Chromosome numbers in other organisms
| Species |
Large
Chromosomes |
Intermediate
Chromosomes |
Small
Chromosomes |
| Trypanosoma brucei |
11 |
6 |
~100 |
The 24 human chromosome territories during prometaphase in fibroblast cells.
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, which is the same in
all body cells.
Sexually reproducing species have somatic
cells (body cells), which are diploid [2n] having two sets of chromosomes, one from the
mother and one from the father. Gametes, reproductive cells, are haploid [n]: they have one set of chromosomes. Gametes are produced by meiosis
of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can
exchange small parts of themselves (crossover), and thus create new chromosomes
that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed.
Some animal and plant species are polyploid [Xn]: they have more than two sets of
homologous chromosomes. Agriculturally important plants 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 wheats are polyploid,
having 28 (tetraploid) and 42 (hexaploid) chromosomes compared to the 14 (diploid) chromosomes in the wild wheat.[10]
Historical note: In 1921, Theophilus Painter claimed, based on his observations,
that human sex cells had 24 chromosomes each, giving humans 48 chromosomes total. It wasn't until 1955 that the number of
chromosomes was clearly shown to be 23.
Prokaryotes
Prokaryote species generally have one copy of each major
chromosome, but most cells can easily survive with multiple copies. Plasmids and plasmid-like small chromosomes are, like in
eukaryotes, very 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, and vice versa.
Karyotype
-
Figure 3: Karyotype of a human male
Karyotyping is a technique used to determine the (diploid) number of nuclear chromosomes of a eukaryotic organism, and may be
used for determining sex and spotting chromosomal abnormalities. 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 karyotype (an ordered set of chromosomes,
Fig. 3), also called karyogram.
Like many sexually reproducing species, humans have special gonosomes
(sex chromosomes, in contrast to autosomes). These are XX in females and XY in males, and can
be seen in the karyotype, Fig. 3.
Chromosomal aberrations
-
The three major single chromosome mutations; deletion (1), duplication (2) and inversion (3).
The two major two-chromosome mutations; insertion (1) and translocation (2).
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, such as Down syndrome. Some chromosome abnormalities do not cause
disease in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of having a child with a
chromosome disorder. Abnormal numbers of chromosomes or chromosome sets, aneuploidy, may be
lethal or give rise to genetic disorders. Genetic counseling is offered for families
that may carry a chromosome rearrangement.
The gain or loss of chromosome material can lead to a variety of genetic disorders.
Human examples include:
- Cri du chat, which is caused by the deletion of part of the short arm of chromosome 5. "Cri du chat" means "cry of the cat" in French,
and the condition was so-named because affected babies make high-pitched cries that sound like a cat. Affected individuals have
wide-set eyes, a small head and jaw and are moderately to severely mentally retarded and very short.
- Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short
arm of chromosome 4. It is characterized by severe growth retardation and severe to profound mental retardation.
- Down's syndrome, usually is caused by an extra copy of chromosome 21 (trisomy 21). Characteristics include decreased muscle tone, asymmetrical skull, slanting eyes and mild to
moderate mental retardation.
- Edwards syndrome, which is the second most common trisomy after Down syndrome. It
is a trisomy of chromosome 18. Symptoms include mental and motor retardation and numerous congenital anomalies causing serious
health problems. Ninety percent die in infancy; however, those who live past their first birthday usually are quite healthy
thereafter. They have a characteristic hand appearance with clenched hands and overlapping fingers.
- Patau Syndrome, also called D-Syndrome or trisomy-13. Symptoms are somewhat similar
to those of trisomy-18, but they do not have the characteristic hand shape.
- Idic15, abbreviation for Isodicentric 15 on chromosome 15; also called the following
names due to various researches, but they all mean the same; IDIC(15), Inverted dupliction 15, extra Marker, Inv dup 15, partial
tetrasomy 15
- Jacobsen syndrome, also called the terminal 11q deletion disorder.[1] This is a very rare disorder. Those affected have normal
intelligence or mild mental retardation, with poor expressive language skills. Most have a bleeding disorder called Paris-Trousseau syndrome.
- Klinefelter's syndrome (XXY). Men with Klinefelter syndrome are usually
sterile, and tend to have longer arms and legs and to be taller than their peers. Boys with the syndrome are often shy and quiet,
and have a higher incidence of speech delay and dyslexia. During puberty, without testosterone
treatment, some of them may develop gynecomastia.
- Turner syndrome (X instead of XX or XY). In Turner syndrome, female sexual
characteristics are present but underdeveloped. People with Turner syndrome often have a short stature, low hairline, abnormal
eye features and bone development and a "caved-in" appearance to the chest.
- XYY syndrome. XYY boys are usually taller than their siblings. Like XXY boys and XXX
girls, they are somewhat more likely to have learning difficulties.
- Triple-X syndrome (XXX). XXX girls tend to be tall and thin. They have a higher
incidence of dyslexia.
- 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.
Chromosomal mutations produce changes in whole chromosomes (more than one gene) or in the number of chromosomes present.
- Deletion- loss of part of a chromosome
- Duplication- extra copies of a part of a chromosome
- Inversion- reverse the direction of a part of a chromosome
- Translocation- part of a chromosome breaks off and attaches to another chromosome
Most mutations are neutral- have little or no effect
A detailed graphical display of all human chromosomes and the diseases annotated at the correct spot may be found at [2].
Human chromosomes
Human cells have 23 pairs of large linear nuclear chromosomes, giving a total of 46 per cell. In addition to these, human
cells have many hundreds of copies of the mitochondrial genome. Sequencing of the human genome has provided a great deal of information about each of the chromosomes.
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.[11] Number of genes is an estimate as it is in part based on gene predictions. Total chromosome length is an estimate as well, based on the estimated size of
unsequenced heterochromatin regions.
| Chromosome |
Genes |
Total bases |
Sequenced bases[12] |
| 1 |
3,148 |
247,200,000 |
224,999,719 |
| 2 |
902 |
242,750,000 |
237,712,649 |
| 3 |
1,436 |
199,450,000 |
194,704,827 |
| 4 |
453 |
191,260,000 |
187,297,063 |
| 5 |
609 |
180,840,000 |
177,702,766 |
| 6 |
1,585 |
170,900,000 |
167,273,992 |
| 7 |
1,824 |
158,820,000 |
154,952,424 |
| 8 |
781 |
146,270,000 |
142,612,826 |
| 9 |
1,229 |
140,440,000 |
120,312,298 |
| 10 |
1,312 |
135,370,000 |
131,624,737 |
| 11 |
405 |
134,450,000 |
131,130,853 |
| 12 |
1,330 |
132,290,000 |
130,303,534 |
| 13 |
623 |
114,130,000 |
95,559,980 |
| 14 |
886 |
106,360,000 |
88,290,585 |
| 15 |
676 |
100,340,000 |
81,341,915 |
| 16 |
898 |
88,820,000 |
78,884,754 |
| 17 |
1,367 |
78,650,000 |
77,800,220 |
| 18 |
365 |
76,120,000 |
74,656,155 |
| 19 |
1,553 |
63,810,000 |
55,785,651 |
| 20 |
816 |
62,440,000 |
59,505,254 |
| 21 |
446 |
46,940,000 |
34,171,998 |
| 22 |
595 |
49,530,000 |
34,893,953 |
| X (sex chromosome) |
1,093 |
154,910,000 |
151,058,754 |
| Y (sex chromosome) |
125 |
57,740,000 |
22,429,293 |
See also
External links
References
- ^ a b c Dubcovsky J, Luo MC, Zhong GY, et al (1996). "Genetic map
of diploid wheat, Triticum monococcum L., and its comparison with maps of Hordeum vulgare L". Genetics 143 (2):
983–99. PMID 8725244.
- ^ Bogin, Barry, Edward Alcamo, Curtis Chubb, William J. Ehmann, Mark R. Feil,
David R. Hershey, Mitchell Leslie, Karel F. Liem, William Thwaites, and Salvatore Tocci. Austin: Holt, Rinehart, and Winston,
1999. 146.
- ^ Bogin, Barry, Edward Alcamo, Curtis Chubb, William J. Ehmann, Mark R. Feil,
David R. Hershey, Mitchell Leslie, Karel F. Liem, William Thwaites, and Salvatore Tocci. Austin: Holt, Rinehart, and Winston,
1999. 146.
- ^ a b
- ^ Houck ML, Kumamoto AT, Gallagher DS,
Benirschke K (2001). "Comparative cytogenetics of the African elephant (Loxodonta africana) and Asiatic elephant (Elephas
maximus)". Cytogenet. Cell Genet. 93 (3-4): 249–52. PMID 11528120.
- ^ Wayne RK, Ostrander EA (1999). "Origin,
genetic diversity, and genome structure of the domestic dog". Bioessays 21 (3): 247–57. PMID
10333734.
- ^ Burt DW (2002). "Origin and evolution of
avian microchromosomes". Cytogenet. Genome Res. 96 (1-4): 97–112. PMID 12438785.
- ^ Ciudad J, Cid E, Velasco A, Lara JM, Aijón
J, Orfao A (2002). "Flow cytometry measurement of the DNA contents of G0/G1 diploid cells from three different teleost fish
species". Cytometry 48 (1): 20–5. PMID 12116377.
- ^ Yasukochi Y, Ashakumary LA, Baba K, Yoshido
A, Sahara K (2006). "A second-generation integrated map of the silkworm reveals synteny and conserved gene order between
lepidopteran insects". Genetics 173 (3): 1319–28. PMID 16547103.
- ^ Sakamura, T. (1918), Kurze Mitteilung uber die Chromosomenzahlen und die
Verwandtschaftsverhaltnisse der Triticum-Arten. Bot. Mag., 32: 151-154.
- ^ http://vega.sanger.ac.uk/Homo_sapiens/index.html All data in this table was derived from this
database, July 7 2007.
- ^ 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. See http://www.ncbi.nlm.nih.gov/genome/seq/ for more information on the Human Genome Project.
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