MT nucleation occurs from centrosomes to which γ-TuRC is recruited via its direct binding partners NEDD1 and GCP9 as well as via the centrosomal proteins Cep192 and CDK5RAP2 (, –99, 102, 107, 174). The first key step to assemble the mitotic spindle is γ-TuRC–dependent microtubule (MT) ( b) nucleation from MT organizing centers. Abbreviations: EB1-GFP, GFP-labeled end-binding protein 1 K-fiber, kinetochore fiber KT, kinetochore. Altogether, these results suggest that MT nucleation combined with local MT transport govern spindle morphogenesis.
#MITOTIC SPINDLE PLUS#
Lastly, femtosecond laser ablation leads to MT depolymerization of newly generated plus ends, a technique used to quantify MT density and identify the locations of MT plus and minus ends (5). Single-molecule speckle imaging has been applied to determine tubulin turnover and MT length distributions in the spindle (78, 79). Markers are lacking for MT minus ends and MT nucleation events, but growing MT plus ends can be tracked via the MT plus-tip tracking protein EB1-GFP (74, 75).
To date, it has not been possible to directly determine the key parameters that describe microtubule (MT) organization within the spindle, namely the origin, dynamics, polarity, and location of each MT, presumably because the high MT density precludes their direct observation. Lastly, advances in studying spindle assembly can be applied to address the molecular mechanisms of how the spindle segregates chromosomes.Ĭell division cytoskeleton microtubule microtubule nucleation microtubule-associated protein γ-tubulin ring complex.Īnatomy of the metaphase spindle. MT nucleation was uncovered as a key principle of spindle assembly, and mechanistic details about MT nucleation pathways and their coordination are starting to be revealed. Recent insight has been gained into the molecular building plan of the metaphase spindle using bulk and single-molecule measurements combined with computational modeling. This is mostly due to the density of MTs in the spindle, which presumably precludes their direct observation. Yet, a mechanistic understanding of how the mitotic spindle is assembled and achieves chromosome segregation is still missing. In anaphase, the spindle fibers pull the chromatids apart toward opposite poles.Life depends on cell proliferation and the accurate segregation of chromosomes, which are mediated by the microtubule (MT)-based mitotic spindle and ∼200 essential MT-associated proteins. In metaphase, they facilitate in aligning the chromosomes to the metaphase plate. The spindle fibers first appear particularly during prophase. The kinetochore microtubules are those that extend from the centrosome to the kinetochore protein in the centromere of the chromosome. The astral microtubules are those that anchor the spindle poles to the cell membrane. The polar microtubules are those that extend across the cell, i.e. The microtubules associated with mitosis are of three groups: the polar microtubules, the astral microtubules, and the kinetochore microtubules. The major components of the mitotic spindle include the spindle fibers (microtubules), microtubule-associated proteins, and the microtubule organizing center. They develop outside the nucleus during mitosis. Spindle fibers are made up of microtubules and they appear as spindle-shaped structures (thus, the name).
The spindle apparatus that forms during mitosis is referred to as mitotic spindle whereas the one that forms during meiosis is called meiotic spindle. These spindle fibers are responsible in moving and segregating the chromosomes during nuclear division. The spindle apparatus is the collective term for all the spindle fibers that form during mitosis or meiosis. The collective term for all the spindle fibers that form during mitosis
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