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Acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during female meiosis in most animals. [5] In this instance, a Ran GTP gradient is the main regulator of spindle microtubule organization and assembly.
Microtubule polymerization is nucleated at the microtubule organizing center. An aster is a cellular structure shaped like a star , consisting of a centrosome and its associated microtubules during the early stages of mitosis in an animal cell.
K fibers have a much longer half life than interpolar microtubules, at between 4 and 8 minutes. [65] During the end of mitoses, the microtubules forming each K fiber begin to disassociate, thus shorting the K fibers. As the K fibers shorten the pair chromosomes are pulled apart right before cytokinesis.
It is widely accepted that the kMTs fiber (the bundle of microtubules bound to the kinetochore) is originated by the capture of MTs polymerized at the centrosomes and spindle poles in mammalian cultured cells. [43] However, MTs directly polymerized at kinetochores might also contribute significantly. [55]
Role of the centrosome in cell cycle progression. Centrosomes are associated with the nuclear membrane during the prophase stage of the cell cycle. During mitosis, the nuclear membrane breaks down, and the centrosome-nucleated microtubules can interact with the chromosomes to build the mitotic spindle.
The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spindle apparatus , which separate the chromosomes during cell division .
This spindle apparatus consists of microtubules, microfilaments and a complex network of various proteins. During metaphase, the chromosomes line up using the spindle apparatus in the middle of the cell along the equatorial plate. The chromosomes move to opposite poles during anaphase and remain attached to the spindle fibers by their centromeres.
The spindle pole body (SPB) is the microtubule organizing center in yeast cells, functionally equivalent to the centrosome. Unlike the centrosome the SPB does not contain centrioles. The SPB organises the microtubule cytoskeleton which plays many roles in the cell. It is important for organising the spindle and thus in cell division.