Questions: Mitotic Spindle Checkpoint and Chromosome Segregation

Question 1

A human cell is in metaphase with 46 chromosome pairs. 45 are properly bi-oriented; one chromosome has both kinetochores attached to microtubules from the same spindle pole. What does the spindle assembly checkpoint predict?

• A. The cell proceeds to anaphase since 45 out of 46 chromosomes are correctly attached
• B. The cell proceeds to anaphase but will produce one aneuploid daughter cell
• C. The cell halts at metaphase — even a single improperly attached kinetochore generates a catalytic inhibitory signal that blocks all anaphase
• D. The cell arrests permanently and triggers apoptosis after detecting the misattachment

Answer: (C)

The SAC is exquisitely sensitive: one unattached or improperly attached kinetochore out of all 46 in a human cell is sufficient to block anaphase entirely. The unattached kinetochore catalytically converts inactive Mad2 to active Mad2, which diffuses throughout the cytoplasm and sequesters Cdc20. Without Cdc20, APC/C cannot activate, and anaphase cannot proceed. The 'voting' metaphor captures this: every kinetochore must signal 'attached' before the checkpoint clears.

Question 2

The catalytic nature of the spindle assembly checkpoint signal means that:

• A. The speed of anaphase onset is proportional to how many kinetochores are correctly attached
• B. One unattached kinetochore acts as a template that converts inactive Mad2 into active Mad2 in quantity, amplifying the inhibitory signal throughout the cell
• C. The checkpoint requires energy proportional to the number of unattached chromosomes to maintain inhibition
• D. Checkpoint proteins must be synthesized de novo each time the cell detects an attachment problem

Answer: (B)

The SAC is not just a sensor — it is a signal amplifier. An unattached kinetochore recruits Mad1 and creates a conformational template that catalyzes the conversion of cytoplasmic Mad2 from inactive (open) to active (closed) form. This active Mad2 then diffuses away, binds Cdc20, and prevents APC/C activation. The catalytic amplification means one kinetochore can produce enough inhibitory signal to halt a cell with 46 chromosomes — a remarkable example of biological signal amplification.

Question 3

The spindle assembly checkpoint prevents anaphase by directly inhibiting separase, the enzyme that cleaves cohesin holding sister chromatids together.

• T. True
• F. False

Answer: False

The SAC does not directly inhibit separase. Instead, it inhibits Cdc20, which prevents APC/C activation. Active APC/C is needed to ubiquitinate and degrade securin — and securin is the direct inhibitor of separase. So the pathway is: SAC → inhibits Cdc20 → APC/C inactive → securin not degraded → separase inhibited. Separase is controlled indirectly, through the APC/C → securin pathway, not by direct checkpoint protein binding.

Question 4

Bi-orientation means each chromosome's two kinetochores are attached to microtubules from opposite spindle poles, creating tension across the centromere.

• T. True
• F. False

Answer: True

Bi-orientation (amphitelic attachment) is the correct attachment state that satisfies the checkpoint. Each sister chromatid has one kinetochore, and for proper segregation, the two kinetochores of a chromosome must attach to opposite poles. The tension generated by opposing forces pulling the chromatids toward each pole is itself a signal that correct attachment has occurred — tension stabilizes microtubule-kinetochore interactions and silences the checkpoint signal at that kinetochore.

Question 5

Why must the APC/C degrade both securin AND cyclin B for anaphase to proceed successfully? What would happen if only one were degraded?

Model answer: Securin degradation releases separase, which cleaves cohesin and allows sister chromatids to separate — this is required for the physical act of segregation. Cyclin B degradation inactivates Cdk1, which is required for mitotic exit — without it, the spindle persists, chromosomes cannot decondense, and the nuclear envelope cannot reform. If only securin is destroyed, chromatids separate but the cell remains trapped in a mitotic state. If only cyclin B is destroyed, the cell exits mitosis with chromatids still attached. Both must be degraded for accurate, complete cell division.

The APC/C functions as a master demolition switch that simultaneously dismantles multiple aspects of the mitotic state. Securin and cyclin B are both degraded by APC/C^Cdc20, but they serve different downstream functions: securin controls the cohesion-dissolution event (the 'cut') and cyclin B controls mitotic exit (the 'reset'). This dual requirement ensures that chromosome segregation and mitotic exit are coordinated — the cell cannot exit mitosis without also completing chromatid separation.