Mitosis

Mitosis (/maɪˈtoʊsɪs/) is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis is an equational division which gives rise to genetically identical cells in which the total number of chromosomes is maintained.^[1][2] Mitosis is preceded by the S phase of interphase (during which DNA replication occurs) and is followed by telophase and cytokinesis, which divide the cytoplasm, organelles, and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components.^[3] The different stages of mitosis altogether define the mitotic phase (M phase) of a cell cycle—the division of the mother cell into two daughter cells genetically identical to each other.^[4]

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

Numerous descriptions of cell division were made during 18th and 19th centuries, with various degrees of accuracy.^[14] In 1835, the German botanist Hugo von Mohl, described cell division in the green algae Cladophora glomerata, stating that multiplication of cells occurs through cell division.^[15][16][17] In 1838, Matthias Jakob Schleiden affirmed that "formation of new cells in their interior was a general rule for cell multiplication in plants", a view later rejected in favour of Mohl's model, due to contributions of Robert Remak and others.^[18]

In animal cells, cell division with mitosis was discovered in frog, rabbit, and cat

Bütschli, Schneider and Fol might have also claimed the discovery of the process presently known as "mitosis".[14] In 1873, the German zoologist Otto Bütschli published data from observations on nematodes. A few years later, he discovered and described mitosis based on those observations.^[21][22][23]

The term "mitosis", coined by

Overview

The primary result of mitosis and cytokinesis is the transfer of a parent cell's

When mitosis begins, the chromosomes condense and become visible. In some eukaryotes, for example animals, the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The nucleolus, which makes ribosomes in the cell, also disappears. Microtubules 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.^[35] 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 each set of daughter chromosomes, which decondense to form interphase nuclei.

During mitotic progression, typically after the anaphase onset, the cell may undergo cytokinesis. In

Diagram of interphase and the following five mitotic stages of the M phase that includes cytokinesis.

Interphase

The interphase is a much longer phase of the

DNA double-strand breaks can be

Interphase helps prepare the cell for mitotic division. It dictates whether the mitotic cell division will occur. It carefully stops the cell from proceeding whenever the cell's DNA is damaged or has not completed an important phase. The interphase is very important as it will determine if mitosis completes successfully. It will reduce the amount of damaged cells produced and the production of cancerous cells. A miscalculation by the key Interphase proteins could be crucial as the latter could potentially create cancerous cells.^[38]

Mitosis

Preprophase (plant cells)

In plant cells only, prophase is preceded by a

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 beginning of prometaphase in animal cells, phosphorylation of

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

During anaphase A, the cohesins that bind sister chromatids together are cleaved, forming two identical daughter chromosomes.^[53] 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.^[54] In late anaphase, chromosomes also reach their overall maximal condensation level, to help chromosome segregation and the re-formation of the nucleus.^[55] In most animal cells, anaphase A precedes anaphase B, but some vertebrate egg cells demonstrate the opposite order of events.^[53]

Telophase

Telophase (from the Greek 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

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 fungi, slime molds, 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.^[58]

Function

The function or significance of mitosis, is the maintenance of the chromosomal set; each formed cell 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.,
  multicellular
  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.
  red blood cells have a short lifespan (only about 3 months) and new RBCs are formed by mitosis.^[60]
• Regeneration: Some organisms can regenerate body parts. The production of new cells in such instances is achieved by mitosis. For example, starfish regenerate lost arms through mitosis.
• Asexual reproduction: Some organisms produce genetically similar offspring through
  vegetative propagation
  in plants.

Variations

Forms of mitosis

The mitosis process in the cells of eukaryotic organisms follows a similar pattern, but with variations in three main details. "Closed" and "open" mitosis can be distinguished on the basis of nuclear envelope remaining intact or breaking down. An intermediate form with partial degradation of the nuclear envelope is called "semiopen" mitosis. With respect to the symmetry of the spindle apparatus during metaphase, an approximately axially symmetric (centered) shape is called "orthomitosis", distinguished from the eccentric spindles of "pleuromitosis", in which mitotic apparatus has bilateral symmetry. Finally, a third criterion is the location of the central spindle in case of closed pleuromitosis: "extranuclear" (spindle located in the cytoplasm) or "intranuclear" (in the nucleus).^[10]

• 
  closed
  intranuclear
  pleuromitosis
• 
  closed
  extranuclear
  pleuromitosis
• 
  closed
  orthomitosis
• 
  semiopen
  pleuromitosis
• 
  semiopen
  orthomitosis
• 
  open
  orthomitosis

Nuclear division takes place only in cells of organisms of the

• Closed intranuclear pleuromitosis is typical of
  ascomycetes), and some Radiolaria (Spumellaria and Acantharia
  ); it seems to be the most primitive type.
• Closed extranuclear pleuromitosis occurs in
  Dinoflagellata
  .
• Closed orthomitosis is found among
  fungi
  .
• Semiopen pleuromitosis is typical of most Apicomplexa.
• Semiopen orthomitosis occurs with different variants in some amoebae (Lobosa) and some green flagellates (e.g., Raphidophyta or Volvox).
• Open orthomitosis is typical in
  land plants
  ; but it also occurs in some protists.

Errors and other variations

Errors can occur during mitosis, especially during early embryonic development in humans.^[65] During each step of mitosis, there are normally checkpoints as well that control the normal outcome of mitosis.^[66] But, occasionally to almost rarely, mistakes will happen. Mitotic errors can create aneuploid cells that have too few or too many of one or more chromosomes, a condition associated with cancer.^[67][68] 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.^[8]

In nondisjunction, sister chromatids fail to separate during anaphase.^[69] 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 trisomy, and the latter will have only one copy, a condition known as monosomy. On occasion, when cells experience nondisjunction, they fail to complete cytokinesis and retain both nuclei in one cell, resulting in binucleated cells.^[70]

Anaphase lag occurs when the movement of one chromatid is impeded during anaphase.^[69] 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.

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

Karyokinesis without cytokinesis originates

coenocytes

.

Diagnostic marker

In histopathology, the mitosis rate (mitotic count or mitotic index) 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.^[75] 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.^[76] Also, the detection of atypical forms of mitosis can be used both as a diagnostic and prognostic marker.^{[citation needed]} For example, lag-type mitosis (non-attached condensed chromatin in the area of the mitotic figure) indicates high risk human papillomavirus infection-related Cervical cancer.^{[citation needed]} In order to improve the reproducibility and accuracy of the mitotic count, automated image analysis using deep learning-based algorithms have been proposed.^[77] However, further research is needed before those algorithms can be used to routine diagnostics.

• 
  Normal and atypical forms of mitosis in cancer cells. A, normal mitosis; B, chromatin bridge; 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

microtubules

green, and membrane and retraction fibers in black. Rounding also occurs in live tissue, as described in the text.

Main article: Mitotic cell rounding

In animal tissue, most cells round up to a near-spherical shape during mitosis.

chromosomes.^[83] Therefore, mitotic cell rounding is thought to play a protective role in ensuring accurate mitosis.^[82][84]

Rounding forces are driven by reorganization of

Cdk1.^[89] Due to its importance in mitosis, the molecular components and dynamics of the mitotic actomyosin cortex

is an area of active research.

Mitotic recombination

Mitotic cells irradiated with X-rays in the G1 phase of the cell cycle repair recombinogenic DNA damages primarily by recombination between homologous chromosomes.^[90] Mitotic cells irradiated in the G2 phase repair such damages preferentially by sister-chromatid recombination.^[90] Mutations in genes encoding enzymes employed in recombination cause cells to have increased sensitivity to being killed by a variety of DNA damaging agents.^[91][92][93] 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, meiosis may have arisen after mitosis.^[94] However, sexual reproduction involving meiosis is also a primitive characteristic of eukaryotes.^[95] Thus meiosis and mitosis may both have evolved, in parallel, from ancestral prokaryotic processes.

While in bacterial cell division, after duplication of DNA, 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.^[10]

Gallery

Mitotic cells can be visualized microscopically by

dyes

.

• 
  Early prophase: Polar microtubules, shown as green strands, have established a matrix around the currently intact nucleus, with the condensing chromosomes in blue. The red nodules are the centromeres.
• 
  Early prometaphase: The nuclear membrane has just disassembled, allowing the microtubules to quickly interact with the kinetochores, which assemble on the centromeres of the condensing chromosomes.
• 
  Metaphase: The centrosomes have moved to the poles of the cell and have established the mitotic spindle. The chromosomes have congressed at the metaphase plate.
• 
  Anaphase: Kinetochore microtubules pull the two sets of chromosomes apart, and lengthening polar microtubules push the halves of the dividing cell further apart, while chromosomes are condensed maximally.
• 
  Telophase: Reversal of prophase and prometaphase events and thus completing the cell cycle.

See also

• Chromosome abnormality
• Cytoskeleton
• DREAM complex
• Mitogen
• Mitosis Promoting Factor
• Mitotic bookmarking

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Further reading

• Morgan DL (2007). The cell cycle: principles of control. London: Published by New Science Press in association with Oxford University Press.
  ISBN 978-0-9539181-2-6
  .
• Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). "Mitosis". Molecular Biology of the Cell (4th ed.). Garland Science. Retrieved 2006-01-22.
• Campbell N, Reece J (December 2001). "The Cell Cycle". Biology (6th ed.). San Francisco: Benjamin Cummings/Addison-Wesley. pp. 217–224.
  ISBN 978-0-8053-6624-2
  .
• Cooper G (2000). "The Events of M Phase". The Cell: A Molecular Approach (2nd ed.). Sinaeur Associates, Inc. Retrieved 2006-01-22.
• Freeman S (2002). "Cell Division". Biological Science. Upper Saddle River, NJ: Prentice Hall. pp. 155–174.
  ISBN 978-0-13-081923-9
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• Lodish H, Berk A, Zipursky L, Matsudaira P, Baltimore D, Darnell J (2000). "Overview of the Cell Cycle and Its Control". Molecular Cell Biology (4th ed.). W. H. Freeman. Retrieved 2006-01-22.

External links

Wikimedia Commons has media related to Mitosis.

Wikiversity has learning resources about Overview of Cell Biology/Mitosis

• A Flash animation comparing Mitosis and Meiosis
• Khan Academy, lecture
• Studying Mitosis in Cultured Mammalian Cells
• General K-12 classroom resources for Mitosis
• The Cell-Cycle Ontology
• WormWeb.org: Interactive Visualization of the C. elegans Cell Lineage – Visualize the entire cell lineage tree and all of the cell divisions of the nematode C. elegans