= Solution
\b[A common transition into <anaphase> both releases <chromosomes> for separation and permits the spindle to organise <cytokinesis>; later safeguards delay final separation when segregation remains incomplete.] Temporal coordination makes the events occur in the right order, while spatial coordination positions division between the separating <chromosome> sets. The two forms of control interact but are experimentally distinguishable.
<S phase> establishes <sister chromatids> and their association through <cohesin>. During <mitosis>, <chromosome condensation> and <DNA topoisomerase II> activity help prepare the copies for separation. Each sister's <kinetochore> should attach to the opposite spindle pole, producing <biorientation>. <Aurora B> contributes to error correction by destabilising inappropriate attachments. Attachment and tension affect this process, but the <spindle checkpoint> should not be described simply as a direct meter of equal tension on every <chromosome>.
Unattached <kinetochores> catalyse formation of the <mitotic checkpoint complex>, involving <Mad2>, <BubR1>, <Bub3> and <Cdc20>. This inhibits productive <anaphase-promoting complex> activity toward key substrates. In a classic causal experiment, destruction of the last unattached <kinetochore> allowed <anaphase> to proceed, with chromosome-arm damage providing an important control. It was the inhibitory signalling structure, rather than the physical presence of one unaligned <chromosome> somewhere in the <biological cell>, that maintained the delay.<The checkpoint delaying anaphase in response to chromosome monoorientation is mediated by an inhibitory signal produced by unattached kinetochores by Conly L. Rieder and others>[Journal of Cell Biology 130, pp. 941–948; laser ablation of the last unattached kinetochore and chromosome-arm controls]{ref}
Once attachment-associated checkpoint signalling is silenced and inhibitory complexes are dismantled, <anaphase-promoting complex> with <Cdc20> promotes <ubiquitin>-dependent destruction of <securin> and mitotic <cyclin B>. <securin> destruction permits <separase> activity, which cleaves the remaining cohesion-bearing kleisin subunit. Non-cleavable <Scc1> blocked sister separation in budding yeast, directly linking regulated proteolysis to the loss of <chromosome> cohesion.<Sister-chromatid separation at anaphase onset is promoted by cleavage of the cohesin subunit Scc1 by Frank Uhlmann and others>[Nature 400, pp. 37–42; cleavage-resistant Scc1 and failure of sister-chromatid separation]{ref} Chromosome movement can then proceed through shortening of kinetochore-associated <microtubules> and separation of the spindle poles.
Destruction of <cyclin B> also lowers <CDK1> activity. This changes the phosphorylation balance, allowing <protein phosphatases> to reverse mitotic modifications and permitting <chromosome> decondensation, nuclear reassembly and cytokinetic organisation. Thus regulated proteolysis produces two coordinated outputs: removal of a physical <chromosome> linkage and removal of the mitotic kinase state. Their sharing of a controller improves ordering, but it does not mean that <cytokinesis> physically waits for a sensor to count every <chromosome> at its destination.
For animal <cytokinesis>, overlapping <microtubules> of the <central spindle> and signals associated with astral <microtubules> specify an equatorial cortical region. <centralspindlin> recruits and regulates the guanine-nucleotide exchange factor <ECT2>, which promotes local <RhoA> activation. <RhoA> stimulates <actin> assembly and <myosin II> activity, organising the <actomyosin ring> that drives furrow ingression. This geometry aligns the division plane between the segregating <chromosome> masses. Spindle position can itself be set by cortical <cell polarity> cues, explaining how the same machinery supports either approximately symmetric or asymmetric division.
Depletion and localisation experiments distinguish the biochemical signal from a simple structural scaffold. Experiments in human <biological cells> showed that <ECT2> is required for cortical <RhoA> and furrow assembly, and that different <centralspindlin> subunits have distinguishable consequences for these events. Loss of central-spindle localisation need not abolish every equatorial signal, because additional spatial inputs can persist.<An ECT2-centralspindlin complex regulates the localization and function of RhoA by Özlem Yüce and others>[Journal of Cell Biology 170, pp. 571–582; ECT2 and centralspindlin depletion, interaction and cortical RhoA assays]{ref} Accordingly, central-spindle disruption should be assessed alongside astral signals, spindle orientation and direct measurements of the cortical activity zone.
Furrow ingression leaves an intercellular bridge with a <midbody>. Final <abscission> involves <ESCRT-III>-dependent membrane-remodelling machinery and must be distinguished from the earlier contractile event. Chromatin trapped in the bridge can sustain <Aurora B>-dependent signalling and delay abscission. Human-cell experiments connected persistent <chromosome> bridges with continued kinase activity and delayed final separation, reducing the risk of unsuccessful division and furrow regression.<Aurora B-mediated abscission checkpoint protects against tetraploidization by Patrick Steigemann and others>[Cell 136, pp. 473–484; chromosome bridges, persistent Aurora B activity and delayed abscission]{ref} This safeguard can provide time to resolve a problem; persistent bridges can still lead to breakage, regression or abnormal descendants.
A decisive experimental series would image labelled <chromosomes>, <microtubules>, <cyclin B>, active <RhoA> and the <biological cell> boundary in the same <biological cells>. Acute checkpoint inhibition predicts premature <anaphase> and increased segregation errors. Stabilised <securin> or cleavage-resistant cohesion should obstruct separation; persistent <cyclin B> should obstruct normal mitotic exit. Acute <ECT2> loss can impair <cytokinesis> even when <chromosome> segregation proceeds, demonstrating separable downstream machinery. Use timed interventions after spindle assembly, independent depletion reagents, appropriately expressed rescue constructs and controls for global <biological cell> damage. Count <chromosome> allocation, furrow initiation, ingression and abscission separately; a binucleate endpoint does not reveal which step failed.
The checkpoint also has blind spots. A <merotelic attachment>, where one kinetochore binds microtubules from both poles, may evade an unattached-kinetochore signal and generate a lagging <chromosome>. \b[Coordination is therefore layered rather than absolute: attachment surveillance, a shared proteolytic transition, spindle-directed cortical signalling and a late abscission checkpoint jointly reduce the chance of partitioning the <genome> incorrectly.] The detailed geometry differs in organisms that divide with a <cell plate> or other cytokinetic mechanisms.
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