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also the dna replication the last check point is located at the site of metaphase where it checks that the chromosomes are correctly connected to the mitotic spindles 20 in s phase the chromosomes are replicated in order for the genetic content to be maintained 21 during g 2 the cell undergoes the final stages of growth before it enters the m phase where spindles are synthesized the m phase can be either mitosis or meiosis depending on the type of cell germ cells or gametes undergo meiosis while somatic cells will undergo mitosis after the cell proceeds successfully through the m phase it may then undergo cell division through cytokinesis the control of each checkpoint is controlled by cyclin and cyclin dependent kinases the progression of interphase is the result of the increased amount of cyclin as the amount of cyclin increases more and more cyclin dependent kinases attach to cyclin signaling the cell further into interphase at the peak of the cyclin attached to the cyclin dependent kinases this system pushes the cell out of interphase and into the m phase where mitosis meiosis and cytokinesis occur 22 there are three transition checkpoints the cell has to go through before entering the m phase the most important being the g 1 s transition checkpoint if the cell does not pass this checkpoint it results in the cell exiting the cell cycle 23 prophase edit prophase is the first stage of division the nuclear envelope begins to be broken down in this stage long strands of chromatin condense to form shorter more visible strands called chromosomes the nucleolus disappears and the mitotic spindle begins to assemble from the two centrosomes 24 microtubules associated with the alignment and separation of chromosomes are referred to as the spindle and spindle fibers chromosomes will also be visible under a microscope and will be connected at the centromere during this condensation and alignment period in meiosis the homologous chromosomes undergo a break in their double stranded dna at the same locations followed by a recombination of the now fragmented parental dna strands into non parental combinations known as crossing over 25 this process is evidenced to be caused in a large part by the highly conserved spo11 protein through a mechanism similar to that seen with topoisomerase in dna replication and transcription 26 prometaphase edit prometaphase is the second stage of cell division this stage begins with the complete breakdown of the nuclear envelope which exposes various structures to the cytoplasm this breakdown then allows the spindle apparatus growing from the centrosome to attach to the kinetochores on the sister chromatids stable attachment of the spindle apparatus to the kinetochores on the sister chromatids will ensure error free chromosome segregation during anaphase 27 prometaphase follows prophase and precedes metaphase metaphase edit in metaphase the centromeres of the chromosomes align themselves on the metaphase plate or equatorial plate an imaginary line that is at equal distances from the two centrosome poles and held together by complexes known as cohesins chromosomes line up in the middle of the cell by microtubule organizing centers mtocs pushing and pulling on centromeres of both chromatids thereby causing the chromosome to move to the center at this point the chromosomes are still condensing and are currently one step away from being the most coiled and condensed they will be and the spindle fibers have already connected to the kinetochores 28 during this phase all the microtubules with the exception of the kinetochores are in a state of instability promoting their progression toward anaphase 29 at this point the chromosomes are ready to split into opposite poles of the cell toward the spindle to which they are connected 30 anaphase edit anaphase is a very short stage of the cell cycle and it occurs after the chromosomes align at the mitotic plate kinetochores emit anaphase inhibition signals until their attachment to the mitotic spindle once the final chromosome is properly aligned and attached the final signal dissipates and triggers the abrupt shift to anaphase 29 this abrupt shift is caused by the activation of the anaphase promoting complex and its function of tagging degradation of proteins important toward the metaphase anaphase transition one of these proteins that is broken down is securin which through its breakdown releases the enzyme separase that cleaves the cohesin rings holding together the sister chromatids thereby leading to the chromosomes separating 31 after the chromosomes line up in the middle of the cell the spindle fibers will pull them apart the chromosomes are split apart while the sister chromatids move to opposite sides of the cell 32 as the sister chromatids are being pulled apart the cell and plasma are elongated by non kinetochore microtubules 33 additionally in this phase the activation of the anaphase promoting complex through the association with cdh 1 begins the degradation of mitotic cyclins 34 telophase edit telophase is the last stage of the cell cycle in which a cleavage furrow splits the cells cytoplasm cytokinesis and chromatin this occurs through the synthesis of a new nuclear envelope that forms around the chromatin gathered at each pole the nucleolus reforms as the chromatin reverts back to the loose state it possessed during interphase 35 36 the division of the cellular contents is not always equal and can vary by cell type as seen with oocyte formation where one of the four daughter cells possess the majority of the duckling 37 cytokinesis edit the last stage of the cell division process is cytokinesis in this stage there is a cytoplasmic division that occurs at the end of either mitosis or meiosis at this stage there is a resulting irreversible separation leading to two daughter cells cell division plays an important role in determining the fate of the cell this is due to there being the possibility of an asymmetric division this as a result leads to cytokinesis producing unequal daughter cells containing completely different amounts or concentrations of fate determining molecules 38 in animals the cytokinesis ends with formation of a contractile ring and thereafter a cleavage but in plants it happen differently at first a cell plate is formed and then a cell wall develops between the two daughter cells 39 in fission yeast s pombe the cytokinesis happens in g1 phase 40 variants edit image of the mitotic spindle in a human cell showing microtubules in green chromosomes dna in blue and kinetochores in red citation needed cells are broadly classified into two main categories simple non nucleated prokaryotic cells and complex nucleated eukaryotic cells due to their structural differences eukaryotic and prokaryotic cells do not divide in the same way also the pattern of cell division that transforms eukaryotic stem cells into gametes sperm cells in males or egg cells in females termed meiosis is different from that of the division of somatic cells in the body cell division over 42 hours the cells were directly imaged in the cell culture vessel using non invasive quantitative phase contrast time lapse microscopy 41 in 2022 scientists discovered a new type of cell division called asynthetic fission found in the squamous epithelial cells in the epidermis of juvenile zebrafish when juvenile zebrafish are growing skin cells must quickly cover the rapidly increasing surface area of the zebrafish these skin cells divide without duplicating their dna the s phase of mitosis causing up to 50 of the cells to have a reduced genome size these cells are later replaced by cells with a standard amount of dna scientists expect to find this type of division in other vertebrates 42 dna damage repair in the cell cycle edit dna damage is detected and repaired at various points in the cell cycle the g1 s checkpoint g2 m checkpoint and the checkpoint between metaphase and anaphase all monitor for dna damage and halt cell division by inhibiting different cyclin cdk complexes the p53 tumor suppressor protein plays a crucial role at the g1 s checkpoint and the g2 m checkpoint activated p53 proteins result in the expression of many proteins that are important in cell cycle arrest repair and apoptosis at the g1 s checkpoint p53 acts to ensure that the cell is ready for dna replication while at the g2 m checkpoint p53 acts to ensure that the cells have properly duplicated their content before entering mitosis 43 specifically when dna damage is present atm and atr kinases are activated activating various checkpoint kinases 44 these checkpoint kinases phosphorylate p53 which stimulates the production of different enzymes associated with dna repair 45 activated p53 also upregulates p21 which inhibits various cyclin cdk complexes these cyclin cdk complexes phosphorylate the retinoblastoma rb protein a tumor suppressor bound with the e2f family of transcription factors the binding of this rb protein ensures that cells do not enter the s phase prematurely however if it is not able to be phosphorylated by these cyclin cdk complexes the protein will remain and the cell will be halted in the g1 phase of the cell cycle 46 if dna is damaged the cell can also alter the akt pathway in which bad is phosphorylated and dissociated from bcl2 thus inhibiting apoptosis if this pathway is altered by a loss of function mutation in akt or bcl2 then the cell with damaged dna will be forced to undergo apoptosis 47 if the dna damage cannot be repaired activated p53 can induce cell death by apoptosis it can do so by activating the p53 upregulated modulator of apoptosis puma puma is a pro apoptotic protein that rapidly induces apoptosis by inhibiting the anti apoptotic bcl 2 family members 48 degradation edit multicellular organisms replace worn out cells through cell division in some animals however cell division eventually halts in humans this occurs on average after 52 divisions known as the hayflick limit the cell is then referred to as senescent with each division the cells telomeres protective sequences of dna on the end of a chromosome that prevent degradation of the chromosomal dna shorten this shortening has been correlated to negative effects such as age related diseases and shortened lifespans in humans 49 50 cancer cells on the other hand are not thought to degrade in this way if at all an enzyme complex called telomerase present in large quantities in cancerous cells rebuilds the telomeres through synthesis of telomeric dna repeats allowing division to continue indefinitely 51 history edit kurt michel with his phase contrast microscope at the beginning of the 19th century various hypotheses circulated about cell proliferation which became observable in plant and animal organisms as a result of advances in microscopy while the proliferation of cells on the inner side of old cells 52 53 the attachment of vesicles to existing cells 54 or crystallization in the intercellular space 55 were postulated as mechanisms of cell proliferation proponents of cell division itself had to fight for its acceptance for decades the belgian botanist barthélemy charles joseph dumortier must be regarded as the first discoverer of cell division in 1832 he described cell division in simple aquatic plants french conferve as follows translated from french to english the development of the conferve is as simple as its structure it takes place by the attachment of new cells to the old and this attachment always takes place from the end the terminal cell elongates more than the deeper cells then the production of a lateral bisector takes place in the inner fluid which tends to divide the cell into two parts of which the deeper one remains stationary while the terminal part elongates again forms a new inner partition and so on is the production of the middle partition originally double or single it is impossible to determine this but it is always true that it later appears double when united and that when two cells naturally separate each of them is closed at both ends 56 in 1835 the german botanist and physician hugo von mohl described plant cell division in much greater detail in his dissertation on freshwater and seawater algae for his phd thesis in medicine and surgery 57 among the most obscure phenomena of plant life is the manner in which the newly developing cells are formed and so there is no lack of manifold descriptions and explanations of this process and that gaps that were found in the observations were filled in by overly bold conclusions and assumptions translated from german to english in 1838 the german physician and botanist franz julius ferdinand meyen confirmed the mechanism of cell division at the root tips of plants 58 the german polish physician robert remak suspected that he had already discovered animal cell division in the blood of chicken embryos in 1841 59 but it was not until 1852 that he was able to confirm animal cell division for the first time in bird embryos frog larvae and mammals 60 in 1943 cell division was filmed for the first time 61 by kurt michel using a phase contrast microscope 62 see also edit cell fusion gametic fusion cell growth cyclin dependent kinase labile cells cells that constantly divide mitotic catastrophe references edit martin ea hine r 2020 a dictionary of biology 6th ed oxford oxford university press isbn 978 0 19 920462 5 oclc 176818780 griffiths aj 2012 introduction to genetic analysis 10th ed new york w h freeman and co isbn 978 1 4292 2943 2 oclc 698085201 10 2 the cell cycle biology 2e openstax openstax org 28 march 2018 retrieved 2020 11 24 gilbert scott f 2000 meiosis developmental biology 6th edition sinauer associates retrieved 2023 09 08 gilbert sf 2000 spermatogenesis developmental biology 6th ed sinauer associates gilbert sf 2000 oogenesis developmental biology 6th ed sinauer associates maton anthea 1997 cells building blocks of life 3rd ed upper saddle river n j prentice hall pp 70 74 isbn 978 0 13 423476 2 oclc 37049921 quammen d april 2008 contagious cancer harper s magazine issn 0017 789x retrieved 2019 04 14 golitsin yuri n krylov mikhail c c 2010 cell division theory variants and degradation new york nova science publishers p 137 isbn 978 1 61122 593 8 oclc 669515286 fletcher daniel a mullins r dyche 28 january 2010 cell mechanics and the cytoskeleton nature 463 7280 485 492 bibcode 2010natur 463 485f doi 10 1038 nature08908 issn 0028 0836 pmc 2851742 pmid 20110992 li shanwei sun tiantian ren haiyun 27 april 2015 the functions of the cytoskeleton and associated proteins during mitosis and cytokinesis in plant cells frontiers in plant science 6 282 bibcode 2015frps 6 282l doi 10 3389 fpls 2015 00282 issn 1664 462x pmc 4410512 pmid 25964792 hohmann tim dehghani faramarz 18 april 2019 the cytoskeleton a complex interacting meshwork cells 8 4 362 doi 10 3390 cells8040362 issn 2073 4409 pmc 6523135 pmid 31003495 hugonnet je mengin 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