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Spindle apparatus
In cell biology, the spindle apparatus refers to the subcellular structure of eukaryotic cells that separates chromosomes between daughter cells during cell division. It is also referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell.
Besides chromosomes, the spindle apparatus is composed of hundreds of proteins.〔Helmke KJ, Heald R, Wilbur JD. Interplay between spindle architecture and function. 2013. Int’l Rev.of Cell and Mol. Biol. 306:83-125. PubMed PMID 24016524.〕 Microtubules comprise the most abundant components of the machinery.
==Spindle structure==
Attachment of microtubules to chromosomes is mediated by kinetochores, which actively monitor spindle formation and prevent premature anaphase onset. Microtubule polymerization and depolymerization dynamics drive chromosome congression. Depolymerization of microtubules generates tension at kinetochores; bipolar attachment of sister kinetochores to microtubules emanating from opposite cell poles couples opposing tension forces, aligning chromosomes at the cell equator and poising them for segregation to daughter cells. Once every chromosome is bi-oriented, anaphase commences and cohesin, which couples sister chromatids, is severed, permitting the transit of the sister chromatids to opposite poles.
The cellular spindle apparatus includes the spindle microtubules, associated proteins, which include kinesin and dynein molecular motors, condensed chromosomes, and any centrosomes or asters that may be present at the spindle poles depending on the cell type. The spindle apparatus is vaguely ellipsoid in cross section and tapers at the ends. 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 female meiosis in most animals. In this instance, a Ran GTP gradient is the main regulator of spindle microtubule organization and assembly. In fungi, spindles form between spindle pole bodies embedded in the nuclear envelope, which does not break down during mitosis.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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