Cell division is the process by which a parent cell divides into two daughter cells. [1] Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there are two distinct types of cell division: a vegetative division (mitosis), producing daughter cells genetically identical to the parent cell, and a cell division that produces haploid gametes for sexual reproduction (meiosis), reducing the number of chromosomes from two of each type in the diploid parent cell to one of each type in the daughter cells. [2] Mitosis 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 replication occurs) and is 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 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.
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, 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]
Bacterial cell division happens through binary fission or 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]
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.[ citation needed ]
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.[ citation needed ]
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 chromatids are separated and distributed in the same way. In humans, other higher animals, and many other organisms, the process of meiosis is called 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.[ citation needed ]
Interphase is the process through which a cell must go before mitosis, meiosis, and cytokinesis. [15] 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. [16] There are checkpoints during interphase that allow the cell to either advance or halt further development. One of the checkpoint is between G1 and S, the purpose for this checkpoint is to check for appropriate cell size and any 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 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. [21] 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. [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 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.
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 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. [26] 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. [28] 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. [29] As the sister chromatids are being pulled apart, the cell and plasma are elongated by non-kinetochore microtubules. [30] Additionally, in this phase, the activation of the anaphase promoting complex through the association with Cdh-1 begins the degradation of mitotic cyclins. [31]
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]
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. [36]
In Fission yeast (S. pombe) the cytokinesis happens in G1 phase. [37]
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.
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. [39]
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. [40]
Specifically, when DNA damage is present, ATM and ATR kinases are activated, activating various checkpoint kinases. [41] These checkpoint kinases phosphorylate p53, which stimulates the production of different enzymes associated with DNA repair. [42] 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. [43]
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. [44] 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. [45]
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. [46] [47] 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. [48]
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, [49] [50] the attachment of vesicles to existing cells, [51] or crystallization in the intercellular space [52] were postulated as mechanisms of cell proliferation, 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." [53]
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: [54]
"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. [55] The German-Polish physician Robert Remak suspected that he had already discovered animal cell division in the blood of chicken embryos in 1841, [56] 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. [57]
In 1943, cell division was filmed for the first time [58] by Kurt Michel using a phase-contrast microscope. [59]
The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA and some of its organelles, and subsequently the partitioning of its cytoplasm, chromosomes and other components into two daughter cells in a process called cell division.
Meiosis (; from Ancient Greek μείωσις 'lessening', is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one copy of each chromosome. Additionally, prior to the division, genetic material from the paternal and maternal copies of each chromosome is crossed over, creating new combinations of code on each chromosome. Later on, during fertilisation, the haploid cells produced by meiosis from a male and a female will fuse to create a zygote, a cell with two copies of each chromosome again.
Mitosis 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. Mitosis is preceded by the S phase of interphase 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. The different stages of mitosis altogether define the mitotic phase of a cell cycle—the division of the mother cell into two daughter cells genetically identical to each other.
Prophase is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin reticulum and the disappearance of the nucleolus.
Anaphase is the stage of mitosis after the process of metaphase, when replicated chromosomes are split and the newly-copied chromosomes are moved to opposite poles of the cell. Chromosomes also reach their overall maximum condensation in late anaphase, to help chromosome segregation and the re-formation of the nucleus.
Interphase is the active portion of the cell cycle that includes the G1, S, and G2 phases, where the cell grows, replicates its DNA, and prepares for mitosis, respectively. Interphase was formerly called the "resting phase," but the cell in interphase is not simply dormant. Calling it so would be misleading since a cell in interphase is very busy synthesizing proteins, transcribing DNA into RNA, engulfing extracellular material, and processing signals, to name just a few activities. The cell is quiescent only in G0. Interphase is the phase of the cell cycle in which a typical cell spends most of its life. Interphase is the "daily living" or metabolic phase of the cell, in which the cell obtains nutrients and metabolizes them, grows, replicates its DNA in preparation for mitosis, and conducts other "normal" cell functions.
Telophase is the final stage in both meiosis and mitosis in a eukaryotic cell. During telophase, the effects of prophase and prometaphase are reversed. As chromosomes reach the cell poles, a nuclear envelope is re-assembled around each set of chromatids, the nucleoli reappear, and chromosomes begin to decondense back into the expanded chromatin that is present during interphase. The mitotic spindle is disassembled and remaining spindle microtubules are depolymerized. Telophase accounts for approximately 2% of the cell cycle's duration.
Anaphase-promoting complex is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cullin (Apc2) and RING (Apc11) subunit much like SCF. Other parts of the APC/C have unknown functions but are highly conserved.
Cyclins are proteins that control the progression of a cell through the cell cycle by activating cyclin-dependent kinases (CDK).
The spindle checkpoint, also known as the metaphase-to-anaphase transition, the spindle assembly checkpoint (SAC), the metaphase checkpoint, or the mitotic checkpoint, is a cell cycle checkpoint during metaphase of mitosis or meiosis that prevents the separation of the duplicated chromosomes (anaphase) until each chromosome is properly attached to the spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles. Only this pattern of attachment will ensure that each daughter cell receives one copy of the chromosome. The defining biochemical feature of this checkpoint is the stimulation of the anaphase-promoting complex by M-phase cyclin-CDK complexes, which in turn causes the proteolytic destruction of cyclins and proteins that hold the sister chromatids together.
A kinetochore is a disc-shaped protein structure associated with duplicated chromatids in eukaryotic cells where the spindle fibers attach during cell division to pull sister chromatids apart. The kinetochore assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis and meiosis. The term kinetochore was first used in a footnote in a 1934 Cytology book by Lester W. Sharp and commonly accepted in 1936. Sharp's footnote reads: "The convenient term kinetochore has been suggested to the author by J. A. Moore", likely referring to John Alexander Moore who had joined Columbia University as a freshman in 1932.
G2 phase, Gap 2 phase, or Growth 2 phase, is the third subphase of interphase in the cell cycle directly preceding mitosis. It follows the successful completion of S phase, during which the cell’s DNA is replicated. G2 phase ends with the onset of prophase, the first phase of mitosis in which the cell’s chromatin condenses into chromosomes.
Cell cycle checkpoints are control mechanisms in the eukaryotic cell cycle which ensure its proper progression. Each checkpoint serves as a potential termination point along the cell cycle, during which the conditions of the cell are assessed, with progression through the various phases of the cell cycle occurring only when favorable conditions are met. There are many checkpoints in the cell cycle, but the three major ones are: the G1 checkpoint, also known as the Start or restriction checkpoint or Major Checkpoint; the G2/M checkpoint; and the metaphase-to-anaphase transition, also known as the spindle checkpoint. Progression through these checkpoints is largely determined by the activation of cyclin-dependent kinases by regulatory protein subunits called cyclins, different forms of which are produced at each stage of the cell cycle to control the specific events that occur therein.
The G1/S transition is a stage in the cell cycle at the boundary between the G1 phase, in which the cell grows, and the S phase, during which DNA is replicated. It is governed by cell cycle checkpoints to ensure cell cycle integrity and the subsequent S phase can pause in response to improperly or partially replicated DNA. During this transition the cell makes decisions to become quiescent, differentiate, make DNA repairs, or proliferate based on environmental cues and molecular signaling inputs. The G1/S transition occurs late in G1 and the absence or improper application of this highly regulated checkpoint can lead to cellular transformation and disease states such as cancer.
MASTL is an official symbol provided by HGNC for human gene whose official name is micro tubule associated serine/threonine kinase like. This gene is 32,1 kbps long. This gene is also known as GW, GWL, THC2, MAST-L, GREATWALL. This is present in mainly mammalian cells like human, house mouse, cattle, monkey, etc. It is in the 10th chromosome of the mammalian nucleus. Recent studies have been carried on zebrafish and frogs. This gene encodes for the protein micro tubule associated serine/threonine kinase and its sub-classes.
Mitotic catastrophe has been defined as either a cellular mechanism to prevent potentially cancerous cells from proliferating or as a mode of cellular death that occurs following improper cell cycle progression or entrance. Mitotic catastrophe can be induced by prolonged activation of the spindle assembly checkpoint, errors in mitosis, or DNA damage and operates to prevent genomic instability. It is a mechanism that is being researched as a potential therapeutic target in cancers, and numerous approved therapeutics induce mitotic catastrophe.
A series of biochemical switches control transitions between and within the various phases of the cell cycle. The cell cycle is a series of complex, ordered, sequential events that control how a single cell divides into two cells, and involves several different phases. The phases include the G1 and G2 phases, DNA replication or S phase, and the actual process of cell division, mitosis or M phase. During the M phase, the chromosomes separate and cytokinesis occurs.
Cdc14 and Cdc14 are a gene and its protein product respectively. Cdc14 is found in most of the eukaryotes. Cdc14 was defined by Hartwell in his famous screen for loci that control the cell cycle of Saccharomyces cerevisiae. Cdc14 was later shown to encode a protein phosphatase. Cdc14 is dual-specificity, which means it has serine/threonine and tyrosine-directed activity. A preference for serines next to proline is reported. Many early studies, especially in the budding yeast Saccharomyces cerevisiae, demonstrated that the protein plays a key role in regulating late mitotic processes. However, more recent work in a range of systems suggests that its cellular function is more complex.
Mitotic exit is an important transition point that signifies the end of mitosis and the onset of new G1 phase for a cell, and the cell needs to rely on specific control mechanisms to ensure that once it exits mitosis, it never returns to mitosis until it has gone through G1, S, and G2 phases and passed all the necessary checkpoints. Many factors including cyclins, cyclin-dependent kinases (CDKs), ubiquitin ligases, inhibitors of cyclin-dependent kinases, and reversible phosphorylations regulate mitotic exit to ensure that cell cycle events occur in correct order with fewest errors. The end of mitosis is characterized by spindle breakdown, shortened kinetochore microtubules, and pronounced outgrowth of astral (non-kinetochore) microtubules. For a normal eukaryotic cell, mitotic exit is irreversible.
Induced cell cycle arrest is the use of a chemical or genetic manipulation to artificially halt progression through the cell cycle. Cellular processes like genome duplication and cell division stop. It can be temporary or permanent. It is an artificial activation of naturally occurring cell cycle checkpoints, induced by exogenous stimuli controlled by an experimenter.