Cell division is the process by which a cell, called the parent cell, divides into two cells, called daughter cells. It is a small part of a large cell cycle. In meiosis, a cell is permanently transformed and cannot divide again. Cell division is the biological basis of life. For simple unicellular organisms such as the Amoeba, one cell division reproduces an entire organism. On a larger scale, cell division can create progeny from multicellular organisms, such as plants that grow from cuttings. Cell division also enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by cell division from gametes. And after growth, cell division allows for continual renewal and repair of the organism. The primary concern of cell division is the maintenance of the original cell’s genome. Before division can occur, the genomic information which is stored in chromosomes must be replicated, and the duplicated genome separated cleanly between cells. A great work of cellular infrastructure is involved in keeping genomic information consistent between “generations”.

Three types of cell division: Cells are classified into two categories: simple, non-nucleated prokaryotic cells, and complex, nucleated eukaryotic cells. By virtue of their structural differences, eukaryotic and prokaryotic cells do not divide in the same way. Furthermore, the pattern of cell division that transforms eukaryotic stem cells into gametes, sperm in males or ova in females is different from that of eukaryotic somatic, non-germ cells. Prokaryotic cells are simpler in structure when compared to eukaryotic cells. They contain non-membranous organelles, lack a cell nucleus, and have a simplistic genome: only one circular chromosome of limited size. Therefore, prokaryotic cell division, a process known as binary fission, is fast. The chromosome is duplicated prior to division. The two copies of the chromosome attach to opposing sides of the cellular membrane. Cytokinesis, the physical separation of the cell, occurs immediately. Mitosis: The division of the nucleus, separating the duplicated genome into two sets identical to the parent’ cell. Cytokinesis: The division of the cytoplasm, separating the organelles and other cellular components. 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. Senescent cells deteriorate and die, causing the body to age. Cells stop dividing because the telomeres, protective bits of DNA on the end of a chromosome, become shorter with each division and eventually can no longer protect the chromosome. Cancer cells, on the other hand, are “immortal.” An enzyme called telomerase, present in large quantities in cancerous cells, rebuilds the telomeres, allowing division to continue indefinitely. During the metaphase stage of mitosis, chromosomes, which become aligned on the equatorial plane, take on the shape of an “X” as a result of a repelling force between chromosomes. The lobes of the chromosome in this shape are called ’sister chromatids. The sister chromatids will be attached by a centromere. During metaphase, centromeres of the chromosomes will be aligned in the centre of the nucleus and spindle fibers will be attached to them. In the beginning of anaphase, spindle fibers contract so that the identical chromatids ,sister chromatids, which where attached by centromere, will be separated. At this stage, each separated chromatid will act as a chromosome, and the two separated chromatids are called daughter chromosome. There are six phases in cell division: the Interphase, Prophase, Anaphase, Telophase, and Cytokinesis. Interphase is a phase of the is phase of cell cycle, defined only by the absence of cell division. During interphase, the cell obtains nutrients, and duplicates its chromatids. Most eukaryotic cells spend most of their time in interphase. For example, human skin cells, which divide about once a day, spend roughly 22 hours in interphase. Cells during interphase may or may not be at any given time, even in an area of rapid cell division such as the tip of a plant root, 90 percent of cells are in interphase. Some cells, such as nerve cells, can stay in interphase for decades. The cell grows and replicates its DNA and centrioles. There are 3 parts of interphase: G1, growth 1 in which the cell creates organelles and begins metabolism) ,S phase, DNA synthesis in which the chromosomes of the cell are copied and G2 growth 2 in which the cell grows in preparation for cell division. Sometimes the cells exit the cell cycle usually from G1 phase and enter the G0 phase. In the G0 phase, cells are alive and metabolically active, but do not divide. In this phase cells do not copy their DNA and do not prepare for cell division. Many cells in the human body, including those in heart muscle, eyes, and brain are in the G0 phase. If these cells are damaged they cannot be replaced. During interphase, the chromosomes are found arranged in the nucleus and appear as a network of long, thin threads, called chromatin. At some point before prophase begins, the chromosomes begin to replicate themselves to form pairs of identical chromosomes. The deoxyribonucleic acid DNA of the chromosomes is in use only during interphase, when the cell is in a stable and self reliant phase. In prophase the two chromatids are still connected by something called the centromere. The sister chromosomes contract tightly. During the late interphase the nucleus is well defined and bounded by the nuclear envelope and the nucleus contains one or more nucleoli. Outside the nucleus are two centrosomes which sprout microtubules by polymerizing free-floating proteins. The centrosomes push themselves to opposite ends of the cell. The network of microtubules forms the beginning of the mitotic spindle. These spindle fibers become visible. The centrioles separate starting to radiate bundles of fibers, called asters. The spindle fibers run from one centriole to the other, at both poles of the cell. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome. DNA has already duplicated back in S phase Prophase is a stage of mitosis in which chromatin condenses into a highly ordered structure called a chromosome (it is at this stage giemsa staining can be applied to elicit G-banding in chromosomes). This process, called chromatin condensation, is mediated by the condensin complex. Since the genetic material has been duplicated, there are two identical copies of each chromosome in the cell. Identical chromosomes, called sister chromosomes, are attached to each other at a DNA element present on every chromosome called the centromere. When chromosomes are paired up and attached, each individual chromosome in the pair is called a chromatid, while the whole unit is called a chromosome. When the chromatids separate, they are no longer called chromatids, but are called chromosomes again. The task of mitosis is to assure that one copy of each sister chromatid - and only one copy - goes to each daughter cell after cell division. An important organelle in mitosis is the centrosome, the microtubule organizing center in metazoans. During prophase, the two centrosomes - which replicate independently of mitosis — have their microtubule-nucleating activity increased due to the recruitement of ?-tubulin. The centrosomes will be pushed apart to opposite ends of the cell nucleus by the action of molecular motors acting on the microtubules. The nuclear envelope breaks down to allow the microtubules to reach the kinetochores on the chromosomes. The nuclear envelope break down marks the end of prophase. Prometaphase, the next step of mitosis will see the chromosome being captured by the microtubules. Metaphase is a stage of mitosis in the eukaryotic cell cycle in which condensed chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells. Preceded by events in prometaphase and followed by anaphase microtubules formed in prophase have already found and attached themselves to kinetochores in metaphase. The centromeres of the chromosomes convene themselves on the metaphase plate, an imaginary line that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug of war between equally strong people. In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. Early events of metaphase can coincide with the later events of prometaphase, as chromosomes with connected kinetochores will start the events of metaphase individually before other chromosomes with unconnected kinetochores that are still lingering in the events of prometaphase. One of the cell cycle checkpoints occurs during prometaphase and metaphase. Only after all chromosomes have become aligned at the metaphase plate, when every kinetochore is properly attached to a bundle of microtubules, does the cell enter anaphase. It is thought that unattached or improperly attached kinetochores generate a signal to prevent premature progression to anaphase, even if most of the kinetochores have been attached and most of the chromosomes have been aligned. Such a signal creates the mitotic spindle checkpoint. This would be accomplished by regulation of the Anaphase Promoting Complex, securin, and separase. The analysis of metaphase chromosomes is one of the main tools of cancer cytogenetics. Malignant cells from solid tumors or leukemia samples are grown in short term culture and dropped onto microscope slides to generate metaphase preparations. Staining of the slides, often with Giemsa or Quinacrine, produces a pattern of in total up to several hundred bands. Inspection of the stained metaphases allows the determination of numerical and structural changes in the tumor cell genome, for example, losses of chromosomal segments or translocations, which may lead to chimeric oncogenes, such as bcr-al in Chronic myelogenous leukemia. Anaphase is the stage of mitosis when chromosomes separate in a eukaryotic cell. Each chromatid moves to opposite poles of the cell, the opposite ends of the mitotic spindle, near the microtubule organizing centers. Anaphase is preceded by metaphase, by the end of which fully condensed sister chromatids are arranged in a straight line down the midline of the cell, defining a structure referred to as the metaphase plate. Spindle fibers, which are microtubules containing ?-tubulin and other Microtubule-associated proteins extend from the poles to the centromeres. The point of contact is a protein complex called the kinetochore, and these fibres are sometimes referred to as kinetochore fibers or k-fibers. Other spindle fibres do not come in contact with the chromosomes but either connect directly with spindle fibres from the opposing pole as overlap microtubules or interpolar microtubules ior with the cell cortex as astral microtubules. Anaphase begins abruptly with the highly-regulated triggering of the metaphase-to-anaphase transition. At this point the Anaphase Promoting Complex, APC becomes activated. This terminates metaphase, M-phase activity by cleaving and inactivating the M-phase cyclin required for the function of M-phase cyclin dependent kinases (M-Cdks). It also cleaves securin, a protein that inhibits the protease known as separase. Separase then cleaves cohesin, a protein responsible for holding sister chromatids together. The consequent separation of chromatids marks the cytological onset of anaphase. After separation they are referred to as daughter chromatids. Within anaphase two distinct processes occur. During early anaphase or Anaphase A, the chromatids abruptly separate and move towards the spindle poles. This is achieved by shortening of the spindle microtubules, and forces are mainly exerted at the kinetochores. When the chromatids are fully separated late anaphase or Anaphase B begins. This involves the polar microtubules elongating and sliding relative to each other to drive the spindle poles further apart. These two processes were originally distinguished by their different sensitivities to drugs, and mechanically they are distinct processes. Early anaphase involves shortening kinetochore mictrotubules by depolymerization at both ends. During this, motor proteins at the kinetochores pull on the kinetochore microtubules. Late anaphase involves both the elongation of overlap microtubules and the use of two distinct sets of motor proteins: one of these pulls overlap microtubules past each other, the other pulls on astral microtubules that have attached to the cell cortex. The contributions of early anaphase and late anaphase to anaphase as a whole vary with cell type. In mammalian cells, late anaphase follows shortly after early anaphase and extends the spindle to around twice its metaphase length; in contrast yeast and certain protozoa use late metaphase as the main means of chromosome separation and can extend the spindle to up to 15 times its metaphase length in the process. Telophase is a stage in either meiosis or mitosis in a eukaryotic cell reversing the effects of prophase and prometaphase events. During those events, the nucleus was dissolved and the chromatin in the cell was condensed into chromosomes. Telophase thus “cleans up” the secondary after-effects of mitosis. At this stage, the non-kinetochore microtubules continue to lengthen, further elongating the cell. Corresponding sister chromosomes, which are the results of anaphase, attach at opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell’s nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Cytokinesis, if slated to occur, usually occurs at the same time the nuclear envelope is reforming, although they are distinct processes. In animal cells, a cleavage furrow develops where the metaphase plate used to be, pinching off the separated nuclei. Cytokinesis is the process whereby the cytoplasm of a single cell is divided to spawn two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a bi-nucleate cell in two to ensure that chromosome number is maintained from one generation to the next. Cytokinesis must happen in both processes. One notable exception to the normal process of cytokinesis is oogenesis the creation of an ovum in the ovarian follicle of the ovary, where the ovum takes almost all the cytoplasm and organelles, leaving very little for the resulting polar bodies, which then die. In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells. During normal proliferative division, animal cell cytokinesis begins shortly after the onset of sister chromatid separation in the anaphase of mitosis. A contractile ring, comprised of non-muscle myosin II and actin filaments, assembles equatorially at the cell cortex (adjacent to the cell membrane). Myosin II uses the free energy released when ATP is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. Continued hydrolysis causes this cleavage furrow to ingress (move inwards), a striking process that is clearly visible down a light microscope. Ingression continues until a so-called midbody structure (composed of electron-dense, proteinaceous material) is formed and the process of abcission then cleaves this midbody to physically pinch one cell into two. Microtubules (non-kinetochore) then reorganize and disappear into a new cytoskeleton as the cell cycle returns to interphase. Simultaneous with contractile ring assembly, a microtubule based structure termed the central spindle, spindle midzone forms when non-kinetochore microtubule fibres are bundled between the spindle poles. A number of different species including H. sapiens, D. and C. elegans require the central spindle in order to efficiently undergo melanogaster cytokinesis, although the specific phenotype described when it is absent varies from one species to the next (for example, certain Drosophila cell types are incapable of forming a cleavage furrow without the central spindle, whereas in both C. elegans embryos and human tissue culture cells a cleavage furrow is observed to form and ingress, but then regress before cytokinesis is complete). Seemingly vital for the formation of the central spindle (and therefore efficient cytokinesis) is a heterotetrameric protein complex called centralspindlin. Along with associated factors such as SPD-1 in C. elegans, centralspindlin might play a role in bundling microtubules to form the spindle midzone during anaphase.