Kinetochoric organizers

Centromere protein A (CENP-A), one of several variants of histone H3, is phosphorylated on Ser 7 by Aurora B kinase which is equivalent to Ser 10 of histone H3 (Zeitlin et al, 2001). Recent studies demonstrate that Aurora A kinase also phosphorylates CENP-A (S7) (Kunitoku et al, 2003) (Table 1). The presence of CENP-A in centromeric nucleosomes is required for kinetochore organization and function (Choo 2001). Loss of CENP-A phosphorylation function at Ser 7 caused a mislocalisation of Aurora B, a putative partner phosphatase (PPl-yl) and inner centromere protein (INCENP). H3.3, another variant of histone H3 is phosphorylated on Ser 31 in vivo (Table 1). H3.3 (S31) is a mitosis-specific modification that is present only in late prometaphase and metaphase. Furthermore, H3.3 (S31) is excluded from centromeres. However it is enriched in distinct chromosomal areas immediately adjacent to centromeres (Hake et al, 2005). 

The critical features of spindle formation are the origin of distinct chromosomal and interpolar fibers and the development of a common spindle axis. Where both polar and kinetochoric organizers are present the interpretation in terms of assembly, parallel alignment, and organizing centers is obvious. But where no polar centers are involved... 

Blower MD, Karpen GH (2001) The role of Drosophila CID in kinetochore formation, cell-cycle progression and heterochromatin interactions. Nat Cell Biol 3 730-739 Blower MD, Sullivan BA, Karpen GH (2002) Conserved organization of centromeric chromatin in flies and humans. Dev Cell 2 319-330... 

The majority of micronuclei formed in this study were kinetochore negative, indicating a predominantly clastogenic effect of deoxycholic acid in these cells. Clastogenicity has previously been reported for organic fractions from human faeces and for ursodeoxycholic acid, utilising Chinese hamster ovary cells and human lymphocytes. 

M. De Brabander, G. Geuens, R. Nuydens, R. Willebrords and J. De Mey, Taxol induces the assembly of free microtubules in living cells and blocks the organizing capacity of the centrosome and kinetochores, Proc. Natl. Acad. Sci. USA 78 (1981) 5608-5612. 

In each half of the spindle, a single centrosome at the pole organizes three distinct sets of microtubules whose (—) ends all point toward the centrosome (Figure 20-3lb). One set, the astral microtubules, forms the aster they radiate outward from the centrosome toward the cortex of the cell, where they help position the mitotic apparatus and later help to determine the cleavage plane in cytokinesis. The other two sets of microtubules compose the spindle. The kinetochore microtubules attach to chromosomes at specialized attachment sites on the chromosomes called kinetochores. Polar microtubules do not interact with chromosomes but instead overlap with polar microtubules from the opposite pole. Two types of interactions hold the spindle halves together to form... 

The mitotic apparatus is basic to mitosis in all organisms, but its appearance and components can vary widely. In the budding yeast Saccharomyces cerevisiae, for instance, the mitotic apparatus consists of just a spindle, which itself is constructed from a minimal number of kinetochore and polar microtubules. These microtubules are organized by spindle pole bodies, trilamlnated structures located In the nuclear membrane, which do not break down during mitosis. Furthermore, because a yeast cell is small, It does not require well-developed asters to assist in mitosis. Although the spindle pole body and centrosome differ structurally, they have proteins such as 7-tubulin In common that act to organize the mitotic spindle. Like yeast cells, most plant cells do not contain visible centrosomes. We consider the unique features of the mitotic apparatus In plant cells at the end of this section. 

Centromeres are the site of organization of kinetochores on mitotic chromosomes where chromosomes c ture the spindle microtubules to ensure faithful chromosomal s regation during mitosis. In mammalian cells, they span tens of megabases and are composed of large arrays of tandemly repeated sequences, the O-satellite in human and the minor sateUite in... 

Only the major features of spindle organization as now known will be considered here. Spindle chemistry, kinetochore structure, and some details of spindle organization will be discussed in the sections which follow on chromosome movement and distribution. 

The proportion of bivalents that achieve bipolar orientation in this fashion will doubtless vary in cells from different organisms. Thus the proportion may be lowered if long bivalents occur and their interkinetochoric chromatin is flexible. Such bivalents are often observably flexed between the kinetochores of partner half-bivalents, which are therefore not constrained to face in opposite directions. As expected from the present interpretation of initial orientation, these bivalents malorient abnormally often (Ostergren, 1951, pp. 140 ff White, 1961 Henderson et al., 1970). Conversely, in cells where individual spindles form around each chromosome and are oriented to each... 

The distribution of chromosomes in meiosis is understandable in terms of initial orientation and reorientation. Direct evidence that initial orientation and reorientation are now explained at the cytological level comes from experiments on bivalents in which orientation and distribution are predictably altered the experimenter 1. can determine the distribution of any chosen chromosome 2. can induce unstable malorientation and 3. can stabilize malorientations and induce nondisjunction. The explanation offered is easily extended to chromosome distribution in mitosis and to most units other than bivalents in meiosis. Testable molecular hypotheses are readily suggested. These would relate features of the cytological explanations to the generally documented organizing center activity of kinetochores and to spindle fiber lability. 

Jokelainen, P. T. 1967. The ultrastructure and spatial organization of the metaphase kinetochore in mitotic rat cells. J. Ultrastruct. Res., 19 19-44. 

Lima-de-Faria, A. 1956. The role of the kinetochore in chromosome organization. Hereditas (Lund), 42 85-160. 

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