Questions: Cell Cycle Modeling

Question 1

The G1/S transition is modeled as a bistable switch. What does bistability mean functionally for cell cycle commitment?

• A. The cell gradually eases into S phase over many hours with no sharp transition point
• B. Once the cell crosses the restriction point, positive feedback between cyclin E-CDK2 and Rb phosphorylation creates a self-reinforcing loop that drives irreversible commitment to S phase — the cell cannot return to G1 without a drastic perturbation
• C. The cell alternates rapidly between G1 and S phase before settling
• D. Bistability means the cell can be in either G1 or S phase randomly at any time

Answer: (B)

Bistability creates two stable states (G1 and S-committed) separated by an unstable threshold. At low cyclin E levels, Rb represses E2F, keeping cyclin E low — a stable G1 state. When growth factor signaling pushes cyclin E above a threshold, cyclin E-CDK2 phosphorylates Rb, releasing E2F, which drives more cyclin E expression — positive feedback pushes the system to the high-cyclin-E stable state (S commitment). The hysteresis inherent in bistable switches means that even if the growth signal is removed after crossing the threshold, the cell remains committed. This irreversibility is essential: once DNA replication begins, returning to G1 would be catastrophic.

Question 2

The cell cycle can be modeled either as a single autonomous oscillator or as a series of linked bistable switches. These two views are contradictory.

• T. True
• F. False

Answer: False

These views are complementary, not contradictory. The Novak-Tyson framework unifies them: the cell cycle is an oscillator composed of linked bistable switches. Each phase transition (G1/S, G2/M, metaphase/anaphase) is a bistable switch that ensures irreversible commitment. But the switches are coupled so that completing one transition sets up the conditions for the next — cyclin accumulation drives entry into a phase, and APC/C-mediated degradation resets the system for the next phase. The result is an autonomous oscillation where each cycle passes through a sequence of irreversible transitions. Bistable switches provide robustness to noise at each transition; the oscillatory coupling provides the periodic timing.

Question 3

What insight did mathematical modeling reveal about cell cycle control that purely experimental approaches had not established?

Model answer: Mathematical modeling revealed that the cell cycle transitions are bistable switches with hysteresis — once the transition is triggered, the system commits irreversibly and cannot return to the previous phase without a large perturbation. Experimental biology had identified the molecular components (cyclins, CDKs, CKIs, APC/C) and many of their interactions, but the dynamical consequence of these interactions — that positive feedback loops create bistability and that this bistability explains the all-or-nothing, irreversible character of phase transitions — was not apparent from the molecular parts list. The models made quantitative predictions (e.g., the threshold cyclin concentration for entry into mitosis, the size of the hysteresis loop) that were subsequently confirmed experimentally.

Pomerening et al. (2003) experimentally demonstrated hysteresis in Xenopus egg extract CDK1 activation — exactly as Novak-Tyson models predicted. The system required a higher cyclin concentration to activate CDK1 than to maintain it, confirming bistability. This bridging of modeling prediction and experimental validation is a paradigm of how systems biology generates biological insight.