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In , mitosis () is a part of the when replicated are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of (during which the DNA is replicated) and is often accompanied or followed by , which divides the , and into two new cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic ( M) phase of an animal cell cycle—the of the mother cell into two daughter cells genetically identical to each other.

The process of mitosis is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are , , , , and . During mitosis, the chromosomes, which have already duplicated, condense and attach to spindle fibers that pull one copy of each chromosome to opposite sides of the cell. The result is two genetically identical daughter nuclei. The rest of the cell may then continue to divide by cytokinesis to produce two daughter cells. Producing three or more daughter cells instead of the normal two is a mitotic error called tripolar mitosis or multipolar mitosis (direct cell triplication / multiplication). Other errors during mitosis can induce (programmed cell death) or cause . Certain types of can arise from such mutations.

Mitosis occurs only in cells. cells, which lack a nucleus, divide by a different process called . Mitosis varies between organisms. For example, cells undergo an "open" mitosis, where the breaks down before the chromosomes separate, whereas undergo a "closed" mitosis, where chromosomes divide within an intact cell nucleus. Most animal cells undergo a shape change, known as mitotic cell rounding, to adopt a near spherical morphology at the start of mitosis. Most human cells are produced by mitotic cell division. Important exceptions include the and cells – which are produced by .


Discovery
Numerous descriptions of were made during 18th and 19th centuries, with various degrees of accuracy. In 1835, the German botanist Hugo von Mohl, described cell division in the , stating that multiplication of cells occurs through cell division.Mohl, Hugo von (1835). Ueber die Vermehrung der Pflanzenzellen durch Theilung. Inaugural-Dissertation, Tübingen. link."Notes and memoranda: The late professor von Mohl". Quarterly Journal of Microscopical Science, v. XV, New Series, p. 178-181, 1875. link. In 1838, Matthias Jakob Schleiden affirmed that the formation of new cells in their interior was a general law for cell multiplication in plants, a view later rejected in favour of Mohl model, due to contributions of and others.Weyers, Wolfgang (2002). 150 Years of cell division. Dermatopathology: Practical & Conceptual, Vol. 8, No. 2. link .

In animal cells, cell division with mitosis was discovered in frog, rabbit, and cat cells in 1873 and described for the first time by the Polish Wacław Mayzel in 1875.

(1981). 9788322318768, Wydawnictwo Interpress.

Bütschli, Schneider and Fol might have also claimed the discovery of the process presently known as "mitosis".Ross, Anna E. "Human Anatomy & Physiology I: A Chronology of the Description of Mitosis". Christian Brothers University. Retrieved 02 May 2018. link . In 1873, the German zoologist Otto Bütschli published data from observations on . A few years later, he discovered and described mitosis based on those observations.Bütschli, O. (1873). Beiträge zur Kenntnis der freilebenden Nematoden. Nova Acta der Kaiserlich Leopoldinisch-Carolinischen Deutschen Akademie der Naturforscher 36, 1-144. link .Bütschli, O. (1876). Studien über die ersten Entwicklungsvorgänge der Eizelle, die Zelleilung und die Conjugation der Infusorien. Abh.d. Senckenb. Naturf. Ges. Frankfurt a. M. 10, 213-452. link .

The term "mitosis", coined by in 1882, is derived from the word μίτος ( mitos, "warp thread"). There are some alternative names for the process,Battaglia, Emilio (2009). Caryoneme alternative to chromosome and a new caryological nomenclature. Caryologia 62 (4): 1–80. link . e.g., "karyokinesis" (nuclear division), a term introduced by Schleicher in 1878,Schleicher, W. (1878). Die Knorpelzelltheilung. Arch. Mirkroskop. Anat. 16, 248-300. link .Toepfer, G. "Karyokinesis". BioConcepts. Retrieved 02 May 2018. link . or "equational division", proposed by in 1887.Battaglia E. (1987). Embryological questions: 12. Have the Polygonum and Allium types been rightly established? Ann Bot (Rome) 45: 81–117. p." However, the term "mitosis" is also used in a broad sense by some authors to refer to karyokinesis and together.Mauseth, J.D. (1991). Botany: an Introduction to Plant Biology. Saunders College Publishing, Philadelphia. p. link. Presently, "equational division" is more commonly used to refer to , the part of meiosis most like mitosis.


Phases

Overview
The primary result of mitosis and cytokinesis is the transfer of a parent cell's into two daughter cells. The genome is composed of a number of chromosomes—complexes of tightly coiled that contain vital for proper cell function. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the of interphase. results in two identical sister chromatids bound together by proteins at the .

When mitosis begins, the chromosomes condense and become visible. In some eukaryotes, for example animals, the , which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The , which makes ribosomes in the cell, also disappears. project from opposite ends of the cell, attach to the centromeres, and align the chromosomes centrally within the cell. The microtubules then contract to pull the sister chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes. As the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase. A new nuclear envelope forms around the separated daughter chromosomes, which decondense to form interphase nuclei.

During mitotic progression, typically after the anaphase onset, the cell may undergo cytokinesis. In , a between the two developing nuclei to produce two new cells. In , a forms between the two nuclei. Cytokinesis does not always occur; coenocytic (a type of multinucleate condition) cells undergo mitosis without cytokinesis.


Interphase
The mitotic phase is a relatively short period of the . It alternates with the much longer , where the cell prepares itself for the process of cell division. Interphase is divided into three phases: G1 (first gap), , and G2 (second gap). During all three parts of interphase, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the . Thus, a cell grows (G1), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G2), and finally divides (M) before restarting the cycle. All these phases in the cell cycle are highly regulated by , cyclin-dependent kinases, and other cell cycle proteins. The phases follow one another in strict order and there are "checkpoints" that give the cell cues to proceed from one phase to another. Cells may also temporarily or permanently leave the cell cycle and enter G0 phase to stop dividing. This can occur when cells become overcrowded (density-dependent inhibition) or when they differentiate to carry out specific functions for the organism, as is the case for human heart muscle cells and . Some G0 cells have the ability to re-enter the cell cycle.

DNA double-strand breaks can be during interphase by two principal processes.Shibata A. Regulation of repair pathway choice at two-ended DNA double-strand breaks. Mutat Res. 2017 Oct;803-805:51-55. doi: 10.1016/j.mrfmmm.2017.07.011. Epub 2017 Jul 29. Review. PMID: 28781144 The first process, non-homologous end joining (NHEJ), can join the two broken ends of DNA in the G1, and G2 phases of interphase. The second process, homologous recombinational repair (HRR), is more accurate than NHEJ in repairing double-strand breaks. HRR is active during the S and G2 phases of interphase when is either partially accomplished or after it is completed, since HRR requires two adjacent .


Mitosis

Preprophase (plant cells)
In plant cells only, prophase is preceded by a pre-prophase stage. In highly plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of a , a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules and filaments (called ) underneath the plasma membrane around the equatorial plane of the future mitotic spindle. This band marks the position where the cell will eventually divide. The cells of higher plants (such as the ) lack ; instead, microtubules form a spindle on the surface of the nucleus and are then organized into a spindle by the chromosomes themselves, after the nuclear envelope breaks down. The preprophase band disappears during nuclear envelope breakdown and spindle formation in prometaphase.
(2020). 9780716710073, W. H. Freeman and Co.. .


Prophase
During prophase, which occurs after G2 interphase, the cell prepares to divide by tightly condensing its chromosomes and initiating mitotic spindle formation. During interphase, the genetic material in the nucleus consists of loosely packed . At the onset of prophase, chromatin fibers condense into discrete chromosomes that are typically visible at high magnification through a . In this stage, chromosomes are long, thin and thread-like. Each chromosome has two chromatids. The two chromatids are joined at the centromere.

Gene transcription ceases during prophase and does not resume until late anaphase to early G1 phase. The also disappears during early prophase.

(2020). 9781461405146, Springer Science & Business Media.
Close to the nucleus of animal cells are structures called , consisting of a pair of surrounded by a loose collection of proteins. The centrosome is the coordinating center for the cell's . A cell inherits a single centrosome at cell division, which is before a new round of mitosis begins, giving a pair of centrosomes. The two centrosomes polymerize to help form a microtubule spindle apparatus. then push the centrosomes along these microtubules to opposite sides of the cell. Although centrosomes help organize microtubule assembly, they are not essential for the formation of the spindle apparatus, since they are absent from plants, and are not absolutely required for animal cell mitosis.


Prometaphase
At the beginning of prometaphase in animal cells, phosphorylation of causes the to disintegrate into small membrane vesicles. As this happens, microtubules invade the nuclear space. This is called open mitosis, and it occurs in some multicellular organisms. Fungi and some , such as or , undergo a variation called closed mitosis where the spindle forms inside the nucleus, or the microtubules penetrate the intact nuclear envelope.

In late prometaphase, kinetochore microtubules begin to search for and attach to chromosomal . A kinetochore is a microtubule-binding structure that forms on the chromosomal centromere during late prophase. A number of polar microtubules find and interact with corresponding polar microtubules from the opposite centrosome to form the mitotic spindle. Although the kinetochore structure and function are not fully understood, it is known that it contains some form of molecular motor. When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to "crawl" up the tube toward the originating centrosome. This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome's two chromatids.


Metaphase
After the microtubules have located and attached to the kinetochores in prometaphase, the two centrosomes begin pulling the chromosomes towards opposite ends of the cell. The resulting tension causes the chromosomes to align along the metaphase plate or equatorial plane, an imaginary line that is centrally located between the two centrosomes (at approximately the midline of the cell). To ensure equitable distribution of chromosomes at the end of mitosis, the metaphase checkpoint guarantees that kinetochores are properly attached to the mitotic spindle and that the chromosomes are aligned along the metaphase plate. If the cell successfully passes through the metaphase checkpoint, it proceeds to anaphase.


Anaphase
During anaphase A, the that bind sister chromatids together are cleaved, forming two identical daughter chromosomes. Shortening of the kinetochore microtubules pulls the newly formed daughter chromosomes to opposite ends of the cell. During anaphase B, polar microtubules push against each other, causing the cell to elongate. In late anaphase, also reach their overall maximal condensation level, to help chromosome segregation and the re-formation of the nucleus. In most animal cells, anaphase A precedes anaphase B, but some vertebrate egg cells demonstrate the opposite order of events.


Telophase
Telophase (from the word τελος meaning "end") is a reversal of prophase and prometaphase events. At telophase, the polar microtubules continue to lengthen, elongating the cell even more. If the nuclear envelope has broken down, a new nuclear envelope forms using the membrane vesicles of the parent cell's old nuclear envelope. The new envelope forms around each set of separated daughter chromosomes (though the membrane does not enclose the centrosomes) and the nucleolus reappears. Both sets of chromosomes, now surrounded by new nuclear membrane, begin to "relax" or decondense. Mitosis is complete. Each daughter nucleus has an identical set of chromosomes. Cell division may or may not occur at this time depending on the organism.


Cytokinesis
Cytokinesis is not a phase of mitosis but rather a separate process, necessary for completing cell division. In animal cells, a (pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei. In both animal and plant cells, cell division is also driven by vesicles derived from the , which move along microtubules to the middle of the cell. In plants, this structure coalesces into a cell plate at the center of the and develops into a cell wall, separating the two nuclei. The phragmoplast is a microtubule structure typical for higher plants, whereas some green algae use a microtubule array during cytokinesis. Each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase.

There are many cells where mitosis and cytokinesis occur separately, forming single cells with multiple nuclei. The most notable occurrence of this is among the , , and coenocytic algae, but the phenomenon is found in various other organisms. Even in animals, cytokinesis and mitosis may occur independently, for instance during certain stages of fruit fly embryonic development.


Function
Mitosis "function" or significance relies on the maintenance of the chromosomal set; each cell formed receives chromosomes that are alike in composition and equal in number to the chromosomes of the parent cell.

Mitosis occurs in the following circumstances:

  • Development and growth: The number of cells within an organism increases by mitosis. This is the basis of the development of a multicellular body from a single cell, i.e., and also the basis of the growth of a body.
  • Cell replacement: In some parts of the body, e.g. skin and digestive tract, cells are constantly sloughed off and replaced by new ones. New cells are formed by mitosis and so are exact copies of the cells being replaced. In like manner, red blood cells have a short lifespan (only about 4 months) and new RBCs are formed by mitosis.
  • Regeneration: Some organisms can regenerate body parts. The production of new cells in such instances is achieved by mitosis. For example, regenerate lost arms through mitosis.
  • Asexual reproduction: Some organisms produce genetically similar offspring through asexual reproduction. For example, the hydra reproduces asexually by budding. The cells at the surface of hydra undergo mitosis and form a mass called a bud. Mitosis continues in the cells of the bud and this grows into a new individual. The same division happens during asexual reproduction or vegetative propagation in plants.


Variations

Forms of mitosis
The mitosis process in the cells of eukaryotic organisms follow a similar pattern, but with variations in three main details. "Closed" and "open" mitosis can be distinguished on the basis of remaining intact or breaking down. An intermediate form with partial degradation of the nuclear envelope is called "semiopen" mitosis. With respesct to the symmetry of the spindle apparatus during metaphase, an approximately axially symmetric (centered) shape is called as "orthomitosis", distinguished from the eccentric spindles of "pleuromitosis", in which mitotic apparatus has bilateral symmetry. Finally, a third criterion is the location of the in case of closed pleuromitosis: "extranuclear" (spindle located in the cytoplasm) or "intranuclear" (in the nucleus).

File:Mitosis classification closed intranuclear pleuromitoses.svg|closed
intranuclear
pleuromitosis File:Mitosis classification closed extranuclear pleuromitoses.svg|closed
extranuclear
pleuromitosis File:Mitosis classification closed orthomitoses.svg|closed
orthomitosis File:Mitosis classification semiopen pleuromitoses.svg|semiopen
pleuromitosis File:Mitosis classification semiopen orthomitoses.svg|semiopen
orthomitosis File:Mitosis classification open orthomitoses.svg|open
orthomitosis

Nuclear division takes place only in cells of organisms of the domain, as and have no nucleus. Within each of the eukaryotic supergroups, mitosis of the open form can be found, as well as closed mitosis, except for , which show exclusively closed mitosis. Following, the occurrence of the forms of mitosis in eukaryotes:R. Desalle, B. Schierwater: Key Transitions in Animal Evolution. CRC Press, 2010, p. 12, link .


Errors and other variations
Errors can occur during mitosis, especially during early development in humans. Mitotic errors can create cells that have too few or too many of one or more chromosomes, a condition associated with . Early human embryos, cancer cells, infected or intoxicated cells can also suffer from pathological division into three or more daughter cells (tripolar or multipolar mitosis), resulting in severe errors in their chromosomal complements.

In , sister chromatids fail to separate during anaphase.

(2020). 9780080463506, Academic Press.
One daughter cell receives both sister chromatids from the nondisjoining chromosome and the other cell receives none. As a result, the former cell gets three copies of the chromosome, a condition known as , and the latter will have only one copy, a condition known as . On occasion, when cells experience nondisjunction, they fail to complete cytokinesis and retain both nuclei in one cell, resulting in binucleated cells.

occurs when the movement of one chromatid is impeded during anaphase. This may be caused by a failure of the mitotic spindle to properly attach to the chromosome. The lagging chromatid is excluded from both nuclei and is lost. Therefore, one of the daughter cells will be monosomic for that chromosome.

Endoreduplication (or endoreplication) occurs when chromosomes duplicate but the cell does not subsequently divide. This results in cells or, if the chromosomes duplicates repeatedly, polytene chromosomes. Endoreduplication is found in many species and appears to be a normal part of development. Endomitosis is a variant of endoreduplication in which cells replicate their chromosomes during S phase and enter, but prematurely terminate, mitosis. Instead of being divided into two new daughter nuclei, the replicated chromosomes are retained within the original nucleus. The cells then re-enter G1 and S phase and replicate their chromosomes again. This may occur multiple times, increasing the chromosome number with each round of replication and endomitosis. -producing go through endomitosis during cell differentiation.

in ciliates and in animal placental tissues results in a random distribution of parental alleles.

Karyokinesis without cytokinesis originates cells called .


Diagnostic marker
In , the mitosis rate is an important parameter in various types of tissue samples, for diagnosis as well as to further specify the aggressiveness of tumors. For example, there is routinely a quantification of mitotic count in breast cancer classification. The mitoses must be counted in an area of the highest mitotic activity. Visually identifying these areas is difficult in tumors with very high mitotic activity. Also, the detection of atypical forms of mitosis can be used both as a diagnostic and prognostic marker. For example, lag-type mitosis (non-attached condensed in the area of the mitotic figure) indicates high risk human papillomavirus infection-related . File:Normal versus atypical mitosis.jpg|Normal and atypical forms of mitosis in cancer cells. A, normal mitosis; B, ; C, multipolar mitosis; D, ring mitosis; E, dispersed mitosis; F, asymmetrical mitosis; G, lag-type mitosis; and H, micronuclei. H&E stain.


Related cell processes

Cell rounding
In animal tissue, most cells round up to a near-spherical shape during mitosis. In and , an efficient rounding process is correlated with proper alignment and subsequent correct positioning of daughter cells. Moreover, researchers have found that if rounding is heavily suppressed it may result in spindle defects, primarily pole splitting and failure to efficiently capture . Therefore, mitotic cell rounding is thought to play a protective role in ensuring accurate mitosis.

Rounding forces are driven by reorganization of and (actomyosin) into a contractile homogeneous that 1) rigidifies the cell periphery and 2) facilitates generation of intracellular hydrostatic pressure (up to 10 fold higher than ). The generation of intracellular pressure is particularly critical under confinement, such as would be important in a tissue scenario, where outward forces must be produced to round up against surrounding cells and/or the extracellular matrix. Generation of pressure is dependent on -mediated nucleation and (ROCK)-mediated contraction, both of which are governed upstream by signaling pathways and ECT2 through the activity of Cdk1. Due to its importance in mitosis, the molecular components and dynamics of the mitotic is an area of active research.


Mitotic recombination
Mitotic cells irradiated with in the G1 phase of the repair recombinogenic DNA damages primarily by recombination between homologous chromosomes. Mitotic cells irradiated in the G2 phase repair such damages preferentially by sister-chromatid recombination. in encoding enzymes employed in recombination cause cells to have increased sensitivity to being killed by a variety of DNA damaging agents. These findings suggest that mitotic recombination is an adaptation for repairing DNA damages including those that are potentially lethal.


Evolution
There are prokaryotic homologs of all the key molecules of eukaryotic mitosis (e.g., actins, tubulins). Being a universal eukaryotic property, mitosis probably arose at the base of the eukaryotic tree. As mitosis is less complex than , meiosis may have arisen after mitosis. However, sexual reproduction involving meiosis is also a primitive characteristic of eukaryotes.Bernstein, H., Bernstein, C. Evolutionary origin and adaptive function of meiosis. In “Meiosis”, Intech Publ (Carol Bernstein and Harris Bernstein editors), Chapter 3: 41-75 (2013). Thus meiosis and mitosis may both have evolved, in parallel, from ancestral prokayotic processes.

While in bacterial cell division, after , two circular chromosomes are attached to a special region of the cell membrane, eukaryotic mitosis is usually characterized by the presence of many linear chromosomes, whose kinetochores attaches to the microtubules of the spindle. In relation to the forms of mitosis, closed intranuclear pleuromitosis seems to be the most primitive type, as it is more similar to bacterial division.


Gallery
Mitotic cells can be visualized microscopically by staining them with and .


See also


Further reading
  • (2020). 9780953918126, Published by New Science Press in association with Oxford University Press.
  • (2001). 9780805366242, Benjamin Cummings/Addison-Wesley. .
  • (2020). 9780130819239, Prentice Hall. .


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