Cell cycle control — non-dividing, terminally differentiated cells Certain specialized human cells (for example, many neurons) permanently exit the division cycle. In which phase/state do such cells remain when they no longer divide?

Introduction / Context:
Not all cells continually cycle. Many differentiated cells withdraw from proliferation and enter a quiescent or terminally differentiated state. Understanding this state is essential for developmental biology and regenerative medicine.

Given Data / Assumptions:

• Cell cycle phases: G1, S, G2, M.
• G0 is a quiescent state outside the canonical cycle.
• Neurons and some muscle cells often remain long-term in G0.

Concept / Approach:

Cells that exit the cycle at the G1 restriction point can enter G0. In G0, they perform specialized functions but do not initiate DNA replication or mitosis. Some cells can re-enter the cycle upon stimulation (reversible quiescence), while others are terminally differentiated (irreversible G0).

Step-by-Step Solution:

Verification / Alternative check:

Histological and molecular markers (e.g., reduced cyclin expression, active Rb pathway) are consistent with G0 in non-proliferative tissues.

Why Other Options Are Wrong:

Prophase and S are active dividing states; G1 is a growth phase preceding S, not a permanent exit. Metaphase arrest is pathological or experimental, not a normal long-term state.

Common Pitfalls:

Confusing quiescent G0 with a prolonged G1; they are distinct regulatory states with different cyclin/CDK activity profiles.

Final Answer:

G0 Phase