Short answer: In most textbooks and standard cell biology courses, the nuclear membrane (nuclear envelope) breaks down during the late prophase or prometaphase of mitosis, with the envelope disappearing as chromatids condense and spindle apparatus forms. Reassembly of the nuclear envelope occurs after chromosomal segregation during telophase and into new daughter nuclei. Details and context:

• Open vs closed mitosis: In many multicellular eukaryotes (higher eukaryotes), mitosis is “open,” meaning the nuclear envelope disassembles to allow spindle access to chromosomes. In some organisms (e.g., many yeasts), mitosis is “closed,” and the envelope remains intact. This distinction is important for understanding that the envelope’s disappearance is not universal across all organisms [envelope disassembly is a hallmark of open mitosis in many animals and plants].

• Timing within mitosis:
  • Prophase: Chromosomes condense, nucleolus disassembles; the nuclear envelope begins to break down in many cells but is not yet completely gone.
  • Prometaphase: The nuclear envelope largely breaks down, nuclear pore complexes disassemble, and microtubules can attach to kinetochores. This is typically when the envelope is effectively gone in open mitosis.

* Metaphase: Chromosomes align at the metaphase plate with spindle microtubules attached.
* Anaphase/Telophase: Chromosomes separate, the nuclear envelope reassembles around each set of chromosomes during telophase, forming two new nuclei. The envelope reformation marks the end of mitosis in the daughter cells.

Common exam-level takeaway:

• "Nuclear envelope breaks down in late prophase to prometaphase in open mitosis; it re-forms during telophase" is a widely tested sequence. Some sources emphasize late prophase as the critical window when the envelope becomes discontinuous or largely absent, leading into prometaphase.

If you’re studying for a specific exam (MCAT, biology course, etc.), note that some sources phrase the breakdown as occurring during prometaphase, while others say late prophase. The practical implication is: the envelope is gone (or effectively gone) by prometaphase in open mitosis, and the reassembly happens in telophase as daughter nuclei form.