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)

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)

The various models of chromatin fiber folding that have been proposed over the years are considered and evaluated. It is concluded that the radial loop / scaffold model is strongly supported by the available evidence, although the term ‘scaffold’ may be an unfortunate one. The scaffold is not a solid rod running the length of the chromatid but rather appears to be an aggregation of discrete anchoring complexes for the loops of the fiber. Despite support for this model, there is (...) a need for more evidence to resolve the questions that remain. (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)

The mechanism of chromosome condensation is one of the classic mysteries of mitosis. A number of years ago, it was suggested that nonhistone proteins of the chromosome scaffold fraction might help chromosomes to condense, possibly by constructing a framework for the condensed structure. Recent results have shown that topoisomerase II and the SMC proteins, two abundant members of the scaffold fraction, are required for chromosome condensation and segregation during mitosis. Topoisomerase II is a well‐characterized enzyme. In contrast, nothing is yet (...) known about the function of the SMC proteins. We summarize evidence suggesting that these proteins may be enzymes whose activity is somehow involved in the establishment and maintenance of mitotic chromosome morphology. (shrink)