Meiosis is a specialized form of nuclear division in which two successive nuclear divisions (meiosis I and II) occur without any chromosomal replication between them. Each division can be divided into 4 phases similar to those of mitosis (pro-, meta-, ana- and telophase). Meiosis occurs during the formation of gametes in animals. Meiosis is a special type of cell division that occurs during the formation of sperm and eggs and provides them with the correct number of chromosomes. Because the sperm and egg unite during fertilization, each need only have half the number of chromosomes as other cells in the body. Otherwise the fertilized cell would have too many. Inside the cells that produce sperm and eggs, chromosomes pair. As they are pressed together, the chromosomes can break apart, and each can exchange a portion of its genetic material with the corresponding portion of its mate. This form of recombination is called crossover. When the chromosomes stick together again and separate, each has picked up new genetic material from the other. The constellation of physical characteristics it determines is now different than before the crossbreeding. In Meiosis 1, the chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step of meiosis that generates genetic diversity. Meiosis 2 is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical due to recombination. Meiosis II separates the chromatids, producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid. During prophase I, homologous chromosomes pair and form synapses. The paired chromosomes are called bivalent and the formation of chiasmata caused by genetic recombination becomes evident. Bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent. In prometaphase I, the nuclear membrane disappears. One kinetochore per chromosome is formed and the chromosomes attached to the spindle fibers begin to move. In metaphase I, bivalents, each composed of two chromosomes, align on the metaphase plate. The orientation is random, with the parent counterpart on one side. This means that there is a 50% chance that the daughter cells will get the mother's or father's homolog for each chromosome. In anaphase I, the chiasmata separate.