<P> In prophase I of meiosis I, each chromosome is aligned with its homologous partner and pairs completely . In prophase I, the DNA has already undergone replication so each chromosome consists of two identical chromatids connected by a common centromere . During the zygotene stage of prophase I, the homologous chromosomes pair up with each other . This pairing occurs by a synapsis process where the synaptonemal complex - a protein scaffold - is assembled and joins the homologous chromosomes along their lengths . Cohesin crosslinking occurs between the homologous chromosomes and helps them resist being pulled apart until anaphase . Genetic crossing - over, a type of recombination, occurs during the pachytene stage of prophase I. In addition, another type of recombination referred to as synthesis - dependent strand annealing (SDSA) frequently occurs . SDSA recombination involves information exchange between paired homologous chromatids, but not physical exchange . SDSA recombination does not cause crossing - over . </P> <P> In the process of crossing - over, genes are exchanged by the breaking and union of homologous portions of the chromosomes' lengths . Structures called chiasmata are the site of the exchange . Chiasmata physically link the homologous chromosomes once crossing over occurs and throughout the process of chromosomal segregation during meiosis . Both the non-crossover and crossover types of recombination function as processes for repairing DNA damage, particularly double - strand breaks . At the diplotene stage of prophase I the synaptonemal complex disassembles before which will allow the homologous chromosomes to separate, while the sister chromatids stay associated by their centromeres . </P> <P> In metaphase I of meiosis I, the pairs of homologous chromosomes, also known as bivalents or tetrads, line up in a random order along the metaphase plate . The random orientation is another way for cells to introduce genetic variation . Meiotic spindles emanating from opposite spindle poles attach to each of the homologs (each pair of sister chromatids) at the kinetochore . </P> <P> In anaphase I of meiosis I the homologous chromosomes are pulled apart from each other . The homologs are cleaved by the enzyme separase to release the cohesin that held the homologous chromosome arms together . This allows the chiasmata to release and the homologs to move to opposite poles of the cell . The homologous chromosomes are now randomly segregated into two daughter cells that will undergo meiosis II to produce four haploid daughter germ cells . </P>

When does separation of pairs of homologous chromosomes occur in meiosis