During the cell cycle of a eukaryotic cell, which of the following processes never occurs during interphase and instead takes place only during the mitotic (M) phase?

Introduction / Context:
The eukaryotic cell cycle is divided into interphase and the mitotic (M) phase. Interphase itself includes the G1, S, and G2 phases and is the time when the cell grows, replicates its DNA, and prepares for division. The M phase includes mitosis and cytokinesis. Many exam questions ask which processes occur in interphase versus which occur only in the M phase. This question specifically asks which process never takes place during interphase.

Given Data / Assumptions:

• The context is a typical eukaryotic cell cycle.
• Interphase includes G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).
• The options list DNA replication, protein synthesis, nuclear division, and cell growth and organelle duplication.
• We assume standard textbook definitions of interphase and mitotic phase.

Concept / Approach:
Interphase is sometimes called the resting phase, but this is misleading because the cell is very active. During G1, the cell grows and synthesises proteins; during S phase, DNA replication occurs; during G2, there is further growth and preparation for mitosis. However, the actual process of nuclear division, in which chromosomes condense, align, separate, and move to opposite poles, occurs only during the mitotic phase. Therefore, nuclear division (mitosis) never occurs in interphase; it is restricted to the M phase.

Step-by-Step Solution:
Step 1: Recall that interphase comprises G1, S, and G2 phases. Step 2: In G1, the cell increases in size, produces RNA and proteins, and carries out normal metabolic functions. Step 3: In S phase, the cell replicates its DNA so that each chromosome now consists of two sister chromatids. Step 4: In G2, the cell continues to grow, synthesises proteins needed for mitosis, and checks for DNA damage. Step 5: The mitotic phase follows interphase and includes prophase, metaphase, anaphase, and telophase, during which the nucleus divides and chromosomes are distributed to daughter cells. Step 6: Therefore, nuclear division (mitosis) is the process that does not occur in interphase and is confined to the M phase.

Verification / Alternative check:
Diagrams of the cell cycle typically show interphase as the long portion of the cycle during which the cell grows, copies its DNA, and prepares for division, followed by a shorter M phase where mitosis and cytokinesis occur. The key features of mitosis, such as chromosome condensation, spindle formation, and separation of chromatids, are only shown in the M phase stages, not in interphase. This visual separation reflects the fact that nuclear division is not part of interphase.

Why Other Options Are Wrong:
DNA replication in preparation for cell division is wrong as the answer because DNA replication occurs in S phase, which is part of interphase. Protein synthesis needed for cell growth and function also occurs throughout interphase, especially in G1 and G2, so option B is not correct. Cell growth and duplication of organelles occur mainly in G1 and G2 phases of interphase as the cell readies itself for division, so option D is also incorrect.

Common Pitfalls:
A common pitfall is to think of interphase as completely inactive and to assume that important processes like DNA replication or protein synthesis must occur only in the mitotic phase. In reality, interphase is where most of the cell's life processes take place. Another mistake is to forget the distinction between DNA replication and nuclear division; replication is copying DNA, while mitosis is separating the copies into daughter nuclei. Remembering that mitosis is synonymous with nuclear division and happens only in M phase will help you choose the correct option.

Final Answer:
The correct choice is Nuclear division (mitosis) in which chromosomes are separated, because this process never occurs during interphase and takes place only during the mitotic phase of the cell cycle.