Y monastrol treatment, kinetochores facing away from the half-spindle can induce microtubule polymerisation. Microtubule nucleation from the kinetochore is also observed in unperturbed budding yeast. In contrast, chromosome arms, although contributing to chromosome congression through binding chromokinesins, appear dispensable for spindle assembly in somatic cells. This is most obvious when mitosis is prematurely induced in the presence of under-replicated chromosomes. Under these conditions, small centromeric fragments containing kinetochores align at the spindle while chromosome arms are excluded. Even in this extreme situation, and despite the PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19853229 capacity of chromosome arms to attract microtubules uncovered by ablation of kinetochores does microtubule nucleation remain restricted to kinetochores. Together, these results suggest that chromosome arms are not a major inducer of spindle formation in somatic cells. During centrosomal mitosis, kinetochore-induced microtubule polymerisation may serve two purposes. First, separation of duplicated centrosomes sometimes occurs belatedly, and chromosome-induced microtubule formation may compensate for the transient lack of a second spindle pole. Second, kinetochore-induced microtubule polymerisation may allow timely attachment of kinetochores to the spindle. Initially, it was thought that kinetochore attachment is achieved by “search and capture” of microtubules originating from centrosomes. However, later analyses indicated such a process to be inefficient and slow. Chromosome-induced microtubule polymerisation may thus ensure the presence of microtubules around kinetochores, and Triptolide price facilitate spindle attachment. These notions are supported by observations that microtubules formed at kinetochores can be captured by the spindle. Evidence for the importance of kinetochore-induced microtubule polymerisation in somatic cells also comes from RanGTP- and CPC perturbations. In most systems, the contribution of the CPC is largely unknown, although in Drosophila S2 cells engineered to lack centrosomes, knockdown of 
CPC subunits Saracatinib cost causes 
small and/or multipolar spindles, and kinetochore microtubule polymerisation after nocodazole washout is defective after knockdown of Survivin in HeLa cells. More is known about RanGTP, which affects centrosomal spindles in all species tested. In human model systems, the involvement of RanGTP in spindle assembly has most extensively been analysed in HeLa cells. Here, uniformly high RanGTP levels result in ectopic microtubule polymerisation, as well as spindle assembly and chromosome alignment delays. Similarly, inhibition of RanGTP results in spindle defects, aberrant chromosome alignment, spindle positioning defects, and defects in kinetochore microtubule formation following nocodazole washout. In agreement with these results, a steep mitotic RanGTP gradient can be detected in many cell lines such as HeLa cells, although other cell lines, and particularly untransformed ones, often do not show clear gradients. The reasons behind this phenomenon appear to be complex, but one major factor may be the cellular DNA content, as fusion of cells of a gradient-free cell line resulted in cells with a gradient. Author Manuscript Author Manuscript Author Manuscript Author Manuscript Bioessays. Author manuscript; available in PMC 2016 October 01. Zierhut and Funabiki Page 7 However, this also resulted in an increase in cell volume, which itself may be important for formation of a RanGTP gra.Y monastrol treatment, kinetochores facing away from the half-spindle can induce microtubule polymerisation. Microtubule nucleation from the kinetochore is also observed in unperturbed budding yeast. In contrast, chromosome arms, although contributing to chromosome congression through binding chromokinesins, appear dispensable for spindle assembly in somatic cells. This is most obvious when mitosis is prematurely induced in the presence of under-replicated chromosomes. Under these conditions, small centromeric fragments containing kinetochores align at the spindle while chromosome arms are excluded. Even in this extreme situation, and despite the PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19853229 capacity of chromosome arms to attract microtubules uncovered by ablation of kinetochores does microtubule nucleation remain restricted to kinetochores. Together, these results suggest that chromosome arms are not a major inducer of spindle formation in somatic cells. During centrosomal mitosis, kinetochore-induced microtubule polymerisation may serve two purposes. First, separation of duplicated centrosomes sometimes occurs belatedly, and chromosome-induced microtubule formation may compensate for the transient lack of a second spindle pole. Second, kinetochore-induced microtubule polymerisation may allow timely attachment of kinetochores to the spindle. Initially, it was thought that kinetochore attachment is achieved by “search and capture” of microtubules originating from centrosomes. However, later analyses indicated such a process to be inefficient and slow. Chromosome-induced microtubule polymerisation may thus ensure the presence of microtubules around kinetochores, and facilitate spindle attachment. These notions are supported by observations that microtubules formed at kinetochores can be captured by the spindle. Evidence for the importance of kinetochore-induced microtubule polymerisation in somatic cells also comes from RanGTP- and CPC perturbations. In most systems, the contribution of the CPC is largely unknown, although in Drosophila S2 cells engineered to lack centrosomes, knockdown of CPC subunits causes small and/or multipolar spindles, and kinetochore microtubule polymerisation after nocodazole washout is defective after knockdown of Survivin in HeLa cells. More is known about RanGTP, which affects centrosomal spindles in all species tested. In human model systems, the involvement of RanGTP in spindle assembly has most extensively been analysed in HeLa cells. Here, uniformly high RanGTP levels result in ectopic microtubule polymerisation, as well as spindle assembly and chromosome alignment delays. Similarly, inhibition of RanGTP results in spindle defects, aberrant chromosome alignment, spindle positioning defects, and defects in kinetochore microtubule formation following nocodazole washout. In agreement with these results, a steep mitotic RanGTP gradient can be detected in many cell lines such as HeLa cells, although other cell lines, and particularly untransformed ones, often do not show clear gradients. The reasons behind this phenomenon appear to be complex, but one major factor may be the cellular DNA content, as fusion of cells of a gradient-free cell line resulted in cells with a gradient. Author Manuscript Author Manuscript Author Manuscript Author Manuscript Bioessays. Author manuscript; available in PMC 2016 October 01. Zierhut and Funabiki Page 7 However, this also resulted in an increase in cell volume, which itself may be important for formation of a RanGTP gra.