During mitosis, in which stage does the nuclear envelope break down so the nucleus is no longer visible as an intact structure?

• A. Prophase (often described more precisely as prometaphase)
• B. Telophase
• C. Anaphase
• D. Metaphase

Answer: (A)

Verified step by step guidance

1. Recall the stages of mitosis: prophase, prometaphase, metaphase, anaphase, and telophase, each with distinct cellular events.
2. Understand that the nuclear envelope is a membrane that encloses the nucleus, keeping it separate from the cytoplasm.
3. Identify that during prophase, chromosomes condense and become visible, but the nuclear envelope is still intact at this point.
4. Recognize that the nuclear envelope breaks down during prometaphase, which is often considered part of late prophase, allowing spindle fibers to interact with chromosomes.
5. Note that in metaphase, anaphase, and telophase, the nuclear envelope is either already broken down or re-forming, so the nucleus is not intact during these stages.