Mitosis

In cell biology, mitosis (/maɪˈtoʊsɪs/) is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the 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 accompanied or followed by cytokinesis, which divides the cytoplasm, organelles and cell membrane into two new cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis 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. The process of mitosis is divided into stages corresponding to the completion of one set of activities and the start of the next. These stages are prophase, prometaphase, metaphase, anaphase, and telophase. During mitosis, the chromosomes, which have already duplicated, condense and attach to spindle fibers that pull one copy of each chromosome to opposite sides of the cell. The result is two genetically identical daughter nuclei. The rest of the cell may then continue to divide by cytokinesis to produce two daughter cells. Producing three or more daughter cells instead of the normal two is a mitotic error called tripolar mitosis or multipolar mitosis (direct cell triplication / multiplication). Other errors during mitosis can induce apoptosis (programmed cell death) or cause mutations. Certain types of cancer can arise from such mutations. Mitosis occurs only in eukaryotic cells. Prokaryotic cells, which lack a nucleus, divide by a different process called binary fission. Mitosis varies between organisms. For example, animal cells undergo an 'open' mitosis, where the nuclear envelope breaks down before the chromosomes separate, whereas fungi undergo a 'closed' mitosis, where chromosomes divide within an intact cell nucleus. Most animal cells undergo a shape change, known as mitotic cell rounding, to adopt a near spherical morphology at the start of mitosis. Most human cells are produced by mitotic cell division. Important exceptions include the gametes – sperm and egg cells – which are produced by meiosis. Numerous descriptions of cell division were made during 18th and 19th centuries, with various degrees of accuracy. In 1835, the German botanist Hugo von Mohl, described cell division in the green alga Cladophora glomerata, stating that multiplication of cells occurs through cell division. In 1838, Schleiden affirmed that the formation of new cells in their interior was a general law for cell multiplication in plants, a view later rejected in favour of Mohl model, due to contributions of Robert Remak and others. In animal cells, cell division with mitosis was discovered in frog, rabbit, and cat cornea cells in 1873 and described for the first time by the Polish histologist Wacław Mayzel in 1875. Bütschli, Schneider and Fol might have also claimed the discovery of the process presently known as 'mitosis'. In 1873, the German zoologist Otto Bütschli published data from observations on nematodes. A few years later, he discovered and described mitosis based on those observations. The term 'mitosis', coined by Walther Flemming in 1882, is derived from the Greek word μίτος (mitos, 'warp thread'). There are some alternative names for the process, e.g., 'karyokinesis' (nuclear division), a term introduced by Schleicher in 1878, or 'equational division', proposed by Weismann in 1887. However, the term 'mitosis' is also used in a broad sense by some authors to refer to karyokinesis and cytokinesis together. Presently, 'equational division' is more commonly used to refer to meiosis II, the part of meiosis most like mitosis. The primary result of mitosis and cytokinesis is the transfer of a parent cell's genome into two daughter cells. The genome is composed of a number of chromosomes—complexes of tightly coiled DNA that contain genetic information vital for proper cell function. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the S phase of interphase. Chromosome duplication results in two identical sister chromatids bound together by cohesin proteins at the centromere.