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Cell Division

Cell division is the process by which a parent cell divides into two or more daughter cells.^[1] Cell division usually occurs as part of a larger cell cycle. In eukaryotes, there are two distinct types of cell division; a vegetative division, whereby each daughter cell is genetically identical to the parent cell (mitosis), and a reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes (meiosis).^[2] In cell biology, mitosis (/maɪˈtoʊsɪs/) is a part of the cell cycle, in which, replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase (during which the DNA is replicated) and is often followed by telophase and cytokinesis; which divides the cytoplasm, organelles and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of Mitosis all together define the mitotic (M) phase of an animal cell cycle—the division of the mother cell into two daughter cells genetically identical to each other ^{[citation needed]}. Meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes are separated in the first division, and sister chromatids are separated in the second division. 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.

Prokaryotes (bacteria and archaea) usually undergo a vegetative cell division known as binary fission, where their genetic material is segregated equally into two daughter cells. While binary fission may be the means of division by most prokaryotes, 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 was produced by meiotic cell division from gametes.^[3]^[4] After growth, cell division by mitosis allows for continual construction and repair of the organism.^[5] The human body experiences about 10 quadrillion cell divisions in a lifetime.^[6]

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 separated cleanly between cells.^[7] A great deal of cellular infrastructure is involved in keeping genomic information consistent between generations.

Cell division in Bacteria

Divisome and elongasome complexes responsible for peptidoglycan synthesis during lateral cell-wall growth and division.^[8]

Bacterial cell division happens through binary fission or budding. The Divisome is a protein complex in bacteria that is responsible for cell division, constriction of inner and outer membranes during division, and peptidoglycan (PG) synthesis at the division site. A tubulin like protein, FtsZ plays a critical role in formation of a contractile ring for the cell division. ^[9]

Cell Division in Eukaryote

Cell division in eukaryote is much more complicated than prokaryote. Depending upon chromosomal number reduced or not; Eukaryotic cell divisions can be classified as Mitosis (equational division) and Meiosis (reductional division). A primitive 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.

Forms of mitosis (of karyokinesis step) in eukaryotes

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

In mitotic metaphase (see below), typically the chromosomes (each with 2 sister chromatid that they developed due to replication in the S phase of interphase) arranged and sister chromatids split and distributed towards daughter cells.

In meiosis, typically in Meiosis-I the homologous chromosomes are paired and then separated and distributed into daughter cells. Meiosis-II is like mitosis where the chromatids are separated. In human and other higher animals and many other organisms, the meiosis is called gametic meiosis, that is the meiosis gives rise to gametes. Whereas in many groups of organisms, especially in plants (observable in lower plants but vestigial stage in higher plants), the meiosis gives rise to the kind of spores that germinate into haploid vegetative phase (gametophyte). This kind of meiosis is called sporic meiosis.

Phases of eukaryotic cell division

Interphase

Interphase is the process through which a cell must go before mitosis, meiosis, and cytokinesis.^[10] Interphase consists of three main phases: G₁, S, and G₂. G₁ is a time of growth for the cell where specialized cellular functions occur in order to prepare the cell for DNA Replication.^[11] There are checkpoints during interphase that allow the cell to either advance or halt further development. In S phase, the chromosomes are replicated in order for the genetic content to be maintained.^[12] 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.^[13] 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.^[14]

Prophase

Prophase is the first stage of division. The nuclear envelope is broken down in this stage, long strands of chromatin condense to form shorter more visible strands called chromosomes, the nucleolus disappears, and microtubules attach to the chromosomes at the disc-shaped kinetochores present in the centromere.^[15] 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.^[16] 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.^[17]

Metaphase

In metaphase, the centromeres of the chromosomes convene 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 complex 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.^[18] During this phase, all the microtubules, except for the kinetochores, are in a state of instability promoting their progression towards anaphase.^[19] At this point, the chromosomes are ready to split into opposite poles of the cell towards the spindle to which they are connected.^[20]

Anaphase

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.^[19] This abrupt shift is caused by the activation of the anaphase-promoting complex and its function of tagging degradation of proteins important towards 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.^[21] 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.^[22] As the sister chromatids are being pulled apart, the cell and plasma are elongated by non-kinetochore microtubules.^[23]

Telophase

Telophase is the last stage of the cell cycle in which a cleavage furrow splits the cell's cytoplasm (cytokinesis) and chromatin. This occurs through the synthesis of a new nuclear envelopes that forms around the chromatin which is gathered at each pole and the reformation of the nucleolus as the chromosomes decidedness their chromatin back to the loose state it possessed during interphase.^[24]^[25] 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 possesses the majority of the cytoplasm.^[26]

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. In 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.^[27]

In animals, the cytokinesis ends with the formation of a contractile ring and thereafter a cleavage. But in plants, it happens differently. First, a cell plate is formed and then a cell wall develops between the 2 daughter cells.

In Fission yeast (S. pombe) the cytokinesis happens in G1 phase ^[28]

Variants

Image of the mitotic spindle in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red.

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. Image of the mitotic spindle in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red.

Cell division over 42. The cells were directly imaged in the cell culture vessel, using non-invasive quantitative phase contrast time-lapse microscopy.^[29]

Degradation

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.^[30]^[31] 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.^[32]

History

A cell division under microscope was first discovered by German botanist Hugo von Mohl in 1835 as he worked over the green alga Cladophora glomerata.^[33]

In 1943, cell division was filmed for the first time^[34] by Kurt Michel using a phase-contrast microscope.^[35]

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