Here's why:
* The nucleolus is a structure within the nucleus responsible for ribosome synthesis. During prophase, the nuclear envelope breaks down, allowing the chromosomes to condense and move freely within the cell.
* The disintegration of the nucleolus is a consequence of the breakdown of the nuclear envelope. The nucleolar components disperse throughout the cytoplasm, and ribosome synthesis is temporarily halted.
* The nucleolus will reassemble during telophase when the nuclear envelope reforms around the chromosomes, and ribosome synthesis resumes.
