These include meiosis in micronucleus cells, amitosis in micronucleus cells, and mitosis in micronucleus cells. Micronucleus cell meiosis involves stretching the genome outside the cell while macronucleus cell amitosis involves a random distribution of the genome.
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C. uncinata goes through asexual reproduction for cell division and duplication called amitosis. As C. uncinata has two nuclei, it goes through two different styles of division of the nuclei. The germ-line nucleus goes through mitosis during asexual division while the somatic nucleus undergoes amitosis. Amitosis is a stochastic process where unlike in mitosis, there is no spindle formation to segregate chromosomes during nuclear division.
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A review of the resulting literature not only affirms the involvement of amitosis in cell proliferation, it also explores the existence of more than one amitotic mechanism capable of producing "progeny nuclei" without the involvement of "mitotic chromosomes." One form of amitosis involves fissioning, a nucleus splitting in two without the involvement of chromosomes, and has been reported in placental tissue as well as in cells grown from that tissue in rats, in human trophoblasts, and in mouse trophoblasts. Amitosis by fissioning has also been reported in mammalian liver cells and human adrenal cells. Chen and Wan not only reported amitosis in rat liver, but also presented a mechanism for a four-stage amitotic process whereby chromatin threads are reproduced and equally distributed to daughter cells as the nucleus splits in two.
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Amitosis (a- + mitosis), also called 'karyostenosis' or direct cell division or binary fission. It is cell proliferation that does not occur by mitosis, the mechanism usually identified as essential for cell division in eukaryotes. The polyploid macronucleus found in ciliates divides amitotically. While normal mitosis results in a precise division of parental alleles, amitosis results in a random distribution of parental alleles.
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The phenomenon of amitosis, even though it is an accepted as occurring in ciliates, continues to meet with skepticism about its role in mammalian cell proliferation, perhaps because it lacks the reassuring iconography of mitosis. Of course the relatively recent discovery of copy number variations (CNV's) in mammalian cells within an organ significantly challenges the age-old assumption that every cell in an organism must inherit an exact copy of the parental genome to be functional. Rather than CNV's resulting from mitosis gone awry, some of this variation may arise from amitosis, and may be both desirable and necessary. Furthermore, it is well to remember that ciliates possess a mechanism for adjusting copy numbers of individual genes during amitosis of the macronucleus.
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Multiple fission at the cellular level occurs in many protists, e.g. sporozoans and algae. The nucleus of the parent cell divides several times by amitosis, producing several nuclei. The cytoplasm then separates, creating multiple daughter cells.
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Despite the passage of more than a century since its description by Walther Flemming (more celebrated for describing mitosis) and others (Child, 1907) the process has not received much attention. Using "mitosis in mammalian cells" as a search term in the Medline data-base calls up more than 10,000 studies dealing with mitosis, whereas "amitosis in mammalian cells" retrieves the titles of fewer than 50 papers. Not surprisingly, this absence of data has led many scientists to conclude that amitosis does not exist, or is minimally important—if any means of proliferation can be deemed "minimally important" while the war on cancer is not yet won. Accordingly, and despite being very much out of fashion, a resurgence of interest in the role of amitosis in mammalian proliferation has been building over the past two to three decades.
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There are also multiple reports of amitosis occurring when nuclei bud out through the plasma membrane of a polyploid cell. Such a process has been shown to occur in amniotic cells transformed by a virus as well as in mouse embryo fibroblast lines exposed to carcinogens. A similar process called extrusion has been described for mink trophoblasts, a tissue in which fissioning is also observed. Asymmetric cell division has also been described in polyploid giant cancer cells and low eukaryotic cells and reported to occur by the amitotic processes of splitting, budding, or burst- like mechanisms. Similarly, two different kinds of amitosis have been described in monolayers of Ishikawa endometrial cells (Fleming, 2014) An example of amitosis particularly suited to the formation of multiple differentiated nuclei in a reasonably short period of time has been shown to occur during the differentiation of fluid-enclosing hemispheres called domes from adherent Ishikawa endometrial monolayer cells during an approximately 20-hour period.
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Division of the macronucleus occurs by amitosis, and the segregation of the chromosomes occurs by a process whose mechanism is unknown. This process is not perfect, and after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation.
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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. Platelet-producing megakaryocytes go through endomitosis during cell differentiation. Amitosis in ciliates and in animal placental tissues results in a random distribution of parental alleles.
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When a female Anopheles mosquito bites an infected person, gametocytes and other stages of the parasite are transferred to the mosquito stomach. Gametocytes ultimately develop into gametes, a process known as gametogony. Microgametocytes become very active, and their nuclei undergo fission (i.e. amitosis) to each give 6-8 daughter nuclei, which becomes arranged at the periphery.
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Cell division in eukaryote is much more complicated than procaryote. Depending upon chromosomal number reduced or not; Eukaryotic cell divisions can be classified as Mitosis (equational division) and Meiosis (reductional division). A premitive form of cell division is also found which is called amitosis. The amitotic or mitotic cell division is more atypical and diverse in the various groups of organisms such as protists (namely diatoms, dinoflagellates etc) and fungi.
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Ciliates reproduce asexually, by various kinds of fission. During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis (except among the Karyorelictean ciliates, whose macronuclei do not divide). The cell then divides in two, and each new cell obtains a copy of the micronucleus and the macronucleus.left Typically, the cell is divided transversally, with the anterior half of the ciliate (the proter) forming one new organism, and the posterior half (the opisthe) forming another.
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Additional examples of non-mitotic proliferation, and important insights into underlying mechanisms, have resulted from extensive work with polyploid cells. Such cells, long acknowledged to exist, were once believed simply to be anomalous. Accumulating research, including in the liver now suggests that cells containing multiple copies of the genome are importantly involved in a cell's ability to adapt to its environment. A couple of decades of research has shown that polyploid cells are frequently "reduced" to diploid cells by amitosis (Zybina et al.).
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However, in karyorelicteans, the macronuclei are unable to divide. Instead, they must be produced by division and differentiation of a micronucleus every time, even during asexual reproduction. Because of their non-dividing somatic macronuclei, the karyorelicteans were thought to represent an intermediate evolutionary stage between the hypothetical ancestor of ciliates that did not have nuclear dualism, and the other more "advanced" ciliates which had both nuclear dualism and macronuclei that could divide by amitosis. The name of the group therefore makes reference to their supposedly "primitive" nuclei.
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Miloš Marić (; ; 20 April 1885, in Ruma, Austria-Hungary, now Serbia – 3 May 1944, in Saratov, Soviet Union, now Russia) was a Russian scientist of Serbian origin, head of the department of histology at the Saratov State University. He entered the history of medicine with his most important research in the field of mitosis and amitosis, which laid the foundation for cloning. His older sister was Mileva Marić, the first wife of Albert Einstein who was also a scientist (physicist) in her own right.
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At the same time, Dr. Marić also held the top Chair of the Histology Department at the Zootechnical- Veterinary School of the same university. He quickly became a respected scientist in Soviet Russia, publishing scientific articles and monographs in the field of mitosis and amitosis. Some Russian scientists are convinced that Milos Marić (in Russia known as Milos Milosevic Maric) laid the foundations of the medical field that is now called cloning. Along with these studies, Marić worked and prepared for the study of the nervous system, but was interrupted by the Great Patriotic War.
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In 1883 Reinke graduated as a matriculation examination, Abitur Reinke began his medical studies in 1883 at the University of Goettingen and the University of Kiel received his medical degree on March 28, 1891 with a specialization in anatomy. His doctoral thesis, Untersuchungen über das Verhältnis der von Arnold beschriebenen Kernformen zur Mitose und Amitose (Investigations into the relationship of the nuclear forms described by Arnold to mitosis and amitosis), was an in-depth study of cell division. Upon graduation, Reinke completed a 6-month internship at the Pathology Institute of the University of Zurich. There, he studied under Edwin Klebs.
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Erenpreisa and colleagues have shown that following treatment of cultured cells with mitosis-inhibiting chemicals (similar to what is used in some chemotherapy), a small population of induced polyploid cells survives. Eventually this population can give rise to "normal" diploid cells by formation of polyploid chromatin bouquets that return to an interphase state, and separate into several secondary nuclei. Intriguing phenomena including controlled autophagic degradation of some DNA as well as production of nuclear envelope-limited sheets accompanies the process. Since neither of these depolyploidizations involves mitotic chromosomes, they conform to the broad definition of amitosis.
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All ciliates, including karyorelicteans, possess two different kinds of nucleus, which separate the functions of gene expression and sexual recombination. The macronuclei, or somatic nuclei, are the site of transcription, while the smaller micronuclei, or germline nuclei, are only active during sexual reproduction, where they first undergo meiosis to form gametic nuclei, which are exchanged when two mating cells conjugate. Two gametic nuclei fuse to form a zygotic nucleus, which divides by mitosis into two daughter nuclei, one of which develops into a new micronucleus and the other into a macronucleus; the old macronucleus typically disintegrates (see main article). In most ciliates, a macronucleus can divide during asexual reproduction to form new daughter macronuclei, through a process called amitosis.
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Or they can fission asymmetrically resulting in one of seven other nuclear morphotypes, five of which appear to be specific to development since they are rarely observed in adult organisms. The research that is accumulating about amitosis suggests that such processes are, indeed, involved in the production of the breathtaking 37 trillion or so cells in humans, perhaps particularly during the fetal and embryonic phases of development when the majority of these cells are produced, perhaps within the complexity of implantation, perhaps when large numbers of cells are being differentiated, and perhaps in cancerous cells. A word of caution: some examples of cell division formerly thought to belong to this "non-mitotic" class, such as the division of some unicellular eukaryotes, may actually occur by the process of "closed mitosis" different from open or semi-closed mitotic processes, all involving mitotic chromosomes and classified by the fate of the nuclear envelope.
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