During mitosis, in which phase does the nuclear envelope (membrane around the nucleus) break down and disappear?

• A. Telophase
• B. Prophase (often completed by prometaphase)
• C. Anaphase
• D. Metaphase

Answer: (B)

Verified step by step guidance

1. Recall the stages of mitosis: prophase, prometaphase, metaphase, anaphase, and telophase.
2. Understand that the nuclear envelope surrounds the nucleus and must disassemble to allow spindle fibers to interact with chromosomes.
3. Identify that during prophase, chromatin condenses into visible chromosomes, but the nuclear envelope is still intact initially.
4. Recognize that the nuclear envelope breaks down during late prophase or prometaphase, allowing spindle microtubules to attach to kinetochores on chromosomes.
5. Note that by metaphase, the nuclear envelope has completely disappeared, and chromosomes align at the metaphase plate.