What happens to the nuclear envelope during prophase?

• A. Breaks down
• B. Reforms
• C. Stays intact
• D. Duplicates

Answer: (A)

The nucleus must open up to allow the chromosomes to interact with the spindle. During prophase the nuclear envelope breaks down, removing the barrier between the chromosomes and the cytoplasm so microtubules can attach to kinetochores and begin moving the chromosomes toward the center. This disassembly is essential for proper chromosome capture and segregation. The envelope does not reform yet and the DNA has already been replicated earlier in the cell cycle, so duplication isn’t the event here. The envelope is later rebuilt during telophase, after chromosomes have been separated.