Chromosome segregation: correcting improperly attached chromosomes

Two new studies show that Aurora B kinase corrects improperly attached chromosomes by recruiting molecules necessary for eliminating the bad attachments and by regulating the turnover of the kinetochore fiber.     

Chromosome segregation

The ability to visualise specific genes and proteins within bacterial cells is revolutionising knowledge of chromosome segregation. The essential elements appear to be the driving force behind DNA replication, which occurs at fixed cellular positions, the condensation of newly replicated DNA by a chromosome condensation machine located at the cell 1/4 and 3/4 positions, and molecular machines that act at midcell to allow chromosome separation after replication and movement of the sister chromosomes away from the division septum prior to cell division. This review attempts to provide a perspective on current views of the bacterial chromosome segregation mechanism and how it relates to other cellular processes.     

Chromosome segregation: centromeres get bent

Work over the last several decades has shown that kinetochores play an active part in chromosome segregation, while the chromatin and, more to the point, the DNA have gathered little attention. In two intriguing papers, the Bloom and Khodjakov groups show that intercentromeric chromatin plays a much more active part in chromosome segregation than previously suspected.     

Chromosome segregation: seeing is believing

For chromosome segregation in mitosis, each centromere directs assembly of a complex, proteinaceous structure - the kinetochore, which connects the chromosome to microtubules of the mitotic spindle. A recent study has provided important new insights into the mechanism by which kinetochores capture spindle microtubules.     

Chromosome segregation: monopolin attracts condensin

To segregate chromosomes properly, the cell must prevent merotely, an error that occurs when a single kinetochore is attached to microtubules emanating from both spindle poles. Recent evidence suggests that cooperation between Pcs1/Mde4 and condensin complexes plays an important role in preventing merotely.     

Chromosome segregation: pushing plasmids apart

The ParM ATPase from Escherichia coli plasmid R1 assembles into F-actin-like filaments which appear to push replicated copies of the plasmid to opposite ends of the cell, ensuring partitioning into daughter cells. Might bacterial chromosomes use a similar mitotic strategy for segregation?     

Chromosome segregation in Eubacteria

It is now clear that bacterial chromosomes rapidly separate in a manner independent of cell elongation, suggesting the existence of a mitotic apparatus in bacteria. Recent studies of bacterial cells reveal filamentous structures similar to the eukaryotic cytoskeleton, proteins that mediate polar chromosome anchoring during Bacillus subtilis sporulation, and SMC interacting proteins that are involved in chromosome condensation. A picture is thereby developing of how bacterial chromosomes are organized within the cell, how they are separated following duplication, and how these processes are coordinated with the cell cycle.     

Chromosome segregation: playing polo in prophase

During mitosis, in most eukaryotes, cohesin is removed from chromosomes in two steps. A paper in the March issue of Molecular Cell identifies Polo-like kinase as a key regulator for the first step that releases much of cohesin during prophase.     

Chromosome segregation: pulling from the poles

Dual wavelength video microscopy has been used to evaluate how chromatids move poleward upon chromosome separation at anaphase. The data reveal that poleward microtubule flux provides the dominant force for separating chromatids in Drosophila embryos during anaphase A.     

Chromosome segregation: Aurora B gets Tousled

Aurora B kinases play important roles during mitosis in eukaryotic cells; new work in Caenorhabditis elegans has identified the Tousled kinase TLK-1 as a substrate activator of the model nematode's Aurora B kinase AIR-2 which acts to ensure proper chromosome segregation during cell division.     

Chromosome segregation: taking the passenger seat

The chromosomal passenger complex (CPC) is a major regulator of mitotic and?meiotic chromosome segregation. Three recent papers now elucidate the mechanisms that determine the localization of the CPC to the inner centromere.     

Chromosome segregation and genomic stability

The acquisition of genomic instability is a crucial step in the development of human cancer. Genomic instability has multiple causes of which chromosomal instability (CIN) and microsatellite instability (MIN) have received the most attention. Whereas the connection between a MIN phenotype and cancer is now proven, the argument that CIN causes cancer remains circumstantial. Nonetheless, the ubiquity of aneuploidy in human cancers, particularly solid tumors, suggests a fundamental link between errors in chromosome segregation and tumorigenesis. Current research in the field is focused on elucidating the molecular basis of CIN, including the possible roles of defects in the spindle checkpoint and other regulators of mitosis.     

Chromosome reproduction: units of DNA for segregation

Evidence is summarized which indicates that the DNA loop anchoring proteins in chromosomes are effectively heterodimers that stack and are fastened into a bilaterally symmetrical array along the chromonemal axis. The evidence consists primarily of the observations made twenty five to thirty years ago on the pattern of sister chromatid exchanges and the way the DNA chains are sorted in the formation of diplochromosomes in cells that have undergone endoreduplication. The evidence indicates that each chain of DNA in the single duplex, which is assumed to run the length of a chromosome, is anchored to a bilaterally symmetrical axis of heterodimers that sort the two original chains among the four derived chromatids of each diplochromosome in a very precise way. These observations are considered in the context of investigations on the nature of scaffold proteins and the loop anchorage sequences, as well as the advances being made on the nature of DNA binding proteins and the roles of topoisomerase II.     

Chromosome segregation: organizing overlap at the midzone

Sets of overlapping microtubules support the segregation of chromosomes by linking the poles of mitotic spindles. Recent work examines the effect of putting these linkages under pressure by the activation of dicentric chromosomes and sheds new light on the structural role of several well-known spindle midzone proteins.     

Chromosome segregation: spindle mechanics come to life

Chromosome segregation is a mechanical process, and the spindle generates, and is subject to, mechanical force. A recent study probes how the mechanical architecture of the spindle allows it to maintain mechanical integrity despite these forces.     

Chromosome segregation: dual control ensures fidelity.

A mitotic checkpoint arrests cell cycle progression in response to spindle damage. It now appears that this checkpoint has two separate arms, one that prevents anaphase and a second that prevents cytokinesis and DNA re-replication.     

Chromosome segregation: clamping down on deviant orientations

Chromosome segregation depends on proper orientation of sister kinetochores. The protein Csm1 is required for mono-orientation of sister kinetochores at meiosis I in budding yeast. Surprisingly, its homologue in fission yeast appears instead of clamp micro-tubule binding sites together on single mitotic kinetochores so that they all face one spindle pole.     

Chromosome segregation: keeping kinetochores in the loop

The Ndc80 complex is a key component of the kinetochore-microtubule interface. Two studies now demonstrate that a conserved loop region within the extended coiled-coil of Ndc80 plays an unexpected role in recruiting proteins to the kinetochore.