The division of the cell nucleus is called either mitosis or meiosis, depending on which type of division has occurred. Meiosis is a type of cell nucleus division specific to the reproduction of the species.
In mitosis, the cell nucleus reproduces itself exactly, with exact copies of the chromosomes transferred to each of the daughter nuclei. The nucleus is thus making an exact copy of itself. Meiosis treats the cell's chromosomes differently. Instead of copying them, the chromosomes are first split in half. The result of the first stage of meiosis is two daughter nuclei, each of which contains half the chromosomes of the original nucleus. The second stage of meiosis is very similar to mitosis, with the new daughter nuclei dividing and replicating themselves.