Q.71 In normal cells, progress through the cell cycle is tightly regulated and each step must be
completed before the next step can begin. There are at least three distinct points in the cell cycle
at which the cell monitors external signals and internal equilibrium before proceeding to the next stage.
These are the G1/S, the G2/M and M checkpoints.
In addition to regulating the cell cycle at the checkpoints, the cell controls progress through
the cell cycle by means of two classes of proteins: cyclins and cyclin-dependent kinases (CDKs).
Based on the above context, answer the following questions:
Given below are two statements:
Statement I: Continuously dividing cells do not exit the cell cycle but they proceed through
G1, S, G2 and M phase.
Statement II: Cancer cells are unable to enter G0 phase.
In the light of the above statements, choose the correct answer from the options given below:
- Both Statement I and Statement II are correct
- Both Statement I and Statement II are incorrect
- Statement I is correct but Statement II is incorrect
- Statement I is incorrect but Statement II is correct
Statement I is correct, but Statement II is incorrect. Continuously dividing cells cycle through G1, S, G2, and M without entering G0, but cancer cells can and often do enter G0, contributing to dormancy and therapy resistance.
Detailed Option Explanation
The cell cycle includes G1, S, G2, M phases, plus G0 (quiescent/resting state outside active division). Checkpoints (G1/S, G2/M, M) ensure orderly progression via cyclins/CDKs.
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Statement I: Continuously dividing cells do not exit the cell cycle but they proceed through G1, S, G2 and M phase: Correct. Stem cells, progenitors, or lab-cultured lines (e.g., HeLa) maintain continuous cycling without G0 entry, driven by growth factors.
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Statement II: Cancer cells are unable to enter G0 phase: Incorrect. Many cancer cells enter G0/G1 (often indistinguishable), especially invading/metastatic ones; they exit upon stimuli, enabling proliferation bursts and chemotherapy evasion.
Options evaluated:
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Both correct: No, II false.
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Both incorrect: No, I true.
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I correct, II incorrect: Yes.
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I incorrect, II correct: No.
Cancer Cells G0 Phase Basics
Cancer cells G0 phase entry allows quiescence, evading division checkpoints (G1/S, G2/M, M) regulated by cyclins/CDKs. Unlike normal cells, tumors exploit this for survival.
Cell Cycle Statements Breakdown
Key statements on cancer cells G0 phase and division:
| Statement |
Correct? |
Reason |
| I: Continuously dividing cells proceed through G1,S,G2,M (no G0 exit) |
Yes |
Lab/tissue examples cycle perpetually |
| II: Cancer cells unable to enter G0 |
No |
Many invade in G0/G1, cycle out later |
Implications for Cancer
Cancer cells G0 phase contributes to heterogeneity—proliferating (cycling) vs. quiescent subpopulations resist therapy. Targeting G0 exit (e.g., via CDK inhibitors) is therapeutic focus.
