Cell cycle regulation — what primarily controls progression? In most eukaryotic cells, progression through the cell cycle is governed predominantly by which mechanism?

Introduction / Context:
The eukaryotic cell cycle is tightly regulated to preserve genome integrity. Central to this control are surveillance systems that halt or permit progression depending on internal and external cues.

Given Data / Assumptions:

• Key checkpoints: G1 (restriction point), S-phase (replication stress), G2/M (DNA damage), and metaphase–anaphase (spindle assembly checkpoint).
• Cyclin–CDK complexes drive transitions and are modulated by checkpoint signaling.
• External mitogens, nutrients, and DNA integrity feed into these checkpoints.

Concept / Approach:

Checkpoints integrate information about growth signals, DNA damage, and spindle attachment, pausing the cycle when necessary. This coordinated system—not a fixed timer or size alone—primarily governs cell-cycle progression in most cells.

Step-by-Step Solution:

Verification / Alternative check:

Genetic or pharmacologic disruption of checkpoint components (e.g., p53, ATM/ATR, spindle checkpoint proteins) leads to unscheduled progression and genomic instability, underscoring their controlling role.

Why Other Options Are Wrong:

Time and size contribute but are insufficient alone. While cell types vary, a conserved checkpoint framework exists across eukaryotes. Random scheduling contradicts observed regulation.

Common Pitfalls:

Overemphasizing a single factor like cell size; in multicellular organisms, mitogen and damage signals dominate through checkpoint pathways.

Final Answer:

A series of checkpoints