Meiosis is the process by which a diploid nucleus divides twice to produce 4 haploid nuclei. The divisions are called meiosis I and meiosis II. In the life cycles of diploid organisms meiosis precedes sexual reproduction. Among animals, the products of meiosis are gametes—eggs or sperm. DNA is replicated prior to the start of meiosis. The identical sister chromatids are joined at the centromere as in mitosis. Unlike in mitosis, homologous chromosomes pair with one another. These pairs intertwine during early prophase of the first meiotic division and may exchange segments. This exchange is called crossing over. During prophase I, the nuclear envelope disappears and the spindle forms. The homologous pairs lie side by side as they reach the midplane of the spindle and attach to spindle fibers in Metaphase I. Metaphase ends and Anaphase I begins as the partners in each pair of homologous chromosomes separate as they are pulled toward opposite poles of the spindle. These chromosomes still consist of sister chromatids joined at their centromeres. During Telophase I the spindle disappears, nuclear membranes may re-form and the 2 nuclei, each containing a haploid set of chromosomes, are separated as cytokinesis divides the cytoplasm. Prophase II begins with the formation of a spindle and the still duplicated chromosomes move toward its mid-plane. At Metaphase II they are lined up and attached to spindle fibers. Anaphase II begins when centromeres separate and sister chromatids, now considered chromosomes, begin moving in opposite directions. During Telophase II the nuclear membrane re-forms, the spindle disappears, and cytokinesis divides the cytoplasm. The result is 4 haploid cells.