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.