Cell division is the process by which a parent cell divides into two daughter cells. 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. 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.
To ensure proper progression through the cell cycle, DNA damage is detected and repaired at various cell cycle checkpoints. 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. 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. After growth from the zygote to the adult, cell division by mitosis allows for continual construction and repair of the organism. The human body experiences about 10 quadrillion cell divisions in a lifetime.
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. A great deal of cellular infrastructure is involved in ensuring consistency of genomic information among generations.
In bacteria
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.Cell Division: The Cycle of the Ring, Lawrence Rothfield and Sheryl Justice, CELL, DOI
In eukaryotes
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.
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 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."
Phases of eukaryotic cell division
Interphase Interphase is the process through which a cell must go before mitosis, meiosis, and cytokinesis. 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. 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. In S phase, the chromosomes are replicated in order for the genetic content to be maintained. 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. 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.
Prophase 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. 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. 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.
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. 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. During this phase all the microtubules, with the exception of the kinetochores, are in a state of instability promoting their progression toward anaphase. At this point, the chromosomes are ready to split into opposite poles of the cell toward the spindle to which they are connected.
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. 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. 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. As the sister chromatids are being pulled apart, the cell and plasma are elongated by non-kinetochore microtubules. Additionally, in this phase, the activation of the anaphase promoting complex through the association with Cdh-1 begins the degradation of mitotic cyclins.
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. 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.
Cytokinesis