In cell biology, the spindle apparatus (also called spindle fibers) is the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells. Depending on the type of cell division, it is also referred to as the mitotic spindle during mitosis and the meiotic spindle during meiosis.
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Shape and components
The cellular spindle apparatus includes the spindle microtubules, associated proteins, and any centrosomes or asters present at the spindle poles.[1] The spindle apparatus is vaguely ellipsoid in shape and tapers at the ends but spreads out in the middle. In the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells. Acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during gametogenesis in animals.[2] In fungi, spindles form between spindle pole bodies embedded in the nuclear envelope. Most plants lack centrosomes or spindle pole bodies and instead spindle microtubules are nucleated on their nuclear envelopes.[3] Take chromosomes through most stages of mitosis. Start to form in early metaphase. Connected to the centrioles at each end of the cell.
Assembly, chromosome attachment and function during cell division
During spindle assembly in prometaphase, some of the spindle's microtubules attach to the kinetochores that assemble on the centromere portion of the chromosomes. After kinetochore "capture" of the spindle fibers, the chromosomes are pulled into alignment along the spindle midzone to form the metaphase plate. Since the center of the spindle specifies the plane along which the cell will divide during cytokinesis, this ensures that each daughter cell will receive one of each chromatid. Spindle formation is complete during metaphase when:
- the chromosomes have been aligned in the metaphase plate of the spindle
- non-kinetochore ("polar") microtubules from opposite spindle poles overlap, and
- the aster microtubules are in contact with the plasma membrane.[1]
Once all the chromosomes are aligned with sister chromatids pointing to opposite ends of the spindle, the cell enters anaphase when proteins holding the sister chromatids together are inactivated.[1] This allows the chromatids to separate into full-fledged chromosomes that start moving toward their respective poles. This movement is mediated by motor proteins on the kinetochores that "walk" the chromosome along the microtubule towards the nearest pole. Similarly, motor proteins attach to non-kinetochore microtubules and walk them away from each other, thus elongating the spindle and pushing apart the spindle poles.[1]
Regulation of spindle assembly
The mitotic kinase aurora A is required for proper spindle assembly and separation.[citation needed] Lamin B is a key component of the spindle matrix helping microtubule assembly, and the mitotic spindle will not form without it.[4]
Polo-like kinase, also known as PLK, especially PLK1 has important roles in the spindle maintenance by regulating the microtubulin dynamics[5].
Mitotic spindle assembly checkpoint
The completion of spindle formation is a crucial transition point in the cell cycle called the spindle assembly checkpoint. If some chromosomes are not properly attached to the mitotic spindle by the time of this checkpoint, the onset of anaphase will be delayed.[6] Failure of this spindle assembly checkpoint can result in aneuploidy and may be involved in aging and the formation of cancer.[7] Abnormal mitotic spindles can produce tripolar mitosis. These are clearly abnormal cases and, if present, are considered definitive evidence that a tumor is malignant rather than benign. Such abnormalities are therefore often searched for in histological assays by pathologists when evaluating the potential malignancy of a tumor mass.
References
- ^ a b c d Campbell, Neil A.; Jane B. Reece (2005). Biology, 7th Edition. San Francisco: Benjamin Cummings. pp. 221–224. ISBN 0-8053-7171-0.
- ^ Manandhar Gf, Schatten H, Sutovsky P (2005). "Centrosome Reduction During Gametogenesis and Its Significance". Biol. Reprod. 72 (1): 2. doi:. PMID 15385423.
- ^ Schmit AC (2002). "Acentrosomal microtubule nucleation inzeff higherd plants". Int. Rev. Cytol. 220: 257–89. doi:. PMID 12224551.
- ^ M. Y. Tsai, S. Wang, J. M. Heidinger, D. K. Shumaker, S. A. Adam, R. D. Goldman & Y. Zheng (2006-03-31). "A mitotic lamin B matrix induced by RanGTP required for spindle assembly". Science 311 (5769): 1887–1193. doi:. PMID 16543417. http://www.sciencemag.org/cgi/content/abstract/1122771v1.
- ^ Peters, U., J. Cherian, et al. ([(2006)]). ""Probing cell-division phenotype space and Polo-like kinase function using small molecules."". Nat Chem Biol 2 (11): 618–26. doi:. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=17028580.
- ^ Raven, Peter H.; Ray F. Evert, Susan E. Eichhorn (2005). Biology of Plants, 7th Edition. New York: W.H. Freeman and Company Publishers. pp. 59. ISBN 0-7167-1007-2.
- ^ Baker DJ, Chen J, van Deursen JM (2005). "The mitotic checkpoint in cancer and aging: what have mice taught us?". Curr. Opin. Cell Biol. 17 (6): 583–9. doi:. PMID 16226453.
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