During the eukaryotic cell cycle, in which specific phase does DNA replication occur so that each chromosome is duplicated before mitosis?

Introduction / Context:
The eukaryotic cell cycle is divided into interphase and the mitotic phase. During interphase, the cell grows and prepares for division. A crucial part of this preparation is the accurate duplication of DNA so that each daughter cell receives a complete set of genetic material. This question asks you to identify the specific phase in which DNA replication occurs.

Given Data / Assumptions:

• The cell cycle phases include G1, S, G2 (interphase), and M (mitosis).
• The options mention S phase, M phase, prophase, and anaphase.
• We assume that you know that mitosis itself is mainly concerned with chromosome separation rather than DNA synthesis.

Concept / Approach:
DNA replication occurs during the S phase (synthesis phase) of interphase. In this phase, each chromosome is duplicated to form two sister chromatids, which remain attached at the centromere. Mitosis, which comprises prophase, metaphase, anaphase, and telophase, follows interphase and is mainly responsible for distributing these sister chromatids to daughter cells. Prophase involves chromosome condensation, and anaphase involves separation of sister chromatids, but the DNA has already been replicated before these stages begin. Therefore, the correct answer is the S phase of interphase.

Step-by-Step Solution:
Step 1: Recall that interphase consists of three subphases: G1 (cell growth), S (DNA synthesis), and G2 (further growth and preparation). Step 2: Remember that during the S phase, the entire DNA content of the cell is replicated, producing identical copies of each chromosome. Step 3: Recognise that the M phase, including prophase and anaphase, deals with segregation of already replicated chromosomes, not with the replication itself. Step 4: Choose S phase of interphase as the phase where DNA replication actually occurs.

Verification / Alternative check:
Cell cycle diagrams in textbooks clearly label S phase as the time of DNA synthesis. Experimental techniques such as incorporation of labelled nucleotides show increased DNA synthesis specifically during S phase. Flow cytometry measurements of DNA content also confirm that cells double their DNA amount during this phase before entering mitosis. All these data support the conclusion that DNA replication is confined to S phase in a normal cell cycle.

Why Other Options Are Wrong:

• M phase: This includes mitosis and cytokinesis, focusing on division and distribution of chromosomes, not their replication.
• Prophase: In prophase, chromatin condenses into visible chromosomes, but the DNA has already been replicated earlier.
• Anaphase: In anaphase, sister chromatids separate and move toward opposite poles, but replication is long complete by this stage.

Common Pitfalls:
Learners sometimes think that because chromosomes become visible in prophase, replication must occur then. In reality, condensation only makes the already replicated DNA visible as distinct structures. Another pitfall is to treat the entire mitotic phase as including synthesis. It is important to remember that synthesis happens earlier in interphase, giving the cell enough time to check and repair DNA before division. Keeping the sequence G1 → S → G2 → M firmly in mind helps avoid these errors.

Final Answer:
DNA replication occurs during the S phase (synthesis phase) of interphase before mitosis begins.