Kinetochores can form and be maintained on DNA sequences that are normally non‐centromeric. The existence of these so‐called neo‐centromeres has posed the problem as to the nature of the epigenetic mechanisms that maintain the centromere. Here we highlight results that indicate that the amount of CENP‐A at human centromeres is tightly regulated. It is also known that kinetochore assembly requires sister chromatid cohesion at mitosis. We therefore suggest that separation or stretching between the sister chromatids at metaphase reciprocally determines (...) the amount of centromere assembly in the subsequent interphase. This reciprocal relationship forms the basis of a negative feedback loop that could precisely control the amount of CENP‐A and faithfully maintain the presence of a kinetochore over many cell divisions. We describe how the feedback loop would work, propose how it could be tested experimentally and suggest possible components of its mechanism. (shrink)

Correct chromosome segregation in mitosis relies on chromosome biorientation, in which sister kinetochores attach to microtubules from opposite spindle poles prior to segregation. To establish biorientation, aberrant kinetochore–microtubule interactions must be resolved through the error correction process. During error correction, kinetochore–microtubule interactions are exchanged (swapped) if aberrant, but the exchange must stop when biorientation is established. In this article, we discuss recent findings in budding yeast, which have revealed fundamental molecular mechanisms promoting this “swap and stop” process for error (...) correction. Where relevant, we also compare the findings in budding yeast with mechanisms in higher eukaryotes. Evidence suggests that Aurora B kinase differentially regulates kinetochore attachments to the microtubule end and its lateral side and switches relative strength of the two kinetochore–microtubule attachment modes, which drives the exchange of kinetochore–microtubule interactions to resolve aberrant interactions. However, Aurora B kinase, recruited to centromeres and inner kinetochores, cannot reach its targets at kinetochore–microtubule interface when tension causes kinetochore stretching, which stops the kinetochore–microtubule exchange once biorientation is established. (shrink)

Segregation of chromosomes during mitosis requires the interaction of dynamic microtubules with the kinetochore, a large protein structure established on the centromere region of sister chromatids. The core microtubule‐binding activity of the kinetochore resides in the KMN network, an outer kinetochore complex. As part of the KMN network, the Ndc80 complex, which is composed of Ndc80, Nuf2, Spc24, and Spc25, is able to bind directly to microtubules and has the ability to track with depolymerizing microtubules to produce chromosome movement. The (...) Ndc80 complex binds directly to microtubules through a calponin homology domain and an unstructured tail in the N terminus of the Ndc80 protein. A recent flurry of papers has highlighted the importance of an internal loop region in Ndc80 in establishing end‐on attachment to microtubules. Here I discuss these recent findings that suggest that the Ndc80 internal loop functions as a binding site for proteins required for kinetochore‐microtubule interactions. (shrink)

We apply molecular code theory to a rule-based model of the human inner kinetochore and study how complex formation in general can give rise to molecular codes. We analyze 105 reaction networks generated from the rule-based inner kinetochore model in two variants: with and without dissociation of complexes. Interestingly, we found codes only when some but not all complexes are allowed to dissociate. We show that this is due to the fact that in the kinetochore model proteins can only bind (...) at kinetochores by attaching to already attached proteins and cannot form complexes in free solution. Using a generalized linear mixed model we study which centromere protein can take which role in a molecular code . By this, associations between CENPs and code roles are found. We observed that CenpA is a major risk factor while CenpQ is a major protection factor . Finally we show, using an abstract model of copolymer formation, that molecular codes can also be realized solely by the formation of stable complexes, which do not dissociate. For example, with particular dimers as context a molecular code mapping from two different monomers to two particular trimers can be realized just by non-selective complex formation. We conclude that the formation of protein complexes can be utilized by the cell to implement molecular codes. Living cells thus facilitate a subsystem allowing for an enormous flexibility in the realization of mappings, which can be used for specific regulatory processes, e.g. via the context of a mapping. (shrink)

We now take for granted that despite the disproportionate contribution of females to initial growth of their progeny, there is little or no asymmetry in the contribution of males and females to the eventual character of their shared offspring. In fact, this key insight was only established towards the end of the eighteenth century by Joseph Koelreuter's pioneering plant breeding experiments. If males and females supply equal amounts of hereditary material, then the latter must double each time an embryo is (...) conceived. How then does the amount of this mysterious stuff not multiply exponentially from generation to generation? A compensatory mechanism for diluting the hereditary material must exist, one that ensures that if each parent contributes one half, each grandparent contributes a quarter, and each great grandparent merely an eighth. An important piece of the puzzle of how hereditary material is diluted at each generation has been elucidated over the past ten years. (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)

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 centromere is a specialized chromosomal structure that dictates kinetochore assembly and, thus, is essential for accurate chromosome segregation. Centromere identity is determined epigenetically by the presence of a centromere‐specific histone H3 variant, CENP‐A, that replaces canonical H3 in centromeric chromatin. Here, we discuss recent work by Roulland et al. that identifies structural elements of the nucleosome as essential determinants of centromere function. In particular, CENP‐A nucleosomes have flexible DNA ends due to the short αN helix of CENP‐A. The higher (...) flexibility of the DNA ends of centromeric nucleosomes impairs binding of linker histones H1, while it facilitates binding of other essential centromeric proteins, such as CENP‐C, and is required for mitotic fidelity. This work extends previous observations indicating that the differential structural properties of CENP‐A nucleosomes are on the basis of its contribution to centromere identity and function. Here, we discuss the implications of this work and the questions arising from it. (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)

Meiotic defects cause abnormal chromosome segregation leading to aneuploidy in mammalian oocytes. Chromosome segregation is particularly error‐prone in human oocytes, but the mechanisms behind such errors remain unclear. To explain the frequent chromosome segregation errors, recent investigations have identified multiple meiotic defects and explained how these defects occur in female meiosis. In particular, we review the causes of cohesin exhaustion, leaky spindle assembly checkpoint (SAC), inherently unstable meiotic spindle, fragmented kinetochores or centromeres, abnormal aurora kinases (AURK), and clinical genetic (...) variants in human oocytes. We mainly focus on meiotic defects in human oocytes, but also refer to the potential defects of female meiosis in mouse models. (shrink)

SummaryMis‐regulation (e.g. overproduction) of the human Ndc80/Hec1 outer kinetochore protein has been associated with aneuploidy and tumourigenesis, but the genetic basis and underlying mechanisms of this phenomenon remain poorly understood. Recent studies have identified the ubiquitous Ndc80 internal loop as a protein‐protein interaction platform. Binding partners include the Ska complex, the replication licensing factor Cdt1, the Dam1 complex, TACC‐TOG microtubule‐associated proteins (MAPs) and kinesin motors. We review the field and propose that the overproduction of Ndc80 may unfavourably absorb these interactors (...) through the internal loop domain and lead to a change in the equilibrium of MAPs and motors in the cells. This sequestration will disrupt microtubule dynamics and the proper segregation of chromosomes in mitosis, leading to aneuploid formation. Further investigation of Ndc80 internal loop‐MAPs interactions will bring new insights into their roles in kinetochore‐microtubule attachment and tumourigenesis. (shrink)

The most striking region of structural differentiation of a eukaryotic chromosome is the kinetochore. This chromosomal domain plays an integral role in the stability and propagation of genetic material to the progeny cells during cell division. The DNA component of this structure, which we refer to as the centromere, has been localized to a small region of 220–250 base pairs within the chromosomes from the yeast Saccharomyces cerevisiae. The centromere DNA (CEN) is organized in a unique structure in the cell (...) nucleus and is required for chromosome stability during both mitotic and meiotic cell cycles. The centromeres from one chromosome can stabilize small circular minichromosomes or other yeast chromosomes. The centromeres may therefore interact with the same components of the segregation apparatus regardless of the chromosome in which they reside. The CEN DNA does not encode any regulatory RNAs or proteins, but rather is a cis‐acting element that provides genetic stability to adjacent DNA sequences. (shrink)

The centromere is the region of the chromosome where the kinetochore forms. Kinetochores are the attachment sites for spindle microtubules that separate duplicated chromosomes in mitosis and meiosis. Kinetochore formation depends on a special chromatin structure containing the histone H3 variant CENP‐A. The epigenetic mechanisms that maintain CENP‐A chromatin throughout the cell cycle have been studied extensively but little is known about the mechanism that targets CENP‐A to naked centromeric DNA templates. In a recent report published in Science,1 such (...) de novo centromere assembly of CENP‐A is shown to be dependent on heterochromatin and the RNA interference pathway. BioEssays 30:526–529, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

Research over the last two decades has identified a group of meiosis‐specific proteins, consisting of budding yeast Spo13, fission yeast Moa1, mouse MEIKIN, and Drosophila Mtrm, with essential functions in meiotic chromosome segregation. These proteins, which we call meiosis I kinase regulators (MOKIRs), mediate two major adaptations to the meiotic cell cycle to allow the generation of haploid gametes from diploid mother cells. Firstly, they promote the segregation of homologous chromosomes in meiosis I (reductional division) by ensuring that sister (...) class='Hi'>kinetochores face towards the same pole (mono‐orientation). Secondly, they safeguard the timely separation of sister chromatids in meiosis II (equational division) by counteracting the premature removal of pericentromeric cohesin, and thus prevent the formation of aneuploid gametes. Although MOKIRs bear no obvious sequence similarity, they appear to play functionally conserved roles in regulating meiotic kinases. Here, the known functions of MOKIRs are reviewed and their possible mechanisms of action are discussed. Also see the video abstract here https://youtu.be/tLE9KL89bwk. (shrink)

A reduction of chromosome number in meiosis is essential for genome transmission in diploid organisms. Reduction depends on a change in kinetochore configuration.1 A recent study2 connects changes in kinetochores with other changes in chromosome structure and raises the intriguing possibility that topoisomerase II, the DNA untangling enzyme, is involved. BioEssays 25:309–312, 2003. © 2003 Wiley Periodicals, Inc.

The assembly of a bipolar spindle is essential for the accurate segregation of replicated chromosomes during cell division. Do chromosomes rely solely on other cellular components to regulate the assembly of the bipolar spindle or are they masters of their own fate? In the Zhang and Nicklas(1) study reviewed here, micromanipulation techniques and video microscopy were used to demonstrate the different roles that chromosome arms, kinetochores and centrosomes play in bipolar spindle assembly.

Chromosome transmission in S. cerevisiae requires the activities of many structural and regulatory proteins required for the replication, repair, recombination and segregation of chromosomal DNA, and co‐ordination of the chromosome cycle with progression through the cell cycle. An important structural domain on each chromosome is the kinetochore (centromere DNA and associated proteins), which provides the site of attachment of chromosomes to the spindle microtubules. Stoler et al.(1) have recently reported the cloning of an essential gene CSE4, mutations in which cause (...) chromosome nondisjunction of a marked chromosome bearing a centromere DNA mutation. The cse4–1 mutation causes cells to arrest in the G2/M phase of the cell cycle with a 2N DNA content in a RAD9 checkpoint‐independent manner. The carboxyl terminus of Cse4p and the human centromere‐localized protein CENP‐A have a high degree of homology to the C‐terminal domain of histone H3. Since both CENP‐A and Cse4p also have biochemical properties similar to histones H3 and H4, it is tempting to speculate that these histone H3‐like proteins are components of specialized nucleosomes, a class of which may be unique to the centromeres. (shrink)

Non‐coding centromeres, which dictate kinetochore formation for proper chromosome segregation, are extremely divergent in DNA sequences across species but are under active transcription carried out by RNA polymerase (RNAP) II. The RNAP II‐mediated centromeric transcription has been shown to facilitate the deposition of the centromere protein A (CENP‐A) to centromeres, establishing a conserved and critical role of centromeric transcription in centromere maintenance. Our recent work revealed another role of centromeric transcription in chromosome segregation: maintaining centromeric cohesion during mitosis. Interestingly, this (...) role appears to be fulfilled through ongoing centromeric transcription rather than centromeric transcripts. In addition, we found that centromeric transcription may not require some of the traditional transcription initiation factors, suggestive of “uniqueness” in its regulation. In this review, we discuss the novel role and regulation of centromeric transcription as well as the potential underlying mechanisms. (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)

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)

The chromosomes which segregate in anaphase I of meiosis are usually physically bound together through chiasmata. This association is necessary for proper segregation, since univalents sort independently from one another in the first meiotic division and this frequently leads to genetically unbalanced offspring. There are, however, a number of species where genetic exchanges in the form of meiotic cross‐overs, the prerequisite of the formation of chiasmata, are routinely missing in one sex or between specific chromosomes. These species nevertheless manage to (...) segregate these non‐exchange chromosomes. There are four direct modes for associating achiasmatic chromosomes: (a) modified SC, (b) adhesion of chromatids comparable to somatic pairing, (c) ‘stickiness’ of heterochromatin or (d) specific ‘segregation bodies’, consisting of material structurally different from chromatin. There is also the possibility that the spindlepossibly joining forces with the kinetochores‐carries out the faithful segregation of univalents which are not directly physically attached to one another. Finally, amphitelic orientation of univalents in metaphase I and pairing of the chromatids in meiosis II appear to ensure correct segregation as well. (shrink)

The chromosome passenger complex (CPC) localizes to chromosomes and microtubules, sometimes simultaneously. The CPC also has multiple domains for interacting with chromatin and microtubules. Interactions between the CPC and both the chromatin and microtubules is important for spindle assembly and error correction. Such dual chromatin‐microtubule interactions may increase the concentration of the CPC necessary for efficient kinase activity while also making it responsive to specific conditions or structures in the cell. CPC‐microtubule dependent functions are considered in the context of the (...) first meiotic division. Acentrosomal spindle assembly is a process that depends on transfer of the CPC from the chromosomes to the microtubules. Furthermore, transfer to the microtubules is not only to position the CPC for a later role in cytokinesis; metaphase I error correction and subsequent bi‐orientation of bivalents may depend on microtubule associated CPC interacting with the kinetochores. (shrink)

Minichromosome maintenance (Mcm) proteins are well‐known for their functions in DNA replication. However, their roles in chromosome segregation are yet to be reviewed in detail. Following the discovery in 1984, a group of Mcm proteins, known as the ARS‐nonspecific group consisting of Mcm13, Mcm16‐19, and Mcm21‐22, were characterized as bonafide kinetochore proteins and were shown to play significant roles in the kinetochore assembly and high‐fidelity chromosome segregation. This review focuses on the structure, function, and evolution of this group of Mcm (...) proteins. Our in silico analysis of the physical interactors of these proteins reveals that they share non‐overlapping functions despite being copurified in biochemically stable complexes. We have discussed the contrasting results reported in the literature and experimental strategies to address them. Taken together, this review focuses on the structure‐function of the ARS‐nonspecific Mcm proteins and their evolutionary flexibility to maintain genome stability in various organisms. (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)

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)

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 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)