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Duncan J Clarke

Publications and source records attributed to Duncan J Clarke.

25 records · Page 2Linked to original sources
PubMed2002

DNA catenations that link sister chromatids until the onset of anaphase are maintained by a checkpoint mechanism.

Treatment of Allium cepa meristematic cells in metaphase with the topoisomerase II inhibitor ICRF-193, results in bridging of the sister chromatids at anaphase. Separation of the sisters in experimentally generated acentric chromosomal fragments was also inhibited by ICRF-193, indicating that some non-centromeric catenations also persist in metaphase chromosomes. Thus, catenations must be resolved by DNA topoisomerase II at the metaphase-to-anaphase transition to allow segregation of sisters. A passive mechanism could maintain catenations holding sisters until the onset of anaphase. At this point the opposite tension exerted on sister chromatids could render the decatenation reaction physically more favorable than catenation. But this possibility was dismissed as acentric chromosome fragments were able to separate their sister chromatids at anaphase. A timing mechanism (a common trigger for two processes taking different times to be completed) could passively couple the resolution of the last remaining catenations to the moment of anaphase onset. This possibility was also discarded as cells arrested in metaphase with microtubule-destabilising drugs still displayed anaphase bridges when released in the presence of ICRF-193. It is possible that a checkpoint mechanism prevents the release of the last catenations linking sisters until the onset of anaphase. To test whether cells are competent to fully resolve catenations before anaphase onset, we generated multinucleate plant cells. In this system, the nuclei within a single multinucleate cell displayed differences in chromosome condensation at metaphase, but initiated anaphase synchronously. When multinucleates were treated with ICRF-193 at the metaphase-toanaphase transition, tangled and untangled anaphases were observed within the same cell. This can only occur if cells are competent to disentangle sister chromatids before the onset of anaphase, but are prevented from doing so by a checkpoint mechanism.

Anaphase↗
PubMedDate not supplied

Proteolysis and the cell cycle.

Without doubt, one of the more dramatic breakthroughs in recent cell cycle history has been the discovery that growth regulators are controlled by proteolysis. This concept blossomed within the last six or seven years, but the story really began when cyclins were discovered, soon followed by the suggestion that proteolysis events might control cell cycle transitions. Proteolytic targets that are now known include most of the cyclins, cyclin dependent kinase inhibitors, DNA replication factors, the securin class of proteins that inhibit loss of sister chromatid cohesion following DNA replication and, of course, the cohesion factor itself. Protein degradation is controlled in various ways including ubiquitin-dependent targeting to proteasomes, activation of ubiquitin ligases by ubiquitin-like molecule conjugation, phosphorylation of proteolytic targets, and activation of the separin class of proteases.

Animals↗
PubMedDate not supplied

Spotlight on mitosis: introduction.

Even before biochemical and genetic approaches were employed, many researchers were captivated and astounded by mitosis. Cell biologists have been enthralled by its sheer beauty for more than a century (Fig. 1). Yanagida, who probably contributed more to the genetic dissection of mitotic processes than any other scientist, described it as a "cellular festival", dramatically thrust between the humdrum chores of daily life. True enough; if the cell cycle were a theatrical performance, mitosis would be the final climatic act. In this Spotlight, a few of the exciting aspects of mitosis are reviewed.

Animals↗
PubMedDate not supplied

A topoisomerase II-dependent checkpoint in G2-phase plant cells can be bypassed by ectopic expression of mitotic cyclin B2.

DNA topoisomerase II is required for mitotic chromosome condensation and segregation. Here we characterize the effects of inhibiting DNA topoisomerase II activity in plant cells using the non-DNA damaging topoisomerase II inhibitor ICRF-193. We report that ICRF-193 abrogated chromosome condensation in cultured alfalfa (Medicago sativa L.) and tobacco (Nicotiana tabaccum L.) mitoses and led to bridged chromosomes at anaphase. Moreover, ICRF-193 treatment delayed entry into mitosis, increasing the frequency of cells having a pre-prophase band of microtubules, a marker of late G2 and prophase, and delaying the activation of cyclin-dependent kinase. These data suggest the existence of a late G2 checkpoint in plant cells that is activated in the absence of topoisomerase II activity. To determine whether the checkpoint-induced delay was a result of reduced cyclindependent kinase activity, mitotic cyclin B2 was ectopically expressed. Cyclin B2 bypassed the ICRF-193-induced delay before mitosis, and correspondingly, reduced the frequency of interphase cells with a pre-prophase band. These data provide evidence that plant cells possess a topoisomerase II-dependent G2 cell cycle checkpoint that transiently inhibits mitotic CDK activation and entry into mitosis, and that is overridden by raising the level of CDK activity through the ectopic expression of a plant mitotic cyclin.

Antineoplastic Agents↗
PubMedDate not supplied

Self-regulating model for control of replication origin firing in budding yeast.

A major research area concentrates on understanding the regulation of replication origin firing. It is now appreciated that checkpoint signaling participates in this controlled process and that defects in such signaling systems affect genome integrity. Inhibition of replication origin firing is most obviously apparent under conditions of replication stress, but origin firing must also be regulated on a minute-by-minute basis as cells progress normally through an unabated S-phase. Here we summarize a straightforward model to account for how origin firing could be controlled by a self-regulating system.

Cell Cycle Proteins↗