Whether whole-chromosome aneuploidy promotes tumorigenesis has been controversial, in large part because of the paucity of insight into underlying mechanisms. Here we identify a mechanism by which mitotic chromosome segregation errors generate DNA breaks via the formation of structures called micronuclei. Whole chromosome-containing micronuclei form when mitotic errors produce lagging chromosomes. We tracked the fate of newly generated micronuclei and found that they undergo defective and asynchronous DNA replication, resulting in DNA damage and frequently pulverization of the chromosome in the micronucleus. Micronuclei can persist in cells over several generations but the chromosome in the micronucleus can also be distributed to daughter nuclei. Thus, chromosome segregation errors potentially lead to mutations and chromosome rearrangements that can integrate into the genome. Pulverization of chromosomes in micronuclei may also be one explanation for “chromothripsis” in cancer and developmental disorders, where isolated chromosomes or chromosome arms undergo massive local DNA breakage and rearrangement.

In yeast, separation of duplicated spindle pole bodies (SPBs) (centrosomes in higher eukaryotes) is an indispensable step in the assembly of mitotic spindle and is triggered by severing of the bridge that connects the sister SPBs. This process requires Cdk1 (Cdc28) activation by Tyrosine 19 dephosphorylation. We show that cells that fail to activate Cdk1 are devoid of spindles due to persistently active APCCdh1, which targets microtubule‐associated proteins Cin8, Kip1 and Ase1 for degradation. Tyrosine 19 dephosphorylation of Cdk1 is necessary to specifically prevent proteolysis of these proteins. Interestingly, SPB separation is dependent on the microtubule‐bundling activity of Cin8 but not on its motor function. Since ectopic expression of proteolysis‐resistant Cin8, Kip1 or Ase1 is sufficient for SPB separation even in the absence of Cdc28‐Clb activity, we suggest that stabilization of these mechanical force‐generating proteins is the predominant role of Cdc28‐Clb in centrosome separation.

Separation of duplicated centrosomes (spindle-pole bodies or SPBs in yeast) is a crucial step in the biogenesis of the mitotic spindle. In vertebrates, centrosome separation requires the BimC family kinesin Eg5 and the activities of Cdk1 and polo kinase; however, the roles of these kinases are not fully understood. In Saccharomyces cerevisiae, SPB separation also requires activated Cdk1 and the plus-end kinesins Cin8 (homologous to vertebrate Eg5) and Kip1. Here we report that polo kinase has a role in the separation of SPBs. We show that adequate accumulation of Cin8 and Kip1 requires inactivation of the anaphase-promoting complex-activator Cdh1 through sequential phosphorylation by Cdk1 and polo kinase. In this process, Cdk1 functions as a priming kinase in that Cdk1-mediated phosphorylation creates a binding site for polo kinase, which further phosphorylates Cdh1. Thus, Cdh1 inactivation through the synergistic action of Cdk1 and polo kinase provides a new model for inactivation of cell-cycle effectors.Centrosomes function as microtubule organizing centres (MTOC) and are responsible for the bipolar nature of the mitotic spindle 1 . Misregulation of the centrosome cycle can produce an abnormal number of centrosomes and multi-polar spindles, and cause chromosome missegregation resulting in aneuploidy. Centrosomal abnormalities and aneuploidy are common in many types of cancers 2 .In mammalian cells, centrosomes consist of a pair of centrioles and the surrounding dense fibrillar mass known as the pericentriolar material 3 . A cell inherits one centrosome from its progenitor but replicates it as it progresses through the cell cycle. The duplicated centrosomes remain tethered to each other by a linker until the time of mitotic entry, when the linkage between the sister centrosomes is severed and they move away from each other, eventually positioning themselves face-to-face, separated by an interdigitated array of microtubules. The structure and duplication of centrosomes have been studied in some detail; however, the process by which they separate is not well understood.3Correspondence should be addressed to U.S. (e-mail: E-mail: mcbucs@imcb.a-star.edu.sg). Note: Supplementary Information is available on the Nature Cell Biology website. AUTHOR CONTRIBUTIONS K.C. and L.H.H. performed all experiments; T.H.G. and M.W. carried out the electron microscopy analysis; U.S. and K.C. planned the project and analysed the data. COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests.Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions/ NIH Public Access The spindle pole body (SPB), a multi-protein assembly, is the centrosome-equivalent in S. cerevisiae 4 . Yeast daughter cells inherit from the mother cells one SPB bearing a half-bridge (a structure that extends from one side of the SPB) and duplicate it as they traverse START (the point in late G1 that precedes DNA replication). The bridge between the duplicated SPBs is severed in late S pha...