= Solution
Both checkpoints couple cell-cycle timing to spatial organization, but they act at different transitions. The budding-yeast <spindle position checkpoint> prevents premature mitotic exit when the anaphase spindle has not been appropriately placed across the mother-bud axis. The fly <centrosome orientation checkpoint> delays entry into mitosis until interphase <centrosomes> have the orientation needed for a properly oriented division.
In <Saccharomyces cerevisiae>, one product of division must enter the bud. If both spindle poles and <chromosome> masses remain in the mother, cytokinesis would generate the wrong nuclear distribution. The <mitotic exit network> promotes sustained activation of <Cdc14> and the decline of mitotic CDK activity. Its upstream <GTPase> <Tem1> is restrained by the <Bub2-Bfa1 complex>. When the spindle is mispositioned, <Kin4> helps preserve that inhibition and opposes the checkpoint-inactivating action of <Cdc5> on Bfa1. Thus spindle mispositioning is connected to continued mitotic arrest rather than simply to an attempt to pull harder on <chromosomes>.<The Protein Kinase Kin4 Inhibits Exit from Mitosis in Response to Spindle Position Defects by Katharine E. D'Aquino and others>[Kin4-dependent inhibition of mitotic exit]{ref}
Spatial regulation makes this pathway sensitive to which compartment a spindle pole occupies. <Kin4> activity is associated with the mother-cell side, whereas the bud environment favours release of the inhibitory state when a pole enters appropriately. <Elm1> contributes to activation of <Kin4>. This is a regulated network involving localization and phosphorylation, rather than an infallible single switch at the bud neck. It is also distinct from the <spindle checkpoint>, which monitors <kinetochore> attachment and restrains anaphase; a spindle can have attached <chromosomes> yet be in the wrong position.<Elm1 kinase activates the spindle position checkpoint kinase Kin4 by Ayse Koca Caydasi and others>[Kin4 activation and regulation of Bub2-Bfa1]{ref}
In male <Drosophila melanogaster> <germline stem cells>, contact with the hub defines the <stem-cell niche>. Normally one <centrosome> is retained toward the hub and the other moves away, preparing a division in which one daughter retains niche contact. Live imaging showed that misoriented <centrosomes> can delay division until orientation is restored, including in aged or dedifferentiated cells. Counting misoriented interphase <centrosomes> alone therefore overestimates the frequency of misoriented mitoses.<Centrosome misorientation reduces stem cell division during aging by Jun Cheng and others>[Interphase misorientation, delayed cycling and reorientation before mitosis]{ref}
The fly checkpoint involves <Centrosomin>, <Par-1>, and regulation of <cyclin A> localization. <Par-1> and the germline <spectrosome> help restrain the mitotic programme; perturbing this organization can uncouple orientation from mitotic entry. <Bazooka> provides a cortical platform associated with <centrosome> anchoring. These observations support spatial control of mitotic competence, not identity of the molecular pathway with yeast <Tem1> signalling.<Cyclin A localization and the centrosome orientation checkpoint by Hebao Yuan and others>[Par-1, spectrosome-associated cyclin A and checkpoint defects]{ref}<The polarity protein Baz forms a platform for the centrosome orientation during asymmetric stem cell division in the Drosophila male germline by Mayu Inaba and others>[Baz-centrosome association and Par-1-dependent regulation]{ref}
A decisive experimental comparison measures both position and transition timing in live cells. A pure orientation defect raises misorientation and prolongs the relevant pause; loss of the checkpoint permits the transition despite misorientation. Rescue with an appropriate wild-type component helps establish specificity. In yeast, score mitotic exit and nuclear inheritance; in fly <stem cells>, score entry into mitosis and daughter contact with the niche. This separates the mechanism that constructs the correct geometry from the mechanism that waits for it.
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