Genetic Variation Meiosis reduces the number of chromosomes and produces haploid cells from diploid cells.

Stages of Meiosis - I Interphase Prophase I DNA is replicated and sister chromatids are joined at the centromere regions Prophase I chromosomes condense and pair crossing-over may occur as DNA segments are exchanged between homologous chromosomes place of crossing over forms a chiasmata (X-like region that holds the homologues together) synaptonemal complex holds homologues together and then degrades by the end of the phase aids in crossing over formed by a specialized protein complex the rest is the same as in mitosis

Stages of Meiosis - I Metaphase I Anaphase I Telophase I (cytokinesis) tetrads are now aligned at the metaphase plate rest like mitosis Anaphase I chromosome pairs (sister chromatids) move toward the poles Telophase I (cytokinesis) each cell is now haploid but still consists of pairs of sister chromatids cytokinesis splits the cell and cells are now haploid at 2n = 46

Stages of Meiosis - II Prophase II Metaphase II Anaphase II spindle fibers again grow and sister chromatid pairs move toward the metaphase plate Metaphase II chromosomes are now at the metaphase plate but are not genetically identical due to crossing over Anaphase II sister chromatid pairs separate at the centromere region and move toward opposite poles Telophase II (cytokinesis) nuclei begin to form and DNA begins to uncondense cytokinesis separates divides the cell there are now 4 haploid cells (n=23) each daughter cell is genetically unique

Genetic Variation Genetic variation is made possible through the following processes: Independent assortment of chromosomes because the arrangement at the metaphase plate is random, each haploid has a 50% chance that it will receive a parental chromosome Crossing over creates recombinant chromosomes that are a mix or both parental chromosomes Random fertilization because of random fertilization each individual contains a genetically unique compliment of chromosomes and genes