The nuclear membrane dissolves during the prometaphase of mitosis, while it disintegrates during prometaphase I and prometaphase II of meiosis. The breakdown of the nuclear envelope is due to the addition of a phosphate group to nuclear lamins by the enzyme M-CDK in a process known as phosphorylation.
The two types of cellular division in most multicellular organisms are mitosis and meiosis. Mitosis is the process of producing two diploid daughter cells, while meiosis is the process of generating four diploid cells called gametes, which are used for sexual reproduction.
Mitosis is divided into several phases, including prophase, prometaphase, metaphase, anaphase and telophase. As the intermediary phase between prophase and metaphase, prometaphase is sometimes considered part of prophase. The disintegration of the nuclear membrane into tiny vesicles signals the end of prophase and the beginning of prometaphase. When the envelope breaks down, the mitotic spindle fibers gain access to the DNA of the cell. The fibers then attach to the chromosomes at their kinetochores and begin moving to polar opposites of the cell.
Meiosis undergoes two nuclear divisions: meiosis I and meiosis II. Meiosis I and meiosis II basically have the same phases as mitosis, except for the Roman numeral designation that indicates the order of division. The breakdown of the nuclear envelope occurs during prometaphase I and prometaphase II, wherein one kinetochore is produced by a single chromosome rather than one kinetochore for every chromatid. The spindle fibers are then connected to the chromosomes to be transported to opposite ends of the cell.