Mitosis is the process by which the contents of the eukaryotic nucleus are separated into 2 genetically identical packages. Chromosomes replicate prior to the beginning of mitosis. As mitosis begins they condense and become visible under a light microscope. They appear as sister chromatids joined at the centromere. Mitosis is divided into 4 stages. During prophase, the nuclear envelope disintegrates and a spindle of microtubules forms. Centrioles may help organize the spindle as in this animal cell. The chromosomes begin to move toward the midplane of the spindle. When they are on the midplane with centromeres attached to spindle fibers, the second stage, metaphase has been reached. Metaphase yields to anaphase as the centromeres separate and the sister chromatids, now termed chromosomes, are pulled toward opposite poles of the spindle. During the final stage, telophase, a nuclear envelope forms around each set of chromosomes, the spindle disappears and the chromosomes decondense. The result is 2 nuclei, each with an identical set of chromosomes.