Cell cycle fundamentals: In which overall phase of the cell cycle are chromosomes replicated to produce two sister chromatids per chromosome?

Introduction / Context:
Each chromosome must be duplicated before cell division to ensure both daughter cells receive complete genetic information. This duplication produces sister chromatids that remain paired until they separate during mitosis or meiosis. The question targets identification of when this duplication occurs within the cell cycle framework.

Given Data / Assumptions:

• Chromosome replication produces sister chromatids.
• Cell cycle phases: G1, S, G2 (collectively interphase) and M phase.
• The query uses the broad phase name rather than the sub-phase.

Concept / Approach:
DNA synthesis happens during S phase, which is part of interphase. Therefore, in the broad categorization, replication occurs in interphase. During M phase (mitosis), the already replicated sister chromatids are separated, not synthesized.

Step-by-Step Solution:

Verification / Alternative check:
Cell cycle diagrams universally place DNA synthesis in S phase within interphase; microscopy shows condensed chromosomes only in M phase.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing “when chromatids appear” with “when they are made.” They appear visibly in M phase but are synthesized earlier during S phase of interphase.

Final Answer:
Interphase.