Eukaryotic DNA replication occurs during which specific cell-cycle phase?
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AG1 phase
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BS phase
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CG2 phase
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DM phase
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EG0 phase
Answer
Correct Answer: S phase
Explanation
Introduction / Context:Cell-cycle control restricts DNA synthesis to a defined window to preserve genome integrity and coordinate replication with division.
Given Data / Assumptions:
- Eukaryotic cell cycle has G1, S, G2, and M phases.
- Checkpoints prevent re-replication and ensure fidelity.
Concept / Approach:Licensing of replication origins occurs in late M/G1, but firing and bulk DNA synthesis are confined to S phase. This temporal separation prevents multiple rounds of replication in one cycle.
Step-by-Step Solution:
Identify the synthesis window: S phase.Confirm that other phases are preparatory (G1), checking (G2), or segregation (M).Thus, select S phase.Verification / Alternative check:DNA content by flow cytometry increases from 2C to 4C specifically during S phase, verifying synthesis timing.
Why Other Options Are Wrong:
- G1: growth and licensing, not bulk synthesis.
- G2: post-replication gap; repair and preparation for mitosis.
- M: chromosome segregation, not synthesis.
- G0: quiescence outside the cycle.
Common Pitfalls:Confusing licensing (G1) with replication (S) or assuming low-level synthesis outside S phase.
Final Answer:S phase