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Asymmetric cell division produces daughter cells with different developmental fates. Unequal size can accompany it, but the defining distinction is the daughters' outcomes. An asymmetry may arise from unequal inheritance of intracellular components or from different access to a stem-cell niche after division. For example, one daughter of a germline stem cell can retain its niche contact while the other begins cell differentiation. This couples cell polarity, spindle apparatus orientation and local signalling. ^[ref]

In the male Drosophila germline, a localized Bazooka patch provides a centrosome docking platform. Reducing Bazooka disrupted centrosome orientation; ectopic patches could associate with misplaced centrosomes. Measurements of both interphase centrosomes and mitotic spindle apparatus helped separate orientation defects from failure of the checkpoint that normally delays division. ^[ref]

A mechanism delaying mitotic entry when interphase centrosomes are misoriented relative to the niche in fly male germline stem cells. It couples geometry to the timing of an oriented division.

A polarity-associated kinase. In fly male germline stem cells, it contributes to the centrosome orientation checkpoint and regulation of cyclin A localization.

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.^[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.^[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.^[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.^[ref][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.