<P> Meiosis is a round of two cell divisions that results in four haploid daughter cells that each contain half the number of chromosomes as the parent cell . It reduces the chromosome number in a germ cell by half by first separating the homologous chromosomes in meiosis I and then the sister chromatids in meiosis II . The process of meiosis I is generally longer than meiosis II because it takes more time for the chromatin to replicate and for the homologous chromosomes to be properly oriented and segregated by the processes of pairing and synapsis in meiosis I. During meiosis, genetic recombination (by random segregation) and crossing over produces daughter cells that each contain different combinations of maternally and paternally coded genes . This recombination of genes allows for the introduction of new allele pairings and genetic variation . Genetic variation among organisms helps make a population more stable by providing a wider range of genetic traits for natural selection to act on . </P> <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 occurs during the pachytene stage of prophase I. In this process, 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 . 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>

A pair of homologous chromosomes each consisting of two sister chromatids is called
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