Questions: Anaphase-Promoting Complex and Cell Cycle Control

Question 1

A researcher treats mitotic cells with a proteasome inhibitor that blocks all protein degradation. What would you expect to observe?

• A. Cells complete mitosis normally, because APC/C can still ubiquitinate its substrates
• B. Cells arrest at metaphase, unable to separate chromosomes, because securin cannot be degraded to release separase
• C. Cells skip anaphase and proceed directly to cytokinesis
• D. Cells re-enter S phase prematurely due to excess cyclin-CDK activity

Answer: (B)

APC/C works by ubiquitinating securin, which targets it for proteasomal degradation. If the proteasome is blocked, securin persists intact, keeping separase inactive. Without active separase, cohesin rings holding sister chromatids together cannot be cleaved — chromosomes cannot separate and the cell arrests at metaphase. This experiment illustrates that ubiquitination alone is not sufficient; it is the proteasomal destruction that does the work.

Question 2

Why does the metaphase-to-anaphase transition use irreversible protein destruction rather than a reversible modification like phosphorylation?

• A. Protein degradation is faster than phosphorylation and allows more rapid transitions
• B. Phosphorylation is energetically too expensive for large-scale cell cycle transitions
• C. Destruction of securin and cyclin B creates an irreversible commitment, preventing re-cohesion of separated chromatids
• D. Degradation is easier for cells to fine-tune through receptor-mediated pathways

Answer: (C)

This is the key design logic of the APC/C switch. Phosphorylation can be reversed by phosphatases, allowing transitions to go backward. But once sister chromatids separate, reattachment would cause catastrophic chromosome missegregation. Protein destruction is permanent (until new synthesis) — so destroying securin and cyclin B makes the commitment irreversible. The cell cycle needs a one-way door at anaphase, and only proteolysis provides it.

Question 3

APC/C is activated as soon as a cell enters mitosis, immediately destroying securin and most mitotic cyclins to initiate chromosome separation.

• T. True
• F. False

Answer: False

APC/C is held inactive during prometaphase and metaphase by the spindle assembly checkpoint, which sequesters the coactivator Cdc20. Only after every chromosome achieves proper bipolar attachment to the spindle does the checkpoint release Cdc20 to activate APC/C. Even then, substrates are destroyed sequentially — securin first (enabling chromosome separation), then cyclin B (enabling mitotic exit). Simultaneous destruction of everything at once would lose the ordered progression APC/C is designed to enforce.

Question 4

After mitosis is complete, APC/C remains active throughout G1, preventing premature re-entry into S phase.

• T. True
• F. False

Answer: True

In G1, the coactivator Cdh1 replaces Cdc20 and keeps APC/C active, sustaining degradation of mitotic cyclins and other S-phase-promoting factors. This APC/C-Cdh1 activity must be actively overcome by rising cyclin levels before the cell can commit to a new round of DNA replication. This is an essential part of the mechanism that prevents rereplication within a single cell cycle.

Question 5

Why is irreversible protein destruction — rather than reversible phosphorylation — the right mechanism for enforcing the metaphase-to-anaphase transition?

Model answer: Because the transition must be one-way: once sister chromatids separate, they cannot safely be reattached. A reversible mechanism like phosphorylation could be undone by phosphatases, potentially allowing the cell to 'reverse' through the transition — which would produce catastrophic errors in chromosome segregation. By permanently destroying securin and cyclin B, APC/C creates an irreversible commitment. The cell can only move forward, and this unidirectionality is exactly what reliable chromosome segregation requires.

This principle — using irreversible molecular events to enforce commitment at critical transitions — appears throughout cell biology. The logic here is that the cost of a 'false positive' (premature irreversibility) is much lower than the cost of reversal (chromosome segregation errors). APC/C therefore acts as a biochemical ratchet: it only turns in one direction.