Crossover occurs during the pachytene stage of meiosis. Meiosis is the mechanism used by the reproductive organs to generate gametes-cells with the haploid number of chromosomes. DNA synthesis occurs before meiotic prophase begins and is followed by a G2 phase. Cells then enter meiotic prophase I. During meiotic prophase I, maternal and paternal' chromosomes are precisely paired, and recombination occurs in each pair of homologous chromosomes. The first meiotic prophase consists of five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis. During metaphase I, there is random segregation of maternal and paternal chromosomes. Homologous chromosomes are aligned on the metaphase plate of the meiotic spindle in metaphase I. The second meiotic division is responsible for the reduction in the chromosome content of the cell by 50%. In meiotic division II, metaphase consists of daughter chromatids of single homologous chromosomes aligned on a metaphase plate (metaphase II). Condensation of the chromatids occurs in leptotene. In zygotene, the synaptonemal complex begins to form, which initiates the close association between chromosomes known as synapsis. The bivalent is formed between the two sets of homologous chromosomes (one set maternal and one set paternal equals a pair of maternal chromatids and a pair of paternal chromatids). The four chromatids form a tetrad (bivalent). Pachytene begins as soon as the synapsis is complete and includes the period of crossover. The fully formed synaptonemal complex is present during the pachytene stage. At each point where crossover has occurred between two chromatids of the homologous chromosomes, an attachment point known as a chiasma forms. The formation of chiasmata and de synapsing (separation of the axes of the synaptonemal complex) occurs in the diplotene stage. Diakinesis is an intermediate phase between diplotene and metaphase of the first meiotic division.