Checkpoint control — where is cell growth assessed? At which major checkpoint is cell growth (size, nutrients, external signals) primarily assessed to determine whether a cell should commit to another round of DNA replication?

Introduction / Context:
Cell cycle checkpoints ensure that conditions are favorable and the genome is intact before progression. A central decision point occurs late in G1, where cells integrate growth cues to commit to DNA replication.

Given Data / Assumptions:

• Restriction point (mammals) or Start (yeast) lies in late G1.
• Inputs include growth factors, nutrient status, and cell size.
• Passing this checkpoint commits the cell to S phase.

Concept / Approach:

The G1 checkpoint governs the decision to enter S phase. Cyclin D/CDK4-6 and Cyclin E/CDK2 activities, along with Rb–E2F regulation, determine whether the cell has achieved adequate growth and signaling to duplicate DNA. Failure to meet criteria results in G1 arrest or entry into G0.

Step-by-Step Solution:

Verification / Alternative check:

Pharmacologic inhibition of CDK4-6 halts cells at G1, demonstrating growth-control integration at this checkpoint.

Why Other Options Are Wrong:

G2 checks replicated DNA before mitosis; S monitors replication forks; metaphase–anaphase checkpoint ensures proper kinetochore attachment; cytokinesis control is post-mitotic.

Common Pitfalls:

Misplacing growth control at G2 instead of G1; G2 focuses on DNA damage repair prior to mitosis, not gross growth cues.

Final Answer:

G1 checkpoint (restriction point/Start)