A chromosome is packaged and organized plasmids or other extrachromosomal DNA.
Compaction of the duplicated chromosomes during mitosis and meiosis results either in a four-arm structure (pictured to the right) if the centromere is located in the middle of the chromosome or a two-arm structure if the centromere is located near one of the ends. Chromosomal recombination during meiosis plays a vital role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe and die, or it may unexpectedly evade apoptosis leading to the progression of cancer.
In prokaryotes (see archaea by homologs to eukaryotic histones, in the case of bacteria by histone-like proteins. Small circular genomes called plasmids are often found in bacteria and also in mitochondria and chloroplasts, reflecting their bacterial origins.
- History of discovery 1
- Structure in sequences 2.1
- DNA packaging 2.2
- Interphase chromatin 3.1.1
- Metaphase chromatin and division 3.1.2
- Human chromosomes 3.2
- Chromatin 3.1
Number of chromosomes in various organisms 4
- Eukaryotes 4.1
- Prokaryotes 4.2
- Historical note 5.1
- Aberrations 6
- See also 7
- Notes and references 8
- External links 9
History of discoveryGreek χρῶμα (chroma, colour) and σῶμα (soma, body). Chromatin and chromosomes, are both very strongly stained by particular dyes.
Schleiden, Virchow and Bütschli were among the first scientists who recognized the structures now so familiar to everyone as chromosomes. The term was coined by von Waldeyer-Hartz, referring to the term chromatin, which was introduced by Walther Flemming.
In a series of experiments beginning in the mid-1880s, Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity. His two principles were the continuity of chromosomes and the individuality of chromosomes. It is the second of these principles that was so original. Wilhelm Roux suggested that each chromosome carries a different genetic load. 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, Walter Sutton and Theophilus Painter were all influenced by Boveri (Wilson and Painter actually worked with him).
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
- 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, Ensembl project, presenting chromosomes, their genes and syntenic loci graphically via the web
- Genographic Project
- 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
- "Structures of virus and virus-like particles". 1 April 2000.
- Coxx, H. J. (1925). Biological Stains - A Handbook on the Nature and Uses of the Dyes Employed in the Biological Laboratory. https://archive.org/stream/biologicalstains00conn/biologicalstains00conn_djvu.txt: Commission on Standardization of Biological Stains.
- Microscopical researches into the accordance in the structure and growth of animals and plants. http://vlp.mpiwg-berlin.mpg.de/library/data/lit28715?.
- Fokin, S.I. (2013). "Otto Bütschli (1848–1920): Where we will genuflect?" Protistology, 8 (1), 22–35, .
- Wilson, E.B. (1925). The Cell in Development and Heredity, Ed. 3. Macmillan, New York. p. 923.
- Mayr, E. (1982). The growth of biological thought. Harvard. p. 749.
- Matthews, Robert. "The bizarre case of the chromosome that never was". Retrieved 13 July 2013.
- Thanbichler M, Shapiro L (2006). "Chromosome organization and segregation in bacteria". J. Struct. Biol. 156 (2): 292–303.
- Nakabachi A, Yamashita A, Toh H, Ishikawa H, Dunbar H, Moran N, Hattori M (2006). "The 160-kilobase genome of the bacterial endosymbiont Carsonella". Science 314 (5797): 267.
- Pradella S, Hans A, Spröer C, Reichenbach H, Gerth K, Beyer S (2002). "Characterisation, genome size and genetic manipulation of the myxobacterium Sorangium cellulosum So ce56". Arch Microbiol 178 (6): 484–92.
- Hinnebusch J, Tilly K (1993). "Linear plasmids and chromosomes in bacteria". Mol Microbiol 10 (5): 917–22.
- Kelman LM, Kelman Z (2004). "Multiple origins of replication in archaea". Trends Microbiol. 12 (9): 399–401.
- Thanbichler M, Wang SC, Shapiro L (2005). "The bacterial nucleoid: a highly organized and dynamic structure". J. Cell. Biochem. 96 (3): 506–21.
- Sandman K, Pereira SL, Reeve JN (1998). "Diversity of prokaryotic chromosomal proteins and the origin of the nucleosome". Cell. Mol. Life Sci. 54 (12): 1350–64.
- Sandman K, Reeve JN (2000). "Structure and functional relationships of archaeal and eukaryal histones and nucleosomes". Arch. Microbiol. 173 (3): 165–9.
- Pereira SL, Grayling RA, Lurz R, Reeve JN (1997). "Archaeal nucleosomes". Proc. Natl. Acad. Sci. U.S.A. 94 (23): 12633–7.
- "Chromosome Mapping: Idiograms" Nature Education - August 13, 2013
- Vega.sanger.ad.uk, all data in this table was derived from this database, November 11, 2008.
- 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.
- Armstrong SJ, Jones GH (January 2003). "Meiotic cytology and chromosome behaviour in wild-type Arabidopsis thaliana". J. Exp. Bot. 54 (380): 1–10.
- Gill BS, Kimber G (April 1974). "The Giemsa C-Banded Karyotype of Rye". Proc. Natl. Acad. Sci. U.S.A. 71 (4): 1247–9.
- Kato A, Lamb JC, Birchler JA (September 2004). "Chromosome painting using repetitive DNA sequences as probes for somatic chromosome identification in maize". Proc. Natl. Acad. Sci. U.S.A. 101 (37): 13554–9.
- 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.
- Kenton A, Parokonny AS, Gleba YY, Bennett MD (August 1993). "Characterization of the Nicotiana tabacum L. genome by molecular cytogenetics". Mol. Gen. Genet. 240 (2): 159–69.
- Leitch IJ, Soltis DE, Soltis PS, Bennett MD (2005). "Evolution of DNA amounts across land plants (embryophyta)". Ann. Bot. 95 (1): 207–17.
- Umeko Semba, Yasuko Umeda, Yoko Shibuya, Hiroaki Okabe, Sumio Tanase and Tetsuro Yamamoto (2004). "Primary structures of guinea pig high- and low-molecular-weight kininogens". International Immunopharmacology 4 (10–11): 1391–1400.
- "The Genetics of the Popular Aquarium Pet - Guppy Fish". Retrieved 2009-12-06.
- Vitturi R, Libertini A, Sineo L, et al. (2005). "Cytogenetics of the land snails Cantareus aspersus and C. mazzullii (Mollusca: Gastropoda: Pulmonata)". Micron 36 (4): 351–7.
- Vitturi R, Colomba MS, Pirrone AM, Mandrioli M (2002). "rDNA (18S-28S and 5S) colocalization and linkage between ribosomal genes and (TTAGGG)(n) telomeric sequence in the earthworm, Octodrilus complanatus (Annelida: Oligochaeta: Lumbricidae), revealed by single- and double-color FISH". J. Hered. 93 (4): 279–82.
- Ambarish, C.N. Sridhar, K.R. (2014). "Cytological and karyological observations of two endemic pill-millipedes Arthrosphaera (Pocock, 1895) (Diplopoda: Sphaerotheriida) of the Western Ghats of India". Caryologia 66 (1).
- Nie W, Wang J, O'Brien PC, et al. (2002). "The genome phylogeny of domestic cat, red panda and five mustelid species revealed by comparative chromosome painting and G-banding". Chromosome Res. 10 (3): 209–22.
- Romanenko, Svetlana A.; Perelman, Polina L.; Serdukova, Natalya A.; Trifonov, Vladimir A.; Biltueva, Larisa S.; Wang, Jinhuan; Li, Tangliang; Nie, Wenhui; O'Brien, Patricia C.M.; Volobouev, Vitaly T.; Stanyon, Roscoe; Ferguson-Smith, Malcolm A.; Yang, Fengtang; Graphodatsky, Alexander S. (2006). "Reciprocal chromosome painting between three laboratory rodent species". Mammalian Genome 17 (12): 1183–92.
- Painter, TS (1928). "A Comparison of the Chromosomes of the Rat and Mouse with Reference to the Question of Chromosome Homology in Mammals". Genetics 13 (2): 180–9.
- Hayes, H.; Rogel-Gaillard, C.; Zijlstra, C.; De Haan, N.A.; Urien, C.; Bourgeaux, N.; Bertaud, M.; Bosma, A.A. (2002). "Establishment of an R-banded rabbit karyotype nomenclature by FISH localization of 23 chromosome-specific genes on both G- and R-banded chromosomes". Cytogenetic and Genome Research 98 (2–3): 199–205.
- T.J. Robinson, F. Yang, W.R. Harrison (2002). "Chromosome painting refines the history of genome evolution in hares and rabbits (order Lagomorpha)". Cytogenic and Genetic Research 96 (1–4): 223–227.
- "Rabbits, Hares and Pikas. Status Survey and Conservation Action Plan". pp. 61–94.
- De Grouchy J (1987). "Chromosome phylogenies of man, great apes, and Old World monkeys". Genetica 73 (1–2): 37–52.
- Houck, M.L.; Kumamoto, A.T.; Gallagher, D.S.; Benirschke, K. (2001). "Comparative cytogenetics of the African elephant (Loxodonta africana) and Asiatic elephant (Elephas maximus)". Cytogenetic and Genome Research 93 (3–4): 249–52.
- Wayne RK, Ostrander EA (1999). "Origin, genetic diversity, and genome structure of the domestic dog". BioEssays 21 (3): 247–57.
- Burt DW (2002). "Origin and evolution of avian microchromosomes". Cytogenet. Genome Res. 96 (1–4): 97–112.
- 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.
- 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.
- Itoh, Masahiro; Ikeuchi, Tatsuro; Shimba, Hachiro; Mori, Michiko; Sasaki, Motomichi; Makino, Sajiro (1969). "A Comparative Karyotype Study in Fourteen Species of Birds". The Japanese journal of genetics 44 (3): 163.
- Smith J, Burt DW (1998). "Parameters of the chicken genome (Gallus gallus)". Anim. Genet. 29 (4): 290–4.
- Sakamura, Tetsu (1918). "Kurze Mitteilung über die Chromosomenzahlen und die Verwandtschaftsverhältnisse der Triticum-Arten". Shokubutsugaku Zasshi 32 (379): 150–3.
- Charlebois R.L. (ed) 1999. Organization of the prokaryote genome. ASM Press, Washington DC.
- Komaki K, Ishikawa H (March 2000). "Genomic copy number of intracellular bacterial symbionts of aphids varies in response to developmental stage and morph of their host". Insect Biochem. Mol. Biol. 30 (3): 253–8.
- Mendell JE, Clements KD, Choat JH, Angert ER (May 2008). "Extreme polyploidy in a large bacterium". Proc. Natl. Acad. Sci. U.S.A. 105 (18): 6730–4.
- White, M. J. D. (1973). The chromosomes (6th ed.). London: Chapman and Hall, distributed by Halsted Press, New York. p. 28.
- von Winiwarter H (1912). "Études sur la spermatogenese humaine". Arch. Biologie 27 (93): 147–9.
- Painter TS (1922). "The spermatogenesis of man". Anat. Res. 23: 129.
- Painter TS (1923). "Studies in mammalian spermatogenesis II. The spermatogenesis of man". J. Exp. Zoology 37 (3): 291–336.
- Tjio JH, Levan A (1956). "The chromosome number of man". Hereditas 42: 1–6.
- Ford C.E, Hamerton J.L (1956). "The Chromosomes of Man". Nature 178 (4541): 1020–1023.
- Hsu T.C. Human and mammalian cytogenetics: a historical perspective. Springer-Verlag, N.Y. p10: "It's amazing that he [Painter] even came close!"
- Miller, Kenneth R. (2000). "9-3". Biology (5th ed.). Upper Saddle River, New Jersey: Prentice Hall. pp. 194–5.
- European Chromosome 11 Network
Notes and references
- Genetic deletion
- 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
- Sex-determination system
- Polytene chromosome
- 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.
- 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.
- 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.
- 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 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.
- 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.
- 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.
- 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.
- XYY syndrome. XYY boys are usually taller than their siblings. Like XXY boys and XXX girls, they are more likely to have learning difficulties.
- 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.
The gain or loss of DNA from chromosomes can lead to a variety of genetic disorders. Human examples include:
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. 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. Abnormal numbers of chromosomes or chromosome sets, called aneuploidy, may be lethal or may give rise to genetic disorders. Genetic counseling is offered for families that may carry a chromosome rearrangement.
It took until 1954 before the human diploid number was confirmed as 46. Considering the techniques of Winiwarter and Painter, their results were quite remarkable. Chimpanzees (the closest living relatives to modern humans) have 48 chromosomes (as well as the other great apes: in humans two chromosomes fused to form chromosome 2).
- Using cells in culture
- Arresting mitosis in metaphase by a solution of colchicine
- Pretreating cells in a hypotonic solution 0.075 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.
New techniques were needed to definitively solve the problem:
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 man is 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.
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: Fig. 3.
Also, variation in karyotype may occur during development from the fertilised egg.
- 1. variation between the two sexes
- 2. variation between the germ-line and soma (between gametes and the rest of the body)
- 3. variation between members of a population, due to balanced genetic polymorphism
- 4. geographical variation between races
- 5. mosaics or otherwise abnormal individuals.
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. 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:
Prokaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. For example, Buchnera, a symbiont of aphids has multiple copies of its chromosome, ranging from 10–400 copies per cell. However, in some large bacteria, such as Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present. 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.
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 wheats are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in the wild wheat.
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. 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.
Asexually reproducing species have one set of chromosomes, which are the same in all body cells. However, asexual species can be either haploid or diploid.
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.
Number of chromosomes in various organisms
|Chromosome||Genes||Total base pairs||Sequenced base pairs||Cumulative (%)|
|X (sex chromosome)||800||154,913,754||151,058,754||99.1|
|Y (sex chromosome)||50||57,741,652||25,121,652||100.0|
|Total||20,000 to 25,000||3,079,843,747||2,857,698,560||100.0|
Chromosomes in humans can be divided into two types: autosomes and sex chromosomes. Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. The autosomes contain the rest of the genetic hereditary information. All act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex 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. 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.
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. 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 (Fig. 2).
In the early stages of mitosis or meiosis (cell division), the chromatin strands become more and more condensed. They cease to function as accessible genetic material (transcription stops) and become a compact transportable form. This 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. 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). This is the only natural context in which individual chromosomes are visible with an optical microscope.
Metaphase chromatin and division
- 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.
Chromatin is the complex of DNA and protein found in the eukaryotic nucleus, which packages chromosomes. The structure of chromatin varies significantly between different stages of the cell cycle, according to the requirements of the DNA.
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.
Eukaryotes (cells with nuclei such as those found in plants, yeast, 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.
In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of chromosomes and chromatin varies through the cell cycle. Chromosomes are even more condensed than chromatin and are an essential unit for cellular division. Chromosomes must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny. Chromosomes may exist as either duplicated or unduplicated. Unduplicated chromosomes are single linear strands, whereas duplicated chromosomes contain two identical copies (called chromatids or sister chromatids) joined by a centromere.
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 have less sequence-based structure than eukaryotes. Bacteria typically have a single point (the operons, and do not usually contain introns, unlike eukaryotes.
Structure in sequences
The prokaryotes – bacteria and archaea – typically have a single circular chromosome, but many variations exist. Most bacteria's chromosome can range in size from only 160,000 base pairs in the endosymbiotic bacterium Candidatus Carsonella ruddii, to 12,200,000 base pairs in the soil-dwelling bacterium Sorangium cellulosum. 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. Some genes, known as Orphons, aren't even in a chromosome at all.
The number of human chromosomes was published in 1923 by Theophilus Painter. By inspection through the microscope he counted 24 pairs which would mean 48 chromosomes. 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.