888), both ascribe color to a non-colored state. Chromosome_sentence_18

History of discovery Chromosome_section_1

The German scientists Schleiden, Virchow and Bütschli were among the first scientists who recognized the structures now familiar as chromosomes. Chromosome_sentence_19

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’. Chromosome_sentence_20

Wilhelm Roux suggested that each chromosome carries a different genetic configuration, and Boveri was able to test and confirm this hypothesis. Chromosome_sentence_21

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. Chromosome_sentence_22

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). Chromosome_sentence_23

In his famous textbook The Cell in Development and Heredity, Wilson linked together the independent work of Boveri and Sutton (both around 1902) by naming the chromosome theory of inheritance the Boveri–Sutton chromosome theory (the names are sometimes reversed). Chromosome_sentence_24

Ernst Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T.H. Chromosome_sentence_25 Morgan, all of a rather dogmatic turn of mind. Chromosome_sentence_26

Eventually, complete proof came from chromosome maps in Morgan's own lab. Chromosome_sentence_27

The number of human chromosomes was published in 1923 by Theophilus Painter. Chromosome_sentence_28

By inspection through the microscope, he counted 24 pairs, which would mean 48 chromosomes. Chromosome_sentence_29

His error was copied by others and it was not until 1956 that the true number, 46, was determined by Indonesia-born cytogeneticist Joe Hin Tjio. Chromosome_sentence_30

Prokaryotes Chromosome_section_2

Main article: Nucleoid Chromosome_sentence_31

The prokaryotes – bacteria and archaea – typically have a single circular chromosome, but many variations exist. Chromosome_sentence_32

The chromosomes of most bacteria, which some authors prefer to call genophores, can range in size from only 130,000 base pairs in the endosymbiotic bacteria Candidatus Hodgkinia cicadicola and Candidatus Tremblaya princeps, to more than 14,000,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum. Chromosome_sentence_33

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. Chromosome_sentence_34

Structure in sequences Chromosome_section_3

Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Chromosome_sentence_35

Bacteria typically have a one-point (the origin of replication) from which replication starts, whereas some archaea contain multiple replication origins. Chromosome_sentence_36

The genes in prokaryotes are often organized in operons, and do not usually contain introns, unlike eukaryotes. Chromosome_sentence_37

DNA packaging Chromosome_section_4

Prokaryotes do not possess nuclei. Chromosome_sentence_38

Instead, their DNA is organized into a structure called the nucleoid. Chromosome_sentence_39

The nucleoid is a distinct structure and occupies a defined region of the bacterial cell. Chromosome_sentence_40

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. Chromosome_sentence_41

In archaea, the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes. Chromosome_sentence_42

Certain bacteria also contain plasmids or other extrachromosomal DNA. Chromosome_sentence_43

These are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. Chromosome_sentence_44

In prokaryotes (see nucleoids) and viruses, the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by histone-like proteins. Chromosome_sentence_45

Bacterial chromosomes tend to be tethered to the plasma membrane of the bacteria. Chromosome_sentence_46

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). Chromosome_sentence_47

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled. Chromosome_sentence_48

The DNA must first be released into its relaxed state for access for transcription, regulation, and replication. Chromosome_sentence_49

Eukaryotes Chromosome_section_5

Main article: Chromatin Chromosome_sentence_50

See also: DNA condensation, Nucleosome, Histone, and Protamine Chromosome_sentence_51

See also: Eukaryotic chromosome fine structure Chromosome_sentence_52

Each eukaryotic chromosome consists of a long linear DNA molecule associated with proteins, forming a compact complex of proteins and DNA called chromatin. Chromosome_sentence_53

Chromatin contains the vast majority of the DNA of an organism, but a small amount inherited maternally, can be found in the mitochondria. Chromosome_sentence_54

It is present in most cells, with a few exceptions, for example, red blood cells. Chromosome_sentence_55

Histones are responsible for the first and most basic unit of chromosome organization, the nucleosome. Chromosome_sentence_56

Eukaryotes (cells with nuclei such as those found in plants, fungi, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Chromosome_sentence_57

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. Chromosome_sentence_58

In addition, most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes. Chromosome_sentence_59

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. Chromosome_sentence_60

Interphase chromatin Chromosome_section_6

The packaging of DNA into nucleosomes causes a 10 nanometer fibre which may further condense up to 30 nm fibres Most of the euchromatin in interphase nuclei appears to be in the form of 30-nm fibers. Chromosome_sentence_61

Chromatin structure is the more decondensed state, i.e. the 10-nm conformation allows transcription. Chromosome_sentence_62

During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished: Chromosome_sentence_63

Chromosome_unordered_list_0

  • Euchromatin, which consists of DNA that is active, e.g., being expressed as protein.Chromosome_item_0_0
  • 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:Chromosome_item_0_1
    • Constitutive heterochromatin, which is never expressed. It is located around the centromere and usually contains repetitive sequences.Chromosome_item_0_2
    • Facultative heterochromatin, which is sometimes expressed.Chromosome_item_0_3

Metaphase chromatin and division Chromosome_section_7

See also: mitosis and meiosis Chromosome_sentence_64

In the early stages of mitosis or meiosis (cell division), the chromatin double helix become more and more condensed. Chromosome_sentence_65

They cease to function as accessible genetic material (transcription stops) and become a compact transportable form. Chromosome_sentence_66

The loops of 30-nm chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells. Chromosome_sentence_67

The DNA is thus condense about 10,000 folds. Chromosome_sentence_68

Chromosome scaffold, which is made of proteins such as condensin, TOP2A and KIF4, play an important role in holding the chromatin into compact chromosome. Chromosome_sentence_69

Loops of 30 nm structure further condense with scaffold into higher order structures. Chromosome_sentence_70

This highly compact form makes the individual chromosomes visible, and they form the classic four arm structure, a pair of sister chromatids attached to each other at the centromere. Chromosome_sentence_71

The shorter arms are called p arms (from the French petit, small) and the longer arms are called q arms (q follows p in the Latin alphabet; q-g "grande"; alternatively it is sometimes said q is short for queue meaning tail in French). Chromosome_sentence_72

This is the only natural context in which individual chromosomes are visible with an optical microscope. Chromosome_sentence_73

Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops. Chromosome_sentence_74

During mitosis, microtubules grow from centrosomes located at opposite ends of the cell and also attach to the centromere at specialized structures called kinetochores, one of which is present on each sister chromatid. Chromosome_sentence_75

A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region. Chromosome_sentence_76

The microtubules then pull the chromatids apart toward the centrosomes, so that each daughter cell inherits one set of chromatids. Chromosome_sentence_77

Once the cells have divided, the chromatids are uncoiled and DNA can again be transcribed. Chromosome_sentence_78

In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus. Chromosome_sentence_79

Human chromosomes Chromosome_section_8

Chromosomes in humans can be divided into two types: autosomes (body chromosome(s)) and allosome (sex chromosome(s)). Chromosome_sentence_80

Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. Chromosome_sentence_81

The autosomes contain the rest of the genetic hereditary information. Chromosome_sentence_82

All act in the same way during cell division. Chromosome_sentence_83

Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell. Chromosome_sentence_84

In addition to these, human cells have many hundreds of copies of the mitochondrial genome. Chromosome_sentence_85

Sequencing of the human genome has provided a great deal of information about each of the chromosomes. Chromosome_sentence_86

Below is a table compiling statistics for the chromosomes, based on the Sanger Institute's human genome information in the Vertebrate Genome Annotation (VEGA) database. Chromosome_sentence_87

Number of genes is an estimate, as it is in part based on gene predictions. Chromosome_sentence_88

Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions. Chromosome_sentence_89

Chromosome_table_general_0

ChromosomeChromosome_header_cell_0_0_0 GenesChromosome_header_cell_0_0_1 Total base pairsChromosome_header_cell_0_0_2 % of basesChromosome_header_cell_0_0_3 Sequenced base pairsChromosome_header_cell_0_0_4 % sequenced base pairsChromosome_header_cell_0_0_5
1Chromosome_cell_0_1_0 2000Chromosome_cell_0_1_1 247,199,719Chromosome_cell_0_1_2 8.0Chromosome_cell_0_1_3 224,999,719Chromosome_cell_0_1_4 91.02%Chromosome_cell_0_1_5
2Chromosome_cell_0_2_0 1300Chromosome_cell_0_2_1 242,751,149Chromosome_cell_0_2_2 7.9Chromosome_cell_0_2_3 237,712,649Chromosome_cell_0_2_4 97.92%Chromosome_cell_0_2_5
3Chromosome_cell_0_3_0 1000Chromosome_cell_0_3_1 199,446,827Chromosome_cell_0_3_2 6.5Chromosome_cell_0_3_3 194,704,827Chromosome_cell_0_3_4 97.62%Chromosome_cell_0_3_5
4Chromosome_cell_0_4_0 1000Chromosome_cell_0_4_1 191,263,063Chromosome_cell_0_4_2 6.2Chromosome_cell_0_4_3 187,297,063Chromosome_cell_0_4_4 97.93%Chromosome_cell_0_4_5
5Chromosome_cell_0_5_0 900Chromosome_cell_0_5_1 180,837,866Chromosome_cell_0_5_2 5.9Chromosome_cell_0_5_3 177,702,766Chromosome_cell_0_5_4 98.27%Chromosome_cell_0_5_5
6Chromosome_cell_0_6_0 1000Chromosome_cell_0_6_1 170,896,993Chromosome_cell_0_6_2 5.5Chromosome_cell_0_6_3 167,273,993Chromosome_cell_0_6_4 97.88%Chromosome_cell_0_6_5
7Chromosome_cell_0_7_0 900Chromosome_cell_0_7_1 158,821,424Chromosome_cell_0_7_2 5.2Chromosome_cell_0_7_3 154,952,424Chromosome_cell_0_7_4 97.56%Chromosome_cell_0_7_5
8Chromosome_cell_0_8_0 700Chromosome_cell_0_8_1 146,274,826Chromosome_cell_0_8_2 4.7Chromosome_cell_0_8_3 142,612,826Chromosome_cell_0_8_4 97.50%Chromosome_cell_0_8_5
9Chromosome_cell_0_9_0 800Chromosome_cell_0_9_1 140,442,298Chromosome_cell_0_9_2 4.6Chromosome_cell_0_9_3 120,312,298Chromosome_cell_0_9_4 85.67%Chromosome_cell_0_9_5
10Chromosome_cell_0_10_0 700Chromosome_cell_0_10_1 135,374,737Chromosome_cell_0_10_2 4.4Chromosome_cell_0_10_3 131,624,737Chromosome_cell_0_10_4 97.23%Chromosome_cell_0_10_5
11Chromosome_cell_0_11_0 1300Chromosome_cell_0_11_1 134,452,384Chromosome_cell_0_11_2 4.4Chromosome_cell_0_11_3 131,130,853Chromosome_cell_0_11_4 97.53%Chromosome_cell_0_11_5
12Chromosome_cell_0_12_0 1100Chromosome_cell_0_12_1 132,289,534Chromosome_cell_0_12_2 4.3Chromosome_cell_0_12_3 130,303,534Chromosome_cell_0_12_4 98.50%Chromosome_cell_0_12_5
13Chromosome_cell_0_13_0 300Chromosome_cell_0_13_1 114,127,980Chromosome_cell_0_13_2 3.7Chromosome_cell_0_13_3 95,559,980Chromosome_cell_0_13_4 83.73%Chromosome_cell_0_13_5
14Chromosome_cell_0_14_0 800Chromosome_cell_0_14_1 106,360,585Chromosome_cell_0_14_2 3.5Chromosome_cell_0_14_3 88,290,585Chromosome_cell_0_14_4 83.01%Chromosome_cell_0_14_5
15Chromosome_cell_0_15_0 600Chromosome_cell_0_15_1 100,338,915Chromosome_cell_0_15_2 3.3Chromosome_cell_0_15_3 81,341,915Chromosome_cell_0_15_4 81.07%Chromosome_cell_0_15_5
16Chromosome_cell_0_16_0 800Chromosome_cell_0_16_1 88,822,254Chromosome_cell_0_16_2 2.9Chromosome_cell_0_16_3 78,884,754Chromosome_cell_0_16_4 88.81%Chromosome_cell_0_16_5
17Chromosome_cell_0_17_0 1200Chromosome_cell_0_17_1 78,654,742Chromosome_cell_0_17_2 2.6Chromosome_cell_0_17_3 77,800,220Chromosome_cell_0_17_4 98.91%Chromosome_cell_0_17_5
18Chromosome_cell_0_18_0 200Chromosome_cell_0_18_1 76,117,153Chromosome_cell_0_18_2 2.5Chromosome_cell_0_18_3 74,656,155Chromosome_cell_0_18_4 98.08%Chromosome_cell_0_18_5
19Chromosome_cell_0_19_0 1500Chromosome_cell_0_19_1 63,806,651Chromosome_cell_0_19_2 2.1Chromosome_cell_0_19_3 55,785,651Chromosome_cell_0_19_4 87.43%Chromosome_cell_0_19_5
20Chromosome_cell_0_20_0 500Chromosome_cell_0_20_1 62,435,965Chromosome_cell_0_20_2 2.0Chromosome_cell_0_20_3 59,505,254Chromosome_cell_0_20_4 95.31%Chromosome_cell_0_20_5
21Chromosome_cell_0_21_0 200Chromosome_cell_0_21_1 46,944,323Chromosome_cell_0_21_2 1.5Chromosome_cell_0_21_3 34,171,998Chromosome_cell_0_21_4 72.79%Chromosome_cell_0_21_5
22Chromosome_cell_0_22_0 500Chromosome_cell_0_22_1 49,528,953Chromosome_cell_0_22_2 1.6Chromosome_cell_0_22_3 34,893,953Chromosome_cell_0_22_4 70.45%Chromosome_cell_0_22_5
X (sex chromosome)Chromosome_cell_0_23_0 800Chromosome_cell_0_23_1 154,913,754Chromosome_cell_0_23_2 5.0Chromosome_cell_0_23_3 151,058,754Chromosome_cell_0_23_4 97.51%Chromosome_cell_0_23_5
Y (sex chromosome)Chromosome_cell_0_24_0 200Chromosome_cell_0_24_1 57,741,652Chromosome_cell_0_24_2 1.9Chromosome_cell_0_24_3 25,121,652Chromosome_cell_0_24_4 43.51%Chromosome_cell_0_24_5
TotalChromosome_header_cell_0_25_0 21,000Chromosome_header_cell_0_25_1 3,079,843,747Chromosome_header_cell_0_25_2 100.0Chromosome_header_cell_0_25_3 2,857,698,560Chromosome_header_cell_0_25_4 92.79%Chromosome_cell_0_25_5

Number in various organisms Chromosome_section_9

Main article: List of organisms by chromosome count Chromosome_sentence_90

In eukaryotes Chromosome_section_10

These tables give the total number of chromosomes (including sex chromosomes) in a cell nucleus. Chromosome_sentence_91

For example, most eukaryotes are diploid, like humans who have 22 different types of autosomes, each present as two homologous pairs, and two sex chromosomes. Chromosome_sentence_92

This gives 46 chromosomes in total. Chromosome_sentence_93

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_sentence_94

Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes (see the table). Chromosome_sentence_95

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. Chromosome_sentence_96

Asexually reproducing species have one set of chromosomes that are the same in all body cells. Chromosome_sentence_97

However, asexual species can be either haploid or diploid. Chromosome_sentence_98

Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes (23 pairs in humans with one set of 23 chromosomes from each parent), one set from the mother and one from the father. Chromosome_sentence_99

Gametes, reproductive cells, are haploid [n]: They have one set of chromosomes. Chromosome_sentence_100

Gametes are produced by meiosis of a diploid germ line cell. Chromosome_sentence_101

During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent. Chromosome_sentence_102

When a male and a female gamete merge (fertilization), a new diploid organism is formed. Chromosome_sentence_103

Some animal and plant species are polyploid [Xn]: They have more than two sets of homologous chromosomes. Chromosome_sentence_104

Plants important in agriculture such as tobacco or wheat are often polyploid, compared to their ancestral species. Chromosome_sentence_105

Wheat has a haploid number of seven chromosomes, still seen in some cultivars as well as the wild progenitors. Chromosome_sentence_106

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. Chromosome_sentence_107

In prokaryotes Chromosome_section_11

Prokaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. Chromosome_sentence_108

For example, Buchnera, a symbiont of aphids has multiple copies of its chromosome, ranging from 10–400 copies per cell. Chromosome_sentence_109

However, in some large bacteria, such as Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present. Chromosome_sentence_110

Plasmids and plasmid-like small chromosomes are, as in eukaryotes, highly variable in copy number. Chromosome_sentence_111

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. Chromosome_sentence_112

Karyotype Chromosome_section_12

Main article: Karyotype Chromosome_sentence_113

In general, the karyotype is the characteristic chromosome complement of a eukaryote species. Chromosome_sentence_114

The preparation and study of karyotypes is part of cytogenetics. Chromosome_sentence_115

Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are often highly variable. Chromosome_sentence_116

There may be variation between species in chromosome number and in detailed organization. Chromosome_sentence_117

In some cases, there is significant variation within species. Chromosome_sentence_118

Often there is: Chromosome_sentence_119

Chromosome_description_list_1

Also, variation in karyotype may occur during development from the fertilized egg. Chromosome_sentence_120

The technique of determining the karyotype is usually called karyotyping. Chromosome_sentence_121

Cells can be locked part-way through division (in metaphase) in vitro (in a reaction vial) with colchicine. Chromosome_sentence_122

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. Chromosome_sentence_123

Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). Chromosome_sentence_124

These are XX in females and XY in males. Chromosome_sentence_125

History and analysis techniques Chromosome_section_13

See also: Argument from authority § Use in science Chromosome_sentence_126

Investigation into the human karyotype took many years to settle the most basic question: How many chromosomes does a normal diploid human cell contain? Chromosome_sentence_127

In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism. Chromosome_sentence_128

Painter in 1922 was not certain whether the diploid number of man is 46 or 48, at first favouring 46. Chromosome_sentence_129

He revised his opinion later from 46 to 48, and he correctly insisted on humans having an XX/XY system. Chromosome_sentence_130

New techniques were needed to definitively solve the problem: Chromosome_sentence_131

Chromosome_ordered_list_2

  1. Using cells in cultureChromosome_item_2_9
  2. Arresting mitosis in metaphase by a solution of colchicineChromosome_item_2_10
  3. Pretreating cells in a hypotonic solution 0.075 M KCl, which swells them and spreads the chromosomesChromosome_item_2_11
  4. Squashing the preparation on the slide forcing the chromosomes into a single planeChromosome_item_2_12
  5. Cutting up a photomicrograph and arranging the result into an indisputable karyogram.Chromosome_item_2_13

It took until 1954 before the human diploid number was confirmed as 46. Chromosome_sentence_132

Considering the techniques of Winiwarter and Painter, their results were quite remarkable. Chromosome_sentence_133

Chimpanzees, the closest living relatives to modern humans, have 48 chromosomes as do the other great apes: in humans two chromosomes fused to form chromosome 2. Chromosome_sentence_134

Aberrations Chromosome_section_14

Chromosomal aberrations are disruptions in the normal chromosomal content of a cell and are a major cause of genetic conditions in humans, such as Down syndrome, although most aberrations have little to no effect. Chromosome_sentence_135

Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of bearing a child with a chromosome disorder. Chromosome_sentence_136

Abnormal numbers of chromosomes or chromosome sets, called aneuploidy, may be lethal or may give rise to genetic disorders. Chromosome_sentence_137

Genetic counseling is offered for families that may carry a chromosome rearrangement. Chromosome_sentence_138

The gain or loss of DNA from chromosomes can lead to a variety of genetic disorders. Chromosome_sentence_139

Human examples include: Chromosome_sentence_140

Chromosome_unordered_list_3

  • Cri du chat, which is caused by the deletion of part of the short arm of chromosome 5. "Cri du chat" means "cry of the cat" in French; the condition was so-named because affected babies make high-pitched cries that sound like those of a cat. Affected individuals have wide-set eyes, a small head and jaw, moderate to severe mental health problems, and are very short.Chromosome_item_3_14
  • Down syndrome, the most common trisomy, usually caused by an extra copy of chromosome 21 (trisomy 21). Characteristics include decreased muscle tone, stockier build, asymmetrical skull, slanting eyes and mild to moderate developmental disability.Chromosome_item_3_15
  • Edwards syndrome, or trisomy-18, the second most common trisomy. Symptoms include motor retardation, developmental disability and numerous congenital anomalies causing serious health problems. Ninety percent of those affected die in infancy. They have characteristic clenched hands and overlapping fingers.Chromosome_item_3_16
  • Isodicentric 15, also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15).Chromosome_item_3_17
  • Jacobsen syndrome, which is very rare. It is also called the terminal 11q deletion disorder. Those affected have normal intelligence or mild developmental disability, with poor expressive language skills. Most have a bleeding disorder called Paris-Trousseau syndrome.Chromosome_item_3_18
  • 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.Chromosome_item_3_19
  • Patau Syndrome, also called D-Syndrome or trisomy-13. Symptoms are somewhat similar to those of trisomy-18, without the characteristic folded hand.Chromosome_item_3_20
  • Small supernumerary marker chromosome. This means there is an extra, abnormal chromosome. Features depend on the origin of the extra genetic material. Cat-eye syndrome and isodicentric chromosome 15 syndrome (or Idic15) are both caused by a supernumerary marker chromosome, as is Pallister–Killian syndrome.Chromosome_item_3_21
  • Triple-X syndrome (XXX). XXX girls tend to be tall and thin and have a higher incidence of dyslexia.Chromosome_item_3_22
  • 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.Chromosome_item_3_23
  • 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.Chromosome_item_3_24
  • XYY syndrome. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.Chromosome_item_3_25

Sperm aneuploidy Chromosome_section_15

Exposure of males to certain lifestyle, environmental and/or occupational hazards may increase the risk of aneuploid spermatozoa. Chromosome_sentence_141

In particular, risk of aneuploidy is increased by tobacco smoking, and occupational exposure to benzene, insecticides, and perfluorinated compounds. Chromosome_sentence_142

Increased aneuploidy is often associated with increased DNA damage in spermatozoa. Chromosome_sentence_143

See also Chromosome_section_16

Chromosome_unordered_list_4


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Chromosome.