In this instance, a Ran GTP gradient is the main regulator of spindle microtubule organization and assembly. Acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during female meiosis in most animals. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells. In the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins. The spindle apparatus is vaguely ellipsoid in cross section and tapers at the ends. 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. 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. 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. Microtubule polymerization and depolymerization dynamic drive chromosome congression. Microtubule polymerization is nucleated at the microtubule organizing center.Īttachment of microtubules to chromosomes is mediated by kinetochores, which actively monitor spindle formation and prevent premature anaphase onset.
Astral microtubules anchor the spindle poles to the cell membrane. Polar microtubules interdigitate at the spindle midzone and push the spindle poles apart via motor proteins. Chromosomes are attached to kinetochore microtubules via a multiprotein complex called the kinetochore. This diagram depicts the organization of a typical mitotic spindle found in animal cells.
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