In addition to monocentric eukaryotes, which have a single localized centromere on each chromosome, there are holocentric species, with extended repeat‐based or repeat‐less centromeres distributed over the entire chromosome length. At least two types of repeat‐based holocentromeres exist, one composed of many small repeat‐based centromere units (small unit‐type), and another one characterized by a few large centromere units (large unit‐type). We hypothesize that the transposable element‐mediated dispersal of hundreds of short satellite arrays formed the small centromere (...) unit‐type holocentromere in Rhynchospora pubera. The large centromere unit‐type of the plant Chionographis japonica is likely a product of simultaneous DNA double‐strand breaks (DSBs), which initiated the de novo formation of repeat‐based holocentromeres via insertion of satellite DNA, derived from extra‐chromosomal circular DNAs (eccDNAs). The number of initial DSBs along the chromosomes must be higher than the number of centromere units since only a portion of the breaks will have incorporated eccDNA at an appropriate position to serve as future centromere unit sites. Subsequently, preferential incorporation of the centromeric histone H3 variant at these positions is assumed. The identification of repeat‐based holocentromeres across lineages will unveil the centromere plasticity and elucidate the mechanisms underlying the diverse formation of holocentromeres. (shrink)

X chromosome centromeric drive may explain the prevalence of polycystic ovary syndrome and contribute to oocyte aneuploidy, menopause, and other conditions. The mammalian X chromosome may be vulnerable to meiotic drive because of X inactivation in the female germline. The human X pericentromeric region contains genes potentially involved in meiotic mechanisms, including multiple SPIN1 and ZXDC paralogs. This is consistent with a multigenic drive system comprising differential modification of the active and inactive X chromosome centromeres in female primordial germ cells (...) and preferential segregation of the previously inactivated X chromosome centromere to the polar body at meiosis I. The drive mechanism may explain differences in X chromosome regulation in the female germlines of the human and mouse and, based on the functions encoded by the genes in the region, the transmission of X pericentromeric genetic or epigenetic variants to progeny could contribute to preeclampsia, autism, and differences in sexual differentiation. (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)

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)

Homologous centromeres compete for segregation to the secondary oocyte nucleus at female meiosis I. Centromeric repeats also compete with each other to populate centromeres in mitotic cells of the germline and have become adapted to use the recombinational machinery present at centromeres to promote their own propagation. Repeats are not needed at centromeres, rather centromeres appear to be hospitable habitats for the colonization and proliferation of repeats. This is probably an indirect consequence of two distinctive features of centromeric DNA. Centromeres (...) are subject to breakage by the mechanical forces exerted by microtubules and meiotic crossing‐over is suppressed. Centromeric proteins acting in trans are under selection to mitigate the costs of centromeric repeats acting in cis. Collateral costs of mitotic competition at centromeres may help to explain the high rates of aneuploidy observed in early human embryos. (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)

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)

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)

Two reports have shown that mammalian artificial chromosomes (MAC) can be constructed from cloned human centromere DNA and telomere repeats, proving the principle that chromosomes can form from naked DNA molecules transfected into human cells. The MACs were mitotically stable, low copy number and bound antibodies associated with active centromeres. As a step toward second-generation MACs, yeast and bacterial cloning systems will have to be adapted to achieve large MAC constructs having a centromere, two telomeres, and genomic copies (...) of mammalian genes. Available construction techniques are discussed along with a new P1 artificial chromosome (PAC)-derived telomere vector (pTAT) that can be joined to other PACs in vitro, avoiding a cloning step during which large repetitive arrays often rearrange. The PAC system can be used as a route to further define the optimal DNA elements required for efficient MAC formation, to investigate the expression of genes on MACs, and possibly to develop efficient MAC-delivery protocols. BioEssays 1999;21:76–83. © 1999 John Wiley & Sons, Inc. (shrink)

Sister chromatid cohesion mediated by the ring‐shaped cohesin complex is essential for faithful chromosome segregation. A tight spatial and temporal control of cohesin release is observed in mitosis and meiosis, and a family of proteins known as shugoshins play a major role in this process. Shugoshin (Sgo) protects centromeric cohesin from dissociation in early mitosis and from cleavage by separase in meiosis I. Three exciting new reports indicate that this is accomplished by recruiting the serine/threonine protein phosphatase 2A (PP2A) to (...) centromeres.(1–3) The proposed targets of PP2A activity include cohesin and Sgo, both of which would otherwise dissociate from chromosomes upon phosphorylation by Polo kinase. Thus, a balance of kinase and phosphatase activities seems to be the key to the conserved mechanism that regulates the stepwise release of cohesin from mitotic and meiotic chromosomes. Additional evidence, however, suggests that this is only part of the story, and that Sgo has also a role independent of PP2A. BioEssays 28: 775–779, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Two reports have shown that mammalian artificial chromosomes (MAC) can be constructed from cloned human centromere DNA and telomere repeats, proving the principle that chromosomes can form from naked DNA molecules transfected into human cells. The MACs were mitotically stable, low copy number and bound antibodies associated with active centromeres. As a step toward second-generation MACs, yeast and bacterial cloning systems will have to be adapted to achieve large MAC constructs having a centromere, two telomeres, and genomic copies (...) of mammalian genes. Available construction techniques are discussed along with a new P1 artificial chromosome (PAC)-derived telomere vector (pTAT) that can be joined to other PACs in vitro, avoiding a cloning step during which large repetitive arrays often rearrange. The PAC system can be used as a route to further define the optimal DNA elements required for efficient MAC formation, to investigate the expression of genes on MACs, and possibly to develop efficient MAC-delivery protocols. BioEssays 1999;21:76–83. © 1999 John Wiley & Sons, Inc. (shrink)

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)

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)

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)

In this review, we discuss how two evolutionarily conserved pathways at the interface of DNA replication and repair, template switching and break-induced replication, lead to the deleterious large-scale expansion of trinucleotide DNA repeats that cause numerous hereditary diseases. We highlight that these pathways, which originated in prokaryotes, may be subsequently hijacked to maintain long DNA microsatellites in eukaryotes. We suggest that the negative mutagenic outcomes of these pathways, exemplified by repeat expansion diseases, are likely outweighed by their positive role in (...) maintaining functional repetitive regions of the genome such as telomeres and centromeres. Break-induced replication and template switching are conserved mechanisms of replication fork restart. They do not cause microsatellite instability in prokaryotes, but promote repeat expansion in eukaryotes. We suggest that TS and BIR persisted in eukaryotes despite their mutagenic potential because they help maintain long repetitive telomeres and centromeres. (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)

Expression of transgenes in mice, when examined with assays that can distinguish individual cells, is often found to be heterocellular, or variegated. Line‐to‐line variations in expression of a transgene may be due largely to differences in the proportion of cells in which it is expressed. Variegated silencing by centromeric heterochromatin is well described, but other factors may also affect transgene silencing in mice. Tandem arrays of transgenes themselves form heterochromatin, and some cell lineages may tend to silence transgenes because of (...) extensive facultative heterochromatin in their nuclei. The cis‐acting transcriptional control elements within a transgene inhibit silencing, and strainspecific differences in chromatin proteins may strongly influence the extent of variegation. The accessibility of multiple differentiated cell lineages in mice suggests that they may provide a tool for dissecting the role of chromatin‐mediated silencing in cell differentiation and tissue‐specific gene expression. (shrink)

Recent studies indicate that mammalian chromosomes contain discretecis‐acting loci that control replication timing, mitotic condensation, and stability of entire chromosomes. Disruption of the large non‐coding RNA gene ASAR6 results in late replication, an under‐condensed appearance during mitosis, and structural instability of human chromosome 6. Similarly, disruption of the mouse Xist gene in adult somatic cells results in a late replication and instability phenotype on the X chromosome. ASAR6 shares many characteristics with Xist, including random mono‐allelic expression and asynchronous replication timing. (...) Additional “chromosome engineering” studies indicate that certain chromosome rearrangements affecting many different chromosomes display this abnormal replication and instability phenotype. These observations suggest that all mammalian chromosomes contain “inactivation/stability centers” that control proper replication, condensation, and stability of individual chromosomes. Therefore, mammalian chromosomes contain four types ofcis‐acting elements, origins, telomeres, centromeres, and “inactivation/stability centers”, all functioning to ensure proper replication, condensation, segregation, and stability of individual chromosomes. (shrink)

The structures of specific chromosome regions, centromeres and telomeres, present a number of puzzles. As functions performed by these regions are ubiquitous and essential, their DNA, proteins and chromatin structure are expected to be conserved. Recent studies of centromeric DNA from human, Drosophila and plant species have demonstrated that a hidden universal centromere‐specific sequence is highly unlikely. The DNA of telomeres is more conserved consisting of a tandemly repeated 6–8 bp Arabidopsis‐like sequence in a majority of organisms as diverse (...) as protozoan, fungi, mammals and plants. However, there are alternatives to short DNA repeats at the ends of chromosomes and for telomere elongation by telomerase. Here we focus on the similarities and diversity that exist among the structural elements, DNA sequences and proteins, that make up terminal domains (telomeres and subtelomeres), and how organisms use these in different ways to fulfil the functions of end‐replication and end‐protection. BioEssays 27:685–697, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Normal meiosis consists of a single round of DNA replication followed by two nuclear divisions. In the 1st division the chromosomes segregate reductionally whereas in the 2nd division they segregate equationally (as they do in mitosis). In certain yeast mutants, a single‐division meiosis takes place, in which some chromosomes segregate reductionally while others divide equationally. This autonomous segregation behaviour of individual chromosomes on a common spindle is determined by the centromeres they carry. The relationship between reductional segregation of a pair (...) of chromosomes and their earlier recombinational history is also discussed. (shrink)

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)

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)

A complex structure, visible by electron microscopy, surrounds each chromosome during mitosis. The organization of this structure is distinct from that of the chromosomes and the cytoplasm. It forms a perichromosomal layer that can be isolated together with the chromosomes. This layer covers the chromosomes except in centromeric regions. The perichromosomal layer includes nuclear and nucleolar proteins as well as ribonucleoproteins (RNPs). The list of proteins and RNAs identified includes nuclear matrix proteins (perichromin, peripherin), nucleolar proteins (perichro‐monucleolin, Ki‐67 antigen, B23 (...) protein, fibrillarin, p103, p52), ribosomal proteins (S1) and snRNAs (U3 RNAs). Only limited information is available about how and when the perichromosomal layer is formed. During early prophase, the proteins extend from the nucleoli towards the periphery of the nucleus. Thin cordon‐like structures reach the nuclear envelope delimiting areas in which chromosomes condense. At telophase, the proteins are associated with the part of the chromosomes remaining condensed and accumulate in newly formed nucleoli in regions where chromatin is already decondensed. The perichromosomal layer contains several different classes of proteins and RNPs and it has been attributed various roles: (1) in chromosome organization, (2) as a barrier around the chromosomes, (3) involvement in compartmentation of the cells in prophase and telophase and (4) a binding site for chromosomal passenger proteins necessary to the early process of nuclear assembly. (shrink)

The objectives of this study were to develop a high-density chromosome bin map of homoeologous group 7 in hexaploid wheat, to identify gene distribution in these chromosomes, and to perform comparative studies of wheat with rice and barley. We mapped 2148 loci from 919 EST clones onto group 7 chromosomes of wheat. In the majority of cases the numbers of loci were significantly lower in the centromeric regions and tended to increase in the distal regions. The level of duplicated loci (...) in this group was 24% with most of these loci being localized toward the distal regions. One hundred nineteen EST probes that hybridized to three fragments and mapped to the three group 7 chromosomes were designated landmark probes and were used to construct a consensus homoeologous group 7 map. An additional 49 probes that mapped to 7AS, 7DS, and the ancestral translocated segment involving 7BS also were designated landmarks. Landmark probe orders and comparative maps of wheat, rice, and barley were produced on the basis of corresponding rice BAC/PAC and genetic markers that mapped on chromosomes 6 and 8 of rice. Identification of landmark ESTs and development of consensus maps may provide a framework of conserved coding regions predating the evolution of wheat genomes. (shrink)

The RNAi machinery is not only involved with post‐transcriptional degradation of messenger RNAs, but also used for targeting of chromatin changes associated with transcriptional silencing. Two recent papers determine the global patterns of gene expression and chromatin modifications produced by the RNAi machinery in fission yeast.(9, 10) The major sites include the outer centromere repeats, the mating‐type locus and subtelomeric regions. By comparison, studies of Arabidopsis heterochromatin also implicate transposons as a major target for silencing. Analyses of siRNA libraries (...) from Drosophila, nematodes and Arabidopsis indicate that major repeats at centromeres, telomeres and transposable elements are likely targets of RNAi. Also, intergenic regions are implicated as targets in Arabidopsis. BioEssays 27:1209–1212, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Supernumerary B chromosomes are dispensable elements of the genome which can be retained in populations at high frequencies, despite being deleterious, through the ability to undergo non‐Mendelian inheritance. Their mode of origin is, however, obscure. Recent work on gynogenetic fish has demonstrated the incorporation of small, unstable, centromere‐containing microchromosomes, probably of interspecific derivation, into an asexual lineage(1). That these resemble B chromosomes both in structure and behaviour is consistent with the proposal that hybridisation between closely related species may be (...) a significant mode of origin for such selfish genetic elements. Additional work on the B chromosome of a parasitoid wasp and observations on patterns of chromosome breakage from somatic cell hybrids also support this hypothesis. (shrink)

Classically, position effect variegation has been studied in Drosophila and results when a euchromatic gene is placed adjacent to either centromeric heterochromatin or to a telomeric domain. In such a circumstance expression of the locus variegates, being active in some cells and silent in others. Over the last few years a comparable phenomenon in yeast has been discovered. This system promises to tell us much about this curious behaviour. Indeed, experiments reported recently(1) indicate that the variegation of a yeast telomeric (...) gene is cell‐cycle regulated. The results suggest the following model. During DNA replication there is a disassembly of chromatin that allows a competition between silencing factors and trans‐activators to take place. Thus, reassembly of the domain may result in either the repression or the expression of the affected gene and, hence, produce a variegating phenotype. (shrink)

Genomes of higher eukaryotes contain abundant non‐coding repeated sequences whose overall biological impact is unclear. They comprise two categories. The first consists of retrotransposon‐derived elements. These are three major families of retroelements (LINEs, SINEs and LTRs). SINEs are clustered in gene‐rich regions and are found in promoters of genes while LINEs are concentrated in gene‐poor regions and are depleted from promoters. The second class consists of non‐coding tandem repeats (satellite DNAs and TTAGGG arrays), which are associated with mammalian centromeres, heterochromatin (...) and telomeres. Terminal TTAGGG arrays are involved in telomere capping and satellite DNAs are located in heterochromatin, which is implicated in transcription silencing by gene repositioning (relocalization). It is unknown whether interstitial TTAGGG sequences, which are present in many vertebrates, have a function. Here, evidence will be presented that retroelements and TTAGGG arrays are involved in regulation of gene expression. Retroelements can provide binding sites for transcription factors and protect promoter CpG islands from repressive chromatin modifications, and may be also involved in nuclear compartmentalization of transcriptionally active and inactive domains. Interstitial telomere‐like sequences can form dynamically maintained three‐dimensional nuclear networks of transcriptionally inactive domains, which may be involved in transcription silencing like classic heterochromatin. BioEssays 30:338–348, 2008. © 2008 Wiley Periodicals, Inc. (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)

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)

DNA topoisomerase II (topo II) is involved in chromosome structure and function, although its exact location and role in mitosis are somewhat controversial. This is due in part to the varied reports of its localization on mitotic chromosomes, which has been described at different times as uniformly distributed, axial on the chromosome arms and predominantly centromeric. These disparate results are probably due to several factors, including use of different preparation and fixation techniques, species differences and changes in distribution during the (...) cell cycle. Recently, several papers have re‐investigated the distribution of topo II on chromosomes as a function of cell cycle and species(1–3). The new studies suggest that Topo II has a dynamic pattern of distribution on the chromosomes, in general becoming axial as chromosomes condense during prophase and then concentrating at centromeres during metaphase. These experiments suggest a novel role for topo II in centromere structure and function. (shrink)

Association between sister chromatids is essential for their attachment and segregation to opposite poles of the spindle in mitosis and meiosis II. Sister‐chromatid cohesion is also likely to be involved in linking homologous chromosomes together in meiosis I. Cytological observations provide evidence that attachment between sister chromatids is different in meiosis and mitosis and suggest that cohesion between the chromatid arms may differ mechanistically from that at the centromere. The physical nature of cohesion is addressed, and proteins that are (...) candidates for holding sister chromatids together are discussed. Dissolution of sister‐chromatid cohesion must be regulated precisely, and potential mechanisms to release cohesion are presented. (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)

A total of 1918 loci, detected by the hybridization of 938 expressed sequence tag unigenes from 26 Triticeae cDNA libraries, were mapped to wheat homoeologous group 4 chromosomes using a set of deletion, ditelosomic, and nulli-tetrasomic lines. The 1918 EST loci were not distributed uniformly among the three group 4 chromosomes; 41, 28, and 31% mapped to chromosomes 4A, 4B, and 4D, respectively. This pattern is in contrast to the cumulative results of EST mapping in all homoeologous groups, as reported (...) elsewhere, that found the highest proportion of loci mapped to the B genome. Sixty-five percent of these 1918 loci mapped to the long arms of homoeologous group 4 chromosomes, while 35% mapped to the short arms. The distal regions of chromosome arms showed higher numbers of loci than the proximal regions, with the exception of 4DL. This study confirmed the complex structure of chromosome 4A that contains two reciprocal translocations and two inversions, previously identified. An additional inversion in the centromeric region of 4A was revealed. A consensus map for homoeologous group 4 was developed from 119 ESTs unique to group 4. Forty-nine percent of these ESTs were found to be homoologous to sequences on rice chromosome 3, 12% had matches with sequences on other rice chromosomes, and 39% had no matches with rice sequences at all. Limited homology was found between wheat ESTs on homoeologous group 4 and the Arabidopsis genome. Forty-two percent of the homoeologous group 4 ESTs could be classified into functional categories on the basis of blastX searches against all protein databases. (shrink)