Eukaryotic DNA replication occurs during which specific cell-cycle phase?

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:

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