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A chromosome is a package of DNA containing part or all of the Genome of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging ; in eukaryotic cells, the most important of these proteins are the . Aided by chaperone proteins, the histones bind to and DNA condensation the DNA molecule to maintain its integrity. These eukaryotic chromosomes display a complex three-dimensional structure that has a significant role in transcriptional regulation.
Normally, chromosomes are 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. (2026). 9780815344544, Garland Science. ISBN 9780815344544 Before this stage occurs, each chromosome is duplicated (S phase), and the two copies are joined by a centromere—resulting in either an X-shaped structure if the centromere is located equatorially, or a two-armed structure if the centromere is located distally; the joined copies are called 'sister chromatids'. During metaphase, the duplicated structure (called a 'metaphase chromosome') is highly condensed and thus easiest to distinguish and study. In animal cells, chromosomes reach their highest compaction level in anaphase during chromosome segregation.
Chromosomal recombination during meiosis and subsequent sexual reproduction plays a crucial role in genetic diversity. If these structures are manipulated incorrectly, through processes known as chromosomal instability and translocation, the cell may undergo mitotic catastrophe. This will usually cause the cell to initiate apoptosis, leading to its own Cell death, but the process is occasionally hampered by cell mutations that result in the progression of cancer.
The term 'chromosome' is sometimes used in a wider sense to refer to the individualized portions of chromatin in cells, which may or may not be visible under light microscopy. In a narrower sense, 'chromosome' can be used to refer to the individualized portions of chromatin during cell division, which are visible under light microscopy due to high condensation.
Some of the early Karyotype terms have become outdated. For example, 'chromatin' (Flemming 1880) and 'chromosom' (Waldeyer 1888) both ascribe color to a non-colored state.
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'.
Wilhelm Roux suggested that every chromosome carries a different Genetic load, 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 experimental work, Boveri identified the connection between the rules of inheritance and the behaviour of the chromosomes. Two generations of American were influenced by Boveri: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton and Theophilus Painter (Wilson, Stevens, 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 'Boveri–Sutton chromosome theory' (sometimes known as the 'Sutton–Boveri chromosome theory').Wilson, E.B. (1925). The Cell in Development and Heredity, Ed. 3. Macmillan, New York. p. 923. Ernst Mayr remarks that the theory was hotly contested by some famous geneticists, including William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T.H. Morgan, all of a rather dogmatic mindset. Eventually, absolute proof came from chromosome maps in Morgan's own laboratory.Mayr, E. (1982). The growth of biological thought. Harvard. p. 749.
The number of human chromosomes was published by Painter in 1923. By inspection through a microscope, he counted 24 pairs of chromosomes, giving 48 in total. His error was copied by others, and it was not until 1956 that the true number (46) was determined by Indonesian-born cytogeneticist Joe Hin Tjio.
Some bacteria have more than one chromosome. For instance, such as Borrelia burgdorferi (causing Lyme disease), contain a single linear chromosome. typically carry two chromosomes of very different size. Genomes of the genus Burkholderia carry one, two, or three chromosomes.
Certain bacteria also contain or other extrachromosomal DNA. These are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer. In prokaryotes 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.
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 supercoiled. The DNA must first be released into its relaxed state for access for transcription, regulation, and DNA replication.
are responsible for the first and most basic unit of chromosome organization, the nucleosome.
(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 chromosomes.
In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around (structural proteins), forming a composite material called chromatin.
During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished:
The chromosome scaffold, which is made of proteins such as condensin, TOP2A and KIF4, plays an important role in holding the chromatin into compact chromosomes. Loops of thirty-nanometer structure further condense with scaffold into higher order structures. (2026). 9780716776017, W. H. Freeman. ISBN 9780716776017
This highly compact form makes the individual chromosomes visible, and they form the classic four-arm structure, a pair of sister attached to each other at the centromere. The shorter arms are called (from the French petit, small) and the longer arms are called ( 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 Mapping: Idiograms" Nature Education – 13 August 2013). 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.
During mitosis, grow from centrosomes located at opposite ends of the cell and also attach to the centromere at specialized structures called , 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.
Accessed 10 January 2026. 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. DNA 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. Vega.sanger.ad.uk, all data in this table was derived from this database, 11 November 2008. Number of genes is an estimate, as it is in part based on . Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions.
| + |
| 8.0 |
| 7.9 |
| 6.5 |
| 6.2 |
| 5.9 |
| 5.5 |
| 5.2 |
| 4.7 |
| 4.6 |
| 4.4 |
| 4.4 |
| 4.3 |
| 3.7 |
| 3.5 |
| 3.3 |
| 2.9 |
| 2.6 |
| 2.5 |
| 2.1 |
| 2.0 |
| 1.5 |
| 1.6 |
| 5.0 |
| 1.9 |
Based on the micrographic characteristics of size, position of the centromere and sometimes the presence of a chromosomal satellite, the human chromosomes are classified into the following groups:The colors of each row match those of the karyogram (see Karyotype section)
| 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) |
Although the DNA 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:
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 ). These are XX in females and XY in males.
New techniques were needed to definitively solve the problem:
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.Hsu T.C. (1979) Human and mammalian cytogenetics: a historical perspective. Springer-Verlag, N.Y. p. 10: "It's amazing that he Painter even came close!" 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.
The gain or loss of DNA from chromosomes can lead to a variety of . Human examples include:
The tables below give the total number of chromosomes (including sex chromosomes) in a cell nucleus for various eukaryotes. Most are diploid, such as humans who have 22 different types of —each present as two homologous pairs—and two , giving 46 chromosomes in total. Some other organisms have more than two copies of their chromosome types, for example bread wheat which is hexaploid, having six copies of seven different chromosome types for a total of 42 chromosomes.
| { class="wikitable sortable" style="float:left; margin:1em 0 1em 1em" | + Chromosome numbers in some plants |
| 10 | |
| 14 | |
| 14 | |
| 20 | |
| 28 | |
| 42 | |
| 48 | |
| approx. 1,200 |
| + Chromosome numbers (2n) in some animals | |
| 6♀, 7♂ | |
| 8 | |
| 30 | |
| 36 | |
| 36 | |
| 38 | |
| 38 | |
| 40 | |
| 42 | |
| 44 | |
| 44 | |
| 46 | |
| 46 | |
| 48 | |
| 48 | |
| Chimpanzee | 48 |
| 54 | |
| 54 | |
| 56 | |
| 56 | |
| 60 | |
| 62 | |
| 64 | |
| 64 | |
| 78 | |
| 90 | |
| 100–104 | |
| 132 |
| + Chromosome numbers in other organisms |
| ≈100 |
| 59–63 |
| 60 |
Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes. Other eukaryotic chromosomes, i.e., mitochondrial and plasmid-like small chromosomes, are much more variable in number, and there may be thousands of copies per cell.
Asexually reproducing species have one set of chromosomes that are the same in all body cells. However, asexual species can be either haploid or diploid.
Sexually reproducing species have (body cells) that are diploid 2n, having two sets of chromosomes (23 pairs in humans), one set from the mother and one from the father. (reproductive cells) are haploid n, having one set of chromosomes. Gametes are produced by meiosis of a diploid germline cell, during which 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 during fertilization, a new diploid organism is formed.
Some animal and plant species are polyploid Xn, having more than two sets of homologous chromosomes. Important crops 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 as well as the wild progenitors. The more common types of pasta and bread wheat are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in wild wheat.
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