Mitotic spindles constitute the machinery responsible for equidistribution of the genetic material into each daughter cell during cell division. They are transient and hence quite labile structures, changing their morphology even while performing their function. Biochemical, immunological and genetic analyses of mitotic cells have allowed us to identify a variety of molecules that are recruited to form the spindle at the onset of mitosis. Evaluation of the roles of these molecules in both the formation and in the (...) dynamics of spindle microtubules should be important for understanding the molecular basis of mitosis and its regulation. We have recently identified a novel mitosis‐specific microtubule‐associated protein (MAP) using a monoclonal antibody probe raised against the mitotic spindles isolated from cultured mammalian cells. This 95/105 kDa antigen represents a unique component of the spindle distinct from any of the other MAPs reported so far. Antibody microinjection resulted in mitotic inhibition in a stage‐specific and dosedependent manner, indicating that the protein is an essential spindle component. (shrink)

The position of the spindle determines the position of the cleavage plane, and is thus crucial for cell division. Although spindle positioning has been extensively studied, the underlying forces ultimately responsible for moving the spindle remain poorly understood. A recent pioneering study by Garzon‐Coral et al. uses magnetic tweezers to perform the first direct measurements of the forces involved in positioning the mitotic spindle. Combining this with molecular perturbations and geometrical effects, they use their data (...) to argue that the forces that keep the spindle in its proper position for cell division arise from astral microtubules growing and pushing against the cell's cortex. Here, we review these ground‐breaking experiments, the various biomechanical models for spindle positioning that they seek to differentiate, and discuss new questions raised by these measurements. (shrink)

The mitotic spindle contains the machinery responsible for sister chromatid segregation. It is composed of a complex and dynamic array of microtubules, which are nucleated from the spindle poles. Studies of yeast spindle functions by molecular genetic analysis and by in vitro functional analysis have identified proteins that are mitosis‐specific and present at very low concentrations in the cell, and have revealed the molecular bases of several processes required for the formation and functioning of the (...) class='Hi'>mitotic spindle. Here I review the current knowledge of the processes that are common to most eukaryotes: microtubule nucleation at the spindle poles, bipolar spindle assembly, maintenance of the spindle structure, chromosome attachment to the spindle and chromosome separation on the spindle. (shrink)

Tissues are shaped and patterned by mechanical and chemical processes. A key mechanical process is the positioning of the mitotic spindle, which determines the size and location of the daughter cells within the tissue. Recent force and position-fluctuation measurements indicate that pushing forces, mediated by the polymerization of astral microtubules against­ the cell cortex, maintain the mitotic spindle at the cell center in Caenorhabditis elegans embryos. The magnitude of the centering forces suggests that the physical limit (...) on the accuracy and precision of this centering mechanism is determined by the number of pushing microtubules rather than by thermally driven fluctuations. In cells that divide asymmetrically, anti-centering, pulling forces generated by cortically located dyneins, in conjunction with microtubule depolymerization, oppose the pushing forces to drive spindle displacements away from the center. Thus, a balance of centering pushing forces and anti-centering pulling forces localize the mitotic spindles within dividing C. elegans cells. We discuss microtubule-based mechanisms of the mitotic spindle positioning, a key mechanical process that shapes and patterns tissues. The magnitude of the centering forces indicates that the physical limit on the accuracy and precision of centering is set by the number of contributing microtubules rather than by thermally driven fluctuations. (shrink)

Tissues are shaped and patterned by mechanical and chemical processes. A key mechanical process is the positioning of the mitotic spindle, which determines the size and location of the daughter cells within the tissue. Recent force and position-fluctuation measurements indicate that pushing forces, mediated by the polymerization of astral microtubules against­ the cell cortex, maintain the mitotic spindle at the cell center in Caenorhabditis elegans embryos. The magnitude of the centering forces suggests that the physical limit (...) on the accuracy and precision of this centering mechanism is determined by the number of pushing microtubules rather than by thermally driven fluctuations. In cells that divide asymmetrically, anti-centering, pulling forces generated by cortically located dyneins, in conjunction with microtubule depolymerization, oppose the pushing forces to drive spindle displacements away from the center. Thus, a balance of centering pushing forces and anti-centering pulling forces localize the mitotic spindles within dividing C. elegans cells. We discuss microtubule-based mechanisms of the mitotic spindle positioning, a key mechanical process that shapes and patterns tissues. The magnitude of the centering forces indicates that the physical limit on the accuracy and precision of centering is set by the number of contributing microtubules rather than by thermally driven fluctuations. (shrink)

Recently, Quyn et al. demonstrated that cells within the stem cell zone of human and mouse intestinal crypts tend to align their mitotic spindles perpendicular to the basal membrane of the crypt. This is associated with asymmetric division, whereby particular proteins and individual chromatids are preferentially segregated to one daughter cell. In colonic mucosa containing a heterozygous adenomatous polyposis coli gene (APC) mutation the asymmetry is lost. Here, we discuss asymmetric stem cell division as an anti‐tumourigenic mechanism. We describe (...) how hierarchical tissue structures suppress somatic evolution, and discuss the relative merits of template strand retention to limit the accumulation of DNA replication errors. We suggest experiments to determine whether somatic mutations resulting in loss of spindle alignment confer an advantage within the stem cell niche. Finally, we discuss whether lack of spindle alignment constitutes an oncogenic event per se, with particular reference to studies in model organisms, and the timing of chromosomal instability in human cancers. (shrink)

Continuous poleward motion of microtubules in metazoan mitotic spindles has been fascinating generations of cell biologists over the last several decades. In human cells, this so‐called poleward flux was recently shown to be driven by the coordinated action of four mitotic kinesins. The sliding activities of kinesin‐5/EG5 and kinesin‐12/KIF15 are sequentially supported by kinesin‐7/CENP‐E at kinetochores and kinesin‐4/KIF4A on chromosome arms, with the individual contributions peaking during prometaphase and metaphase, respectively. Although recent data elucidate the molecular mechanism underlying (...) this cellular phenomenon, the functional roles of microtubule poleward flux during cell division remain largely elusive. Here, we discuss potential contribution of microtubule flux engine to various essential processes at different stages of mitosis. (shrink)

Chromosomes are not only carriers of the genetic material, but also actively regulate the assembly of complex intracellular architectures. During mitosis, chromosome‐induced microtubule polymerisation ensures spindle assembly in cells without centrosomes and plays a supportive role in centrosome‐containing cells. Chromosomal signals also mediate post‐mitotic nuclear envelope (NE) re‐formation. Recent studies using novel approaches to manipulate histones in oocytes, where functions can be analysed in the absence of transcription, have established that nucleosomes, but not DNA alone, mediate the chromosomal (...) regulation of spindle assembly and NE formation. Both processes require the generation of RanGTP by RCC1 recruited to nucleosomes but nucleosomes also acquire cell cycle stage specific regulators, Aurora B in mitosis and ELYS, the initiator of nuclear pore complex assembly, at mitotic exit. Here, we review the mechanisms by which nucleosomes control assembly and functions of the spindle and the NE, and discuss their implications for genome maintenance. (shrink)

This bibliographical review of the modelling of the mitotic apparatus covers a period of one hundred and twenty years, from the discovery of the bipolar mitotic spindle up to the present day. Without attempting to be fully comprehensive, it will describe the evolution of the main ideas that have left their mark on a century of experimental and theoretical research. Fol and Bütschli's first writings date back to 1873, at a time when Schleiden and Schwann's cell theory (...) was rapidly gaining ground throughout Germany. Both mitosis and chromosomes were to be discovered within the space of thirty years, along with the two key events in the animal and plant reproductive cycle, namely fecondation and meiosis. The mitotic pole, a term still in use to this day, was employed to describe a morphological fact which was noted as early as 1876, namely that the lines and the dots of the karyokinetic figure, with its spindle and asters, looks remarkably like the lines of force around a bar magnet. This was to lead to models designed to explain the movements of chromosomes which take place when the cell nucleus appears to cease to exist as an organelle during mitosis. The nature of those mechanisms and the origin of the forces behind the chromosomes' ordered movements were central to the debate. Auguste Prenant, in a remarkable bibliographical synthesis published in 1910, summed up the opposing viewpoints of the vitalists, on the one hand, who favoured the theory of contractility or extensility in spindle fibres, and of those who believed in models based on physical phenomena, on the other. The latter subdivided into two groups: some, like Bütschli, Rhumbler or Leduc, referred to diffusion, osmosis and superficial tension, whilst the others, led by Gallardo and Hartog, focussed on the laws of electromagnetism. Lillie, Kuwada and Darlington followed up this line of research. The mid-20th century was a major turning point. Most of the modelling mentioned above was criticized and fell into disuse after disappearing from research publications and textbooks.This marked the onset of a new era, as electron microscopes made possible the materialization and detailed study of the macromolecular elements of the fibres, filaments and microtubules of the cytoskeleton. The successive phases of (a) de Harven and Bernhard's 1956 discovery of the centriole's ultrastructure, (b) its identification with the basal body of the cilia and flagella, confirming the theory set out by Henneguy and von Lenhossek (1898–99), (c) the universal presence of microtubules in animal, vegetal and eukaryotic protist cells, (d) the polymerization-depolymerization induced reversible transformations of the tubulin pool in mitosing cells (Inoue, 1960), (e) ultrastructural comparative studies of the mitotic apparatus of eukaryotes illustrating the Pickett-Heaps integrating concept of the MTOC (microtubule-organizing centre), (f) the possibility ofin vitro experiments on mtocs or on microtubules, brings us upon the present day, which has seen the focus placed on the concept of motor-proteins (kinesin, dynein) and on cell cycle models. The latter are based on a close coincidence between the observable modifications of the mitotic apparatus and the periodic variations in intracellular concentrations of calcium or of certain enzymes (cyclins, Cdc2) during the main transitions of the cell cycle. (shrink)

Polo‐like kinase 1 (PLK1) is a serine/threonine kinase that plays multiple and essential roles during the cell division cycle. Its inhibition in cultured cells leads to severe mitotic aberrancies and cell death. Whereas previous reports suggested that Plk1 depletion in mice leads to a non‐mitotic arrest in early embryos, we show here that the bi‐allelic Plk1 depletion in mice certainly results in embryonic lethality due to extensive mitotic aberrations at the morula stage, including multi‐ and mono‐polar spindles, (...) impaired chromosome segregation and cytokinesis failure. In addition, the conditional depletion of Plk1 during mid‐gestation leads also to severe mitotic aberrancies. Our data also confirms that Plk1 is completely dispensable for mitotic entry in vivo. On the other hand, Plk1 haploinsufficient mice are viable, and Plk1‐heterozygous fibroblasts do not harbor any cell cycle alterations. Plk1 is overexpressed in many human tumors, suggesting a therapeutic benefit of inhibiting Plk1, and specific small‐molecule inhibitors for this kinase are now being evaluated in clinical trials. Therefore, the different Plk1 mouse models here presented are a valuable tool to reexamine the relevance of the mitotic kinase Plk1 during mammalian development and animal physiology. (shrink)

Although alterations in Ras signalling are found in about 30% of human cancers, the transforming activity of oncogenic Ras is not fully understood. In a recent paper, a putative Ras1 effector in S. pombe, named Scd1, was reported to localize to mitotic apindies. Scd1 physically associates with Moe1, a factor that may contribute to the inherent inatability of microtubules (MTs) and appears to be needed for proper apindle function. Altered MT dynamics within the spindle are likely to affect (...) spindle assembly and chromosome capture, processes that need to be delicately controlled if cells are to guard against genome instability adn transformation. BloEssays 23: 307‐310,2001.©2001 John Willey & Sons, Inc. (shrink)

Asymmetric cell division generates cell diversity and contributes to cellular aging and rejuvenation. Here, we review the molecular mechanisms enabling budding yeast to recognize spindle pole bodies (SPB, centrosome equivalent) based on their age, and guide their non‐random mitotic segregation: SPB inheritance requires the distinction of old from new SPBs and is regulated by the SPB‐inheritance network (SPIN) and the mitotic exit network (MEN). The SPIN marks the pre‐existing SPB as old and the MEN recognizes these marks (...) translating them into spindle orientation. We next revisit other molecules and structures that partition depending on their age rather than their abundance at mitosis as, for example, DNA, centrosomes, mitochondria, and histones in yeast and other systems. The recurrence of this differential behavior suggests a functional significance for numerous cell types, which we then discuss. We conclude that non‐random segregation may facilitate asymmetric cell fate determination and thereby indirectly aging and rejuvenation. (shrink)

The fate of cells arrested in mitosis by antimitotic compounds is complex but is influenced by competition between pathways promoting cell death and pathways promoting mitotic exit. As components of both of these pathways are regulated by Cdc20‐dependent degradation, I hypothesize that variations in Cdc20 protein levels, rather than mutations in checkpoint genes, could affect cell fate during prolonged mitotic arrest. This hypothesis is supported by experiments where manipulation of Cdc20 levels affects the response to antimitotic compounds. The (...) observed differences in Cdc20 levels between cell lines likely reflects differences in the rate of synthesis or degradation of the protein; therefore, understanding these pathways at a molecular level could pave the way for modulating the activity of Cdc20, in turn presenting novel therapeutic possibilities. (shrink)

Beta‐catenin is a multifunctional protein with critical roles in cell‐cell adhesion, Wnt‐signaling and the centrosome cycle. Whereas the roles of β‐catenin in cell‐cell adhesion and Wnt‐signaling have been studied extensively, the mechanism(s) involving β‐catenin in centrosome functions are poorly understood. β‐Catenin localizes to centrosomes and promotes mitotic progression. NIMA‐related protein kinase 2 (Nek2), which stimulates centrosome separation, binds to and phosphorylates β‐catenin. β‐Catenin interacting proteins involved in Wnt signaling such as adenomatous polyposis coli, Axin, and GSK3β, are also localized (...) at centrosomes and play roles in promoting mitotic progression. Additionally, proteins associated with cell‐cell adhesion sites, such as dynein, regulate mitotic spindle positioning. These roles of proteins at the cell cortex and Wnt signaling that involve β‐catenin indicate a cross‐talk between different sub‐cellular sites in the cell at mitosis, and that different pools of β‐catenin may co‐ordinate centrosome functions and cell cycle progression. (shrink)

Faithful DNA replication and accurate chromosome segregation are the key machineries of genetic transmission. Disruption of these processes represents a hallmark of cancer and often results from loss of tumor suppressors. PTEN is an important tumor suppressor that is frequently mutated or deleted in human cancer. Loss of PTEN has been associated with aneuploidy and poor prognosis in cancer patients. In mice, Pten deletion or mutation drives genomic instability and tumor development. PTEN deficiency induces DNA replication stress, confers stress tolerance, (...) and disrupts mitotic spindle architecture, leading to accumulation of structural and numerical chromosome instability. Therefore, PTEN guards the genome by controlling multiple processes of chromosome inheritance. Here, we summarize current understanding of the PTEN function in promoting high-fidelity transmission of genetic information. We also discuss the PTEN pathways of genome maintenance and highlight potential targets for cancer treatment. PTEN is a guardian of the genome. However, the role of PTEN in guarding the genome has not been revealed until recently. PTEN controls multiple fundamental processes of genomic transmission. By physically interacting with key molecules in DNA replication, DNA repair/decatenation, and chromosome segregation during the cell cycle, PTEN maintains genomicintegrity and fitness. (shrink)

Zygote cytokinesis produces two symmetric blastomeres, which contain one copy of each parental genome. Contrary to this dogma, we recently discovered that mammalian zygotes can spontaneously segregate entire parental genomes into different blastomeres and coined this novel form of genome segregation heterogoneic division. The molecular mechanisms underlying the emergence of blastomeres with different parental genomes during the first mitotic cycle remain to be elucidated. Here, we speculate on which parental genome asymmetries could provide a mechanistic foundation for these remarkable (...) zygote divisions. In reviewing the field and considering our findings, we revisit the architecture of the first zygotic metaphase by invoking asymmetric interactions between the mitotic spindle and the parental kinetochores. We also speculate on how asynchronous parental cell cycles can be a source of heterogoneic zygote divisions through the formation of parental genome private spindles. (shrink)

How do cells direct the microtubule motor protein dynein to move cellular components to the right place at the right time? Recent studies in budding yeast shed light on a new mechanism for directing dynein, involving the protein She1. She1 restricts where and when dynein moves the nucleus and mitotic spindle. Experiments with purified proteins show that She1 binds to microtubules and inhibits dynein by stalling the motor on its track. Here I describe what we have learned so (...) far about She1, based on a combination of genetic, cell biology, and biophysical approaches. These findings set the stage for further interrogation of the She1 mechanism, and raise the question of whether similar mechanisms exist in other species. (shrink)

Recent evidence shows that dynamic instability is the dominant mechanism for the assembly of pure tubulin in vitro and for the great majority of microtubules in the mitotic spindle and the interphase cytoplasmic microtubule complex. The basic concepts of this model provide a framework for future characterization of the molecular basis of spatial and temporal regulation of microtubule dynamics in the cell and the function of microtubule dynamics in motile processes such as chromosome movement.

The mammalian centromere is a multifunctional chromosomal domain with a complexity that is reflected in its higher order structure, DNA sequence organization and protein composition. The centromere plays a major role during cell division where it functions as the site for the integration of the chromosome with the mitotic spindle, the site of the mechanochemical motor responsible for the movement of chromosomes and the major and last point of interaction between sister chromatids. Recent studies have focused on characterizing (...) the components of the centromere and establishing their relationship to its function. The following brief review summarizes some selected aspects of this recent work. (shrink)

Cell cycle arrest in M phase can be induced by the failure of a single chromosome to attach properly to the mitotic spindle. The same cell cycle checkpoint mediates M phase arrest when cells are treated with drugs that either disrupt or hyperstabilize spindle microtubules. Study of yeast mutants that fail to arrest in the presence of microtubule disruptors identified a set of genes important in this checkpoint pathway. Two recent papers report the cloning of human and (...) Xenopus homologues of one of these yeast genes, called MAD2 (for mitotic arrest deficient‐2)(1,2). Introduction of antibodies to the MAD2 protein into living mammalian cells or Xenopus egg extracts abrogates the M phase arrest induced by microtubule inhibitors. This and other recent developments suggest a model for the M phase checkpoint in which unattached kinetochores inhibit the ubiquitination of proteins whose proteolysis is necessary for chromatid separation and exit from mitosis. (shrink)

The nature of the forces that move chromosomes in mitosis is beginning to be revealed. The kinetochore, a specialized structure situated at the primary constriction of the chromosome, appears to translocate in both directions along the microtubules of the mitotic spindle. One or more members of the newly described families of microtubule motor molecules may power these movements. Microtubules of the mitotic spindle undergo rapid cycles of assembly and disassembly. These microtubule dynamics may contribute toward generating (...) force and regulating direction in chromosome movement. (shrink)

Hameroff: I became interested in understanding consciousness as an undergraduate at the University of Pittsburgh in the late 60's. In my third year of medical school at Hahnemann in Philadelphia I did a research elective in professor Ben Kahn's hematology-oncology lab. They were studying various types of malignant blood cells, and I became interested in mitosis-looking under the microscope at normal and abnormal cell division. I became fascinated by centrioles and mitotic spindles pulling apart the chromosomes, doing this little (...) dance, dividing the cytoplasm, establishing the daughter-cell architecture, and beginning differentiation. I remember wondering to myself how these centrioles and mitotic spindles "knew" where to go and what to do. What kind of intelligence was running the show at the cellular level? (shrink)

The molecular signals that determine the position and timing of the furrow that forms during mammalian cell cytokinesis are presently unknown. It is apparent, however, that these signals are generated by the mitotic spindle after the onset of anaphase. Recently we have described a structure that bisects the cell during telophase at the position of the cytokinetic furrow. This structure, the telephase disc, appears to the templated by the motitc spindle during anaphase, and precedes the formation of (...) the cytokinetic furrow. The relationship of the telephase disc to the myosin and actin based furrowing mechanism is discussed here. We propose that the telophase disc may determine the position and timing of cleavage by recruitment and alignment of myosin. (shrink)

Cytolkinesis ensures proper partitioning of the nucleocytoplasmic contents into two daughter cells. It has generally been thought that cytokinesis is accomplished differently in animals and plants because of the differences in the preparatory phases, into the centrosomal or acentrosomal nature of the process, the presence or absence of rigid cell walls, and on the basis of 'outside-in' or 'inside-out' mechanism. However, this long-standing paradigm needs further reevaluation based on new findings. Recent advances reveal that plant cells, similarly to animal cells, (...) possess astral microtubuies that regulate the cell division plane. Furthermore, endocytosis has been found to be important for cytokinesis in animal and plant cells: vesicles containing endocytosed cargo provide material for the cell plate formation in plants and for closure of the midbody channel in animals. Thus, although the preparatory phases of the cell division process differ between plant and animal cells, the later phases show similarities. We unify these findings in a model that suggests a conserved mode of cytokinesis. (shrink)

Balancing self‐renewal and differentiation of stem cells is an important issue in stem cell and cancer biology. Recently, the Drosophila neuroblast (NB), neural stem cell has emerged as an excellent model for stem cell self‐renewal and tumorigenesis. It is of great interest to understand how defects in the asymmetric division of neural stem cells lead to tumor formation. Here, we review recent advances in asymmetric division and the self‐renewal control of Drosophila NBs. We summarize molecular mechanisms of asymmetric cell division (...) and discuss how the defects in asymmetric division lead to tumor formation. Gain‐of‐function or loss‐of‐function of various proteins in the asymmetric machinery can drive NB overgrowth and tumor formation. These proteins control either the asymmetric protein localization or mitotic spindle orientation of NBs. We also discuss other mechanisms of brain tumor suppression that are beyond the control of asymmetric division. (shrink)

The cellular processes of transport, division and, possibly, early development all involve microtubule‐based motors. Recent work shows that, unexpectedly, many of these cellular functions are carried out by different types of kinesin and kinesin‐related motor proteins. The kinesin proteins are a large and rapidly growing family of microtubule‐motor proteins that share a 340‐amino‐acid motor domain. Phylogenetic analysis of the conserved motor domains groups the kinesin proteins into a number of subfamilies, the members of which exhibit a common molecular organization and (...) related functions. The kinesin proteins that belong to different subfamilies differ in their rates and polarity of movement along microtubules, and probably in the particles/organelles that they transport. The kinesins arose early in eukaryotic evolution and gene duplication has allowed functional specialization to occur, resulting in a surprisingly large number of different classes of these proteins adapted for intracellular transport of vesicles and organelles, and for assembly and force generation in the meiotic and mitotic spindles. (shrink)

Cytokinesis in animal cells is accomplished by the active constriction of the equatorial regions of a cell by an actomyosin‐containing contractile ring. The mitotic apparatus specifies the position and orientation of the furrow such that the mitotic spindle is always bisected. Global cortical contractions occur in the cortex of a cell prior to cytokinesis that are independent of the presence of the mitotic apparatus. It was proposed some years ago that the asters of the mitotic (...) apparatus could act to relax the preformed cortical tension in their vicinity. This would produce a differential in tension between the equatorial regions and the adjacent regions of the cortex so that the equatorial regions would contract, forming a cleavage furrow. It can be shown that, as it stands, this theory cannot explain cleavage. However, if cortical contractile elements are assumed to be laterally mobile in the plane of the cortex, then the astral relaxation theory can account for many of the aspects of cleavage, including the formation of the contractile ring. Similar schemes may account for the behaviour of the lamellapodia of motile cells. (shrink)

To ensure accurate chromosome segregation during mitosis, the spindle checkpoint monitors chromosome alignment on the mitotic spindle. Indjeian and colleagues have investigated the precise role of the shugoshin 1 protein (Sgo1p) in this process in budding yeast.1 The Sgo proteins were originally identified as highly conserved proteins that protect cohesion at centromeres during the first meiotic division. Together with other recent findings,2 the study highlighted here has identified Sgo1 as a component that informs the mitotic (...) class='Hi'>spindle checkpoint when spindle tension is perturbed. This discovery has provided a molecular link between sister chromatid cohesion and tension‐sensing at the kinetochore–microtubule interface. BioEssays 27:588–591, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

The human spindle and kinetochore associated complex is required for proper mitotic progression. Extensive studies have demonstrated its important functions in both stable kinetochore-microtubule interactions and spindle checkpoint silencing. We suggest a model to explain how various Ska functions might be fulfilled by distinct pools of Ska at kinetochores. The Ndc80-loop pool of Ska is recruited by the Ndc80 loop, or together with some of its flanking sequences, and the recruitment is also dependent on Cdk1-mediated Ska3 phosphorylation. (...) This pool seems to play a more important role in silencing the spindle checkpoint than stabilizing kinetochore-microtubule interactions. In contrast, the Ndc80-N-terminus pool of Ska is recruited by the N-terminal domains of Ndc80 and appears to be more important for stabilizing kinetochore-microtubule interactions. Here, we review and discuss the evidence that supports this model and suggest further experiments to test the functioning mechanisms of the Ska complex. The human spindle and kinetochore associated complex plays essential functions in proper mitotic progression by promoting stable kinetochore-microtubule interactions and spindle checkpoint silencing. We propose that “distinct pools” of Ska—the Ndc80-loop and Ndc80-N-terminus pools—might fulfill these various functions. (shrink)

The relationship between kinetochores and nuclear pore complexes (NPCs) is intimate but poorly understood. Several NPC components and associated proteins are relocated to mitotic kinetochores to assist in different activities that ensure faithful chromosome segregation. Such is the case of the Mad1‐c‐Mad2 complex, the catalytic core of the spindle assembly checkpoint (SAC), a surveillance pathway that delays anaphase until all kinetochores are attached to spindle microtubules. Mad1‐c‐Mad2 is recruited to discrete domains of unattached kinetochores from where it (...) promotes the rate‐limiting step in the assembly of anaphase‐inhibitory complexes. SAC proficiency further requires Mad1‐c‐Mad2 to be anchored at NPCs during interphase. However, the mechanistic relevance of this arrangement for SAC function remains ill‐defined. Recent studies uncover the molecular underpinnings that coordinate the release of Mad1‐c‐Mad2 from NPCs with its prompt recruitment to kinetochores. Here, current knowledge on Mad1‐c‐Mad2 function and spatiotemporal regulation is reviewed and the critical questions that remain unanswered are highlighted. (shrink)

Here we discuss a “chromosome separation checkpoint” that might regulate the anaphase‐telophase transition. The concept of cell cycle checkpoints was originally proposed to account for extrinsic control mechanisms that ensure the order of cell cycle events. Several checkpoints have been shown to regulate major cell cycle transitions, namely at G1‐S and G2‐M. At the onset of mitosis, the prophase‐prometaphase transition is controlled by several potential checkpoints, including the antephase checkpoint, while the spindle assembly checkpoint guards the metaphase‐anaphase transition. Our (...) hypothesis is based on the recently uncovered feedback control mechanism that delays chromosome decondensation and nuclear envelope reassembly until effective separation of sister chromatids during anaphase is achieved. A central player in this potential checkpoint is the establishment of a constitutive, midzone‐based Aurora B phosphorylation gradient that monitors the position of chromosomes along the spindle axis. We propose that this surveillance mechanism represents an additional step towards ensuring mitotic fidelity. (shrink)

In eukaryotic cells a specialized organelle called the microtubule organizing center (MTOC) is responsible for disposition of microtubules in a radial, polarized array in interphase cells and in the spindle in mitotic cells. Eukaryotic cells across different species, and different cell types within single species, have morphologically diverse MTOCs, but these share a common function of organizing microtubule arrays. MTOCs effect microtubule organization by initiating microtubule assembly and anchoring microtubules by their slowly growing minus ends, thus ensuring that (...) the rapidly growing plus ends extend distally in each microtubule array. The goal is to define molecular components of the MTOC responsible for regulating microtubule assembly. One approach to defining the molecules responsible for MTOC function is to look for molecules common to all MTOCs. A newly discovered centrosomal protein, γ‐tubulin, is found in MTOCs in cells from many different organisms, and has several properties which make it a candidate for both initiation of microtubule assembly and anchorage. The hypothesis that γ‐tubulin plays a role in MTOCs in microtubule initiation and anchorage is currently being tested by a variety of experimental approaches. (shrink)

In budding yeast, commitment to meiosis is attained when meiotic cells cannot return to the mitotic cell cycle even if the triggering cue (nutrients deprivation) is withdrawn. Commitment is arrived at gradually, and different aspects of meiosis may be committed at different times. Cells become fully committed to meiosis at the end of Prophase I, long after DNA replication and just before the first meiotic division (MI). Whole‐genome gene expression analysis has shown that committed cells have a distinct and (...) rapid response to nutrients, and are not simply insulated from environmental signals. Thus becoming committed to meiosis is an active process. The cellular event most likely to be associated with commitment to meiosis is the separation of the duplicated spindle‐pole bodies (SPBs) and the formation of the spindle. Commitment to the mitotic cell cycle is also associated with the separation of SPBs, although it occurs in G1, before DNA replication. (shrink)

Within the last decade, the study of mitosis has evolved into a multidisciplinary science in which findings from fields as diverse as chromosome biology and cytoskeletal architecture have converged to present a more cohesive understanding of the complex events that occur when cells divide. The largest strides have been made in the identification and characterization of regulatory enzymes (kinases and phosphatases) that modulate mitotic activity, as well as a number of the proteins and structural components (spindle, chromosomes, nuclear (...) envelope) which carry out the mitotic instructions. One emerging theme appears to be that molecular signalling, which can involve modification of components (such as phosphorylation) or even their specific destruction, monitors the state of the mitotic cell at all stages. One of the major challenges for the future will be the identification of addititonal targets of the signalling machinery, as well as new regulatory components and their targets on the chromosomes, on the spindle, and at the cleavage furrow. (shrink)

A framework for understanding the complex movements of mitosis and meiosis has been provided by the recent discovery of microtubule motor proteins, required for the proper distribution of chromosomes or the structural integrity of the mitotic or meiotic spindle. Although overall features of mitosis and meiosis are often assumed to be similar in mechanism, it is now clear that they differ in several important aspects. These include spindle structure and assembly, and timing of chromosome segregation to opposite (...) poles. Here we review progress in the functional characterization of several newly identified microtubule motor proteins, emphasizing their possible roles in spindle structure and function. (shrink)

During mitosis, cells comprehensively restructure their interior to promote the faithful inheritance of DNA and cytoplasmic contents. In metazoans, this restructuring entails disassembly of the nuclear envelope, redistribution of its components into the endoplasmic reticulum (ER) and eventually nuclear envelope reassembly around the segregated chromosomes. The microtubule cytoskeleton has recently emerged as a critical regulator of mitotic nuclear envelope and ER dynamics. Microtubules and associated molecular motors tear open the nuclear envelope in prophase and remove nuclear envelope remnants from (...) chromatin. Additionally, two distinct mechanisms of microtubule‐based regulation of ER dynamics operate later in mitosis. First, association of the ER with microtubules is reduced, preventing invasion of ER into the spindle area, and second, organelle membrane is actively cleared from metaphase chromosomes. However, we are only beginning to understand the role of microtubules in shaping and distributing ER and other organelles during mitosis. (shrink)

Chromosome ends have been implicated in the meiotic processes of the nematode Caenorhabditis elegans. Cytological observations have shown that chromosome ends attach to the nuclear membrane and adopt kinetochore functions. In this organism, centromeric activity is highly regulated, switching from multiple spindle attachments all along the chromosome during mitotic division to a single attachment during meiosis. C. elegans chromosomes are functionally monocentric during meiosis. Earlier genetic studies demonstrated that the terminal regions of the chromosomes are not equivalent in (...) their meiotic potentials. There are asymmetries in the abilities of the ends to recombine when duplicated or deleted. In addition, mutations in single genes have been identified that mimic the meiotic effects of a terminal truncation of the X chromosome. The recent cloning and characterization of the C. elegans telomeres has provided a starting point for the study of chromosomal elements mediating the meiotic process. (shrink)

Sister chromatid separation in anaphase is an important event in the cell's transmission of genetic information to a descendent. It has been investigated from different aspects: cell cycle regulation, spindle and chromosome dynamics within the three‐dimensional cell architecture, transmission fidelity control and cellular signaling. Integrated studies directed toward unified understanding are possible using multidisciplinary methods with model organisms. Ubiquitin‐dependent proteolysis, protein dephosphorylation, an unknown function by the TPR repeat proteins, chromosome transport by microtubule‐based motors and DNA topological change by (...) DNA topoisomerase II are all necessary for progression from metaphase to anaphase. Chromosome condensation, mitotic kinetochore function and spindle formation require a large number of proteins, which are prerequisites for successful sister chromatid separation. Factors that help to retain sister chromatid connection after replication and prevent premature separation remain to be determined. Although sister chromatid separation occurs in anaphase, gene functions in other cell cycle stages also ensure the progression of correct chromatid separation. (shrink)

Stable maintenance of genetic information during meiosis and mitosis is dependent on accurate chromosome transmission. The centromere is a key component of the segregational machinery that couples chromosomes with the spindle apparatus. Most of what is known about the structure and function of the centromeres has been derived from studies on yeast cells. In Saccharomyces cerevisiae, the centromere DNA requirements for mitotic centromere function have been defined and some of the proteins required for an active complex have been (...) identified. Centromere DNA and the centromere proteins form a complex that has been studied extensively at the chromatin level. Finally, recent findings suggest that assembly and activation of the centromere are integrated in tethe cell cycle. (shrink)

Realism about mental disorders is a perennial area of dispute, but the controversy burns especially intensely for Attention Deficit Hyperactivity Disorder. In this dissertation, I clarify what is at issue in these debates, surveying how realists have typically argued for mental disorder realism: the definitional debate about health and illness. I argue that the realist need not be committed to the terms of the definitional debate and recommend that a better approach is to show that mental disorders are natural kinds. (...) While there are many accounts of kind-hood on offer, I adopt Richard Boyd’s homeostatic property cluster theory of kinds, which I interpret through the philosophy of neuroscience literature on mechanisms. In sum, I conclude that if ADHD is a natural kind – and thus real – then individuals diagnosed with the disorder should be sufficiently similar with respect to an underlying cognitive neurobiological mechanism. To determine whether ADHD individuals are similar in this way, I consider the question through Russell Barkley’s Executive Function Model of ADHD. Relying primarily on the cognitive neurobiological research, I argue that there is now reasonable evidence to conclude that the DSM classification of ADHD corresponds not to a single natural kind, but several. Thus, ADHD is thus real. (shrink)

This study surveyed investors to determine the extent to which they preferred ethical behavior to profits and their interest in having information about corporate ethical behavior reported in the corporate annual report. First, investors were asked to determine what penalties should be assessed against employees who engage in profitable, but unethical, behavior. Second, investors were asked about their interest in using the annual report to disclose the ethical performance of the corporation and company officials. Finally, investors were asked if they (...) felt that ethics reports should be audited.The survey results indicate that many shareholders (42%) do not expect a high level of ethical behavior from corporate employees or officers. There is a significant amount of interest in disclosure of ethical issues (72%) and unwillingness to trust management to provide unbiased reports of ethical behavior. If such reports are included with the financial statements, 32 percent of the investors surveyed would prefer to have them audited to provide independent verification. (shrink)

A completely mitotic computably enumerable degree is a c.e. degree in which every c.e. set is mitotic, or equivalently in which every c.e. set is autoreducible. There are known to be low, low2, and high completely mitotic degrees, though the degrees containing non-mitotic sets are dense in the c.e. degrees. We show that there exists an upper cone of c.e. degrees each of which contains a non-mitotic set, and that the completely mitotic c.e. degrees (...) are nowhere dense in the c.e. degrees. We also show that there is a set computably enumerable in and above 0′ which is not the jump of any completely mitotic degree. (shrink)

How eukaryotic genomes are packaged into compact cylindrical chromosomes in preparation for cell divisions has remained one of the major unsolved questions of cell biology. Novel approaches to study the topology of DNA helices inside the nuclei of intact cells, paired with computational modeling and precise biomechanical measurements of isolated chromosomes, have advanced our understanding of mitotic chromosome architecture. In this Review Essay, we discuss – in light of these recent insights – the role of chromatin architecture and the (...) functions and possible mechanisms of SMC protein complexes and other molecular machines in the formation of mitotic chromosomes. Based on the information available, we propose a stepwise model of mitotic chromosome condensation that envisions the sequential generation of intra‐chromosomal linkages by condensin complexes in the context of cohesin‐mediated inter‐chromosomal linkages, assisted by topoisomerase II. The described scenario results in rod‐shaped metaphase chromosomes ready for their segregation to the cell poles. (shrink)

Studies in the yeast Saccharomyces cerevisiae have validated the major features of the double‐strand break repair (DSBR) model as an accurate representation of the pathway through which meiotic crossovers (COs) are produced. This success has led to this model being invoked to explain double‐strand break (DSB) repair in other contexts. However, most non‐crossover (NCO) recombinants generated during S. cerevisiae meiosis do not arise via a DSBR pathway. Furthermore, it is becoming increasingly clear that DSBR is a minor pathway for recombinational (...) repair of DSBs that occur in mitotically‐proliferating cells and that the synthesis‐dependent strand annealing (SDSA) model appears to describe mitotic DSB repair more accurately. Fundamental dissimilarities between meiotic and mitotic recombination are not unexpected, since meiotic recombination serves a very different purpose (accurate chromosome segregation, which requires COs) than mitotic recombination (repair of DNA damage, which typically generates NCOs). (shrink)