Cell division

Source: Wikipedia, the free encyclopedia.
Not to be confused with cellular differentiation.
binary fission) and eukaryotes (mitosis and meiosis
). The thick lines are chromosomes, and the thin blue lines are fibers pulling on the chromosomes and pushing the ends of the cell apart.
The cell cycle in eukaryotes: I = Interphase, M = Mitosis, G0 = Gap 0, G1 = Gap 1, G2 = Gap 2, S = Synthesis, G3 = Gap 3.

Cell division is the process by which a parent

M phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells.[3] To ensure proper progression through the cell cycle, DNA damage is detected and repaired at various checkpoints throughout the cycle. These checkpoints can halt progression through the cell cycle by inhibiting certain cyclin-CDK complexes. Meiosis undergoes two divisions resulting in four haploid daughter cells. Homologous chromosomes are separated in the first division of meiosis, such that each daughter cell has one copy of each chromosome. These chromosomes have already been replicated and have two sister chromatids which are then separated during the second division of meiosis. [4]
Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.

binary fission, where their genetic material is segregated equally into two daughter cells, but there are alternative manners of division, such as budding
, that have been observed. All cell divisions, regardless of organism, are preceded by a single round of DNA replication.

For simple unicellular microorganisms such as the amoeba, one cell division is equivalent to reproduction – an entire new organism is created. On a larger scale, mitotic cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. Mitotic cell division enables sexually reproducing organisms to develop from the one-celled zygote, which itself is produced by fusion of two gametes, each having been produced by meiotic cell division.[5][6] After growth from the zygote to the adult, cell division by mitosis allows for continual construction and repair of the organism.[7] The human body experiences about 10 quadrillion cell divisions in a lifetime.[8]

The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information that is stored in chromosomes must be replicated, and the duplicated genome must be cleanly divided between progeny cells.[9] A great deal of cellular infrastructure is involved in ensuring consistency of genomic information among generations.[10][11][12]

In bacteria

Divisome and elongasome complexes responsible for peptidoglycan synthesis during lateral cell-wall growth and division.[13]

Bacterial cell division happens through binary fission or sometimes through budding. The divisome is a protein complex in bacteria that is responsible for cell division, constriction of inner and outer membranes during division, and remodeling of the peptidoglycan cell wall at the division site. A tubulin-like protein, FtsZ plays a critical role in formation of a contractile ring for the cell division.[14]

In eukaryotes

See also: Alternation of generations

Cell division in eukaryotes is more complicated than in prokaryotes. If the chromosomal number is reduced, eukaryotic cell division is classified as meiosis (reductional division). If the chromosomal number is not reduced, eukaryotic cell division is classified as mitosis (equational division). A primitive form of cell division, called amitosis, also exists. The amitotic or mitotic cell divisions are more atypical and diverse among the various groups of organisms, such as protists (namely diatoms, dinoflagellates, etc.) and fungi.

Forms of mitosis (of karyokinesis step) in eukaryotes
  • closed intranuclear pleuromitosis
    closed
    intranuclear
    pleuromitosis
  • closed extranuclear pleuromitosis
    closed
    extranuclear
    pleuromitosis
  • closed orthomitosis
    closed
    orthomitosis
  • semiopen pleuromitosis
    semiopen
    pleuromitosis
  • semiopen orthomitosis
    semiopen
    orthomitosis
  • open orthomitosis
    open
    orthomitosis

In the mitotic metaphase (see below), typically the chromosomes (each containing 2 sister chromatids that developed during replication in the S phase of interphase) align themselves on the metaphase plate. Then, the sister chromatids split and are distributed between two daughter cells.

In meiosis I, the homologous chromosomes are paired before being separated and distributed between two daughter cells. On the other hand, meiosis II is similar to mitosis. The

gametic meiosis, during which meiosis produces four gametes. Whereas, in several other groups of organisms, especially in plants (observable during meiosis in lower plants, but during the vestigial stage in higher plants), meiosis gives rise to spores that germinate
into the haploid vegetative phase (gametophyte). This kind of meiosis is called sporic meiosis.

Phases of eukaryotic cell division

The phases (ordered counter-clockwise) of cell division (mitosis) and the cell cycle in animal cells.

Interphase

DNA damage . The second check point is in the G2 phase, this checkpoint also checks for cell size but 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.[17] In S phase, the chromosomes are replicated in order for the genetic content to be maintained.[18] During G2, 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.[19] There are three transition checkpoints the cell has to go through before entering the M phase. The most important being the G1-S transition checkpoint. If the cell does not pass this checkpoint, it results in the cell exiting the cell cycle.[20]

Prophase

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.[22] 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.[23]

Prometaphase

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.[24] Prometaphase follows prophase and precedes metaphase.

Metaphase

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.[25] During this phase all the microtubules, with the exception of the kinetochores, are in a state of instability promoting their progression toward anaphase.[26] At this point, the chromosomes are ready to split into opposite poles of the cell toward the spindle to which they are connected.[27]

Anaphase

Cdh-1 begins the degradation of mitotic cyclins.[31]

Telophase

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.[32][33] 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.[34]

Cytokinesis

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.[35]

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.[citation needed]

In Fission yeast (S. pombe) the cytokinesis happens in G1 phase. [36]

Variants

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

egg cells in females), termed meiosis, is different from that of the division of somatic
cells in the body.

quantitative phase contrast time-lapse microscopy.[37]

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.[38]

DNA Damage Repair in the Cell Cycle

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. [39]

Specifically, when DNA damage is present,

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.[42]

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. [43] 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.[44]

Degradation

Multicellular organisms replace worn-out cells through cell division. In some animals, however, cell division eventually halts. In

shorten. This shortening has been correlated to negative effects such as age-related diseases and shortened lifespans in humans.[45][46] 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.[47]

History

Kurt Michel with his phase-contrast microscope

A cell division under

green alga Cladophora glomerata.[48]

In 1943, cell division was filmed for the first time[49] by Kurt Michel using a phase-contrast microscope.[50]

See also

References

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Further reading

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