Mitosis
In 1835, the German botanist Hugo von Mohl described cell division in the green algae Cladophora glomerata. He stated that multiplication of cells occurs through cell division. This observation appeared in a publication titled Notes and memoranda: The late professor von Mohl within the Quarterly Journal of Microscopical Science. Matthias Jakob Schleiden affirmed in 1838 that formation of new cells in their interior was a general rule for cell multiplication in plants. Later researchers rejected this view in favor of Mohl's model due to contributions from Robert Remak and others. In animal cells, cell division with mitosis was discovered in frog, rabbit, and cat cornea cells in 1873. Polish histologist Wacław Mayzel described this process for the first time in 1875. German zoologist Otto Bütschli published data from observations on nematodes in 1873. A few years later, he discovered and described mitosis based on those observations. Walther Flemming coined the term mitosis in 1882. The word derives from the Greek word mitos meaning warp thread. Alternative names included karyokinesis introduced by Schleicher in 1878. August Weismann proposed equational division in 1887.
During prophase which occurs after G2 interphase, the cell prepares to divide by tightly condensing its chromosomes. Chromatin fibers condense into discrete chromosomes that are typically visible at high magnification through a light microscope. Each chromosome has two chromatids joined at the centromere. Gene transcription ceases during prophase and does not resume until late anaphase to early G1 phase. The nucleolus also disappears during early prophase. Close to the nucleus of an animal cell are structures called centrosomes consisting of a pair of centrioles surrounded by loose collection of proteins. The centrosome is the coordinating center for the cells microtubules. Motor proteins then push the centrosomes along these microtubules to opposite sides of the cell. At the beginning of prometaphase in animal cells phosphorylation of nuclear lamins causes the nuclear envelope to disintegrate into small membrane vesicles. Microtubules invade the nuclear space. This process is called open mitosis. In late prometaphase kinetochore microtubules begin to search for and attach to chromosomal kinetochores. A kinetochore is a proteinaceous microtubule-binding structure that forms on the chromosomal centromere during late prophase. When a microtubule connects with the kinetochore, the motor activates using energy from ATP to crawl up the tube toward the originating centrosome.
The interphase is a much longer phase of the cell cycle than the relatively short M phase. During interphase the cell prepares itself for the process of cell division. Interphase is divided into three subphases: G1 first gap, S synthesis, and G2 second gap. During all three parts of interphase, the cell grows by producing proteins and cytoplasmic organelles. However chromosomes are replicated only during the S phase. Thus a cell grows G1 continues to grow as it duplicates its chromosomes S grows more and prepares for mitosis G2 and finally divides M before restarting the cycle. All these phases in the cell cycle are highly regulated by cyclins cyclin-dependent kinases and other cell cycle proteins. The phases follow one another in strict order and there are cell cycle checkpoints that give the cell cues to proceed or not from one phase to another. Cells may also temporarily or permanently leave the cell cycle and enter G0 phase to stop dividing. This can occur when cells become overcrowded due to density-dependent inhibition or when they differentiate to carry out specific functions for the organism. Some G0 cells have the ability to re-enter the cell cycle. DNA double-strand breaks can be repaired during interphase by two principal processes.
Mitosis varies between organisms. Animal cells generally undergo an open mitosis where the nuclear envelope breaks down before the chromosomes separate. Fungal cells generally undergo closed mitosis where chromosomes divide within an intact cell nucleus. Most animal cells undergo shape change known as mitotic cell rounding to adopt near spherical morphology at start of mitosis. Most human cells are produced by mitotic cell division. Important exceptions include gametes sperm and egg cells which are produced by meiosis. Prokaryotes bacteria and archaea lack true nucleus and divide by different process called binary fission. Closed intranuclear pleuromitosis is typical of Foraminifera some Prasinomonadida some Kinetoplastida Oxymonadida Haplosporidia many fungi chytrids oomycetes zygomycetes ascomycetes and some Radiolaria Spumellaria and Acantharia. It seems to be most primitive type. Closed extranuclear pleuromitosis occurs in Trichomonadida and Dinoflagellata. Closed orthomitosis found among diatoms ciliates some Microsporidia unicellular yeasts and some multicellular fungi. Semiopen pleuromitosis typical of most Apicomplexa. Semiopen orthomitosis occurs with different variants in some amoebae Lobosa and some green flagellates Raphidophyta or Volvox.
An error in mitosis can result in production of three or more daughter cells instead of normal two. This condition is called tripolar mitosis and multipolar mitosis respectively. These errors can cause non-viable embryos that fail to implant. Other errors during mitosis can induce mitotic catastrophe apoptosis programmed cell death or cause mutations. Certain types of cancers arise from such mutations. Mitotic errors create aneuploid cells that have too few or too many of one or more chromosomes. Condition associated with cancer includes early human embryos cancer cells infected or intoxicated cells suffering pathological division into three or more daughter cells resulting in severe errors in chromosomal complements. In nondisjunction sister chromatids fail to separate during anaphase. One daughter cell receives both sister chromatids from nondisjoining chromosome and other cell receives none. Former cell gets three copies of chromosome known as trisomy latter will have only one copy known as monosomy. On occasion when cells experience nondisjunction they fail to complete cytokinesis and retain both nuclei in one cell resulting in binucleated cells. Anaphase lag occurs when movement of one chromatid impeded during anaphase caused by failure of mitotic spindle to properly attach to chromosome.
In histopathology the mitosis rate mitotic count or mitotic index important parameter in various types of tissue samples for diagnosis as well to further specify aggressiveness of tumors. There is routinely quantification of mitotic count in breast cancer classification. The mitoses must be counted in area of highest mitotic activity. Visually identifying these areas difficult in tumors with very high mitotic activity. Detection of atypical forms of mitosis can used both as diagnostic and prognostic marker. Lag-type mitosis non-attached condensed chromatin in area of mitotic figure indicates high risk human papillomavirus infection-related cervical cancer. In order to improve reproducibility and accuracy of mitotic count automated image analysis using deep learning-based algorithms proposed. Further research needed before those algorithms can used to routine diagnostics. Mitoses appear in breast cancer images showing abnormal division patterns. Researchers use these visual markers to classify tumor severity and predict patient outcomes based on cellular behavior observed under microscope.
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Common questions
Who described cell division in green algae Cladophora glomerata in 1835?
Hugo von Mohl described cell division in the green algae Cladophora glomerata in 1835. He stated that multiplication of cells occurs through cell division and published this observation in Notes and memoranda: The late professor von Mohl within the Quarterly Journal of Microscopical Science.
When did Walther Flemming coin the term mitosis and what is its origin?
Walther Flemming coined the term mitosis in 1882. The word derives from the Greek word mitos meaning warp thread.
What happens to chromosomes during prophase after G2 interphase?
During prophase which occurs after G2 interphase, chromatin fibers condense into discrete chromosomes that are typically visible at high magnification through a light microscope. Each chromosome has two chromatids joined at the centromere and gene transcription ceases until late anaphase to early G1 phase.
How do animal cells differ from fungal cells regarding nuclear envelope behavior during mitosis?
Animal cells generally undergo open mitosis where the nuclear envelope breaks down before the chromosomes separate. Fungal cells generally undergo closed mitosis where chromosomes divide within an intact cell nucleus.
What condition results when sister chromatids fail to separate during anaphase?
Nondisjunction occurs when sister chromatids fail to separate during anaphase. One daughter cell receives both sister chromatids resulting in trisomy while the other cell receives none resulting in monosomy.