What phase of the cell cycle occurs when the nucleus is not dividing?

The phase of the cell cycle when the nucleus is not dividing is interphase. During interphase, the cell is engaged in growth, performing its normal functions, and preparing for cell division. This phase is subdivided into three stages: G1 (first gap), S (synthesis), and G2 (second gap). It is essential for the cell to complete these stages before entering mitosis, where the nucleus divides. In interphase, the genetic material is also replicated, ensuring that each daughter cell will have the necessary DNA after division.

In contrast, the other phase options represent stages of mitosis where nuclear division is actively occurring. Prophase is the first step of mitosis, where chromosomes condense and the nuclear envelope begins to break down. Metaphase follows, characterized by chromosomes aligning at the cell's equatorial plane. Telophase marks the final stage of mitosis, where the chromosomes have been pulled apart and the nuclear envelope reforms around each set of chromosomes. Thus, interphase is indeed the correct phase when the nucleus is not undergoing division.