Meiosis Meiosis – M phase that occurs in the ovaries (females) or testes (males) of an organism to create gametes or sex cells Produces 4 haploid cells using 2 stages of meiosis (Meiosis I and Meiosis II) Makes 4 n sperm cells in males (spermatogenesis) Makes 1 n egg cell and 3 n polar bodies in females (oogenesis)

Meiosis I Prophase I Nuclear membrane and nucleolus dissolves Chromatin finishes coiling into Chromosomes Centrioles move to opposite poles and begin forming spindle fibers Homologous chromosomes pair together through synapsis – Independent assortment occurs (no way to know which cell will get which chromosome from each pair) – Crossing over of genes may occur with the tetrad pair creating genetic recombination

Homologous chromosomes - chromosome pairs, one from each parent, that are the same in length, gene position, and centromere location The position of the genes on each homologous chromosome is the same, however the genes may contain different alleles Called a tetrad

Metaphase I Spindle fibers are attached to the chromosomes at their centromeres Tetrads (homologous chromosome pairs) are pulled to the metaphase plate or center of the cell be the spindle fibers Anaphase I Spindle fibers pull the homologous chromosomes to opposite sides of the cell Each chromosome still has two sister chromatids Nondisjunction can occur here

Nondisjunction - failure of homologous chromosomes or sister chromatids to separate properly during cell division Telophase I Chromosomes stay in their coiled up shape Nuclear membrane may or may not reform; depends on the species Cytokinesis occurs (Cleavage furrow forms in animal cells; cell plate forms in plant cells) Produces 2 diploid daughter cells The cells will enter a brief interphase but they DO NOT replicate their DNA again. It is just long enough to prepare for Meiosis II. How long depends on the species (can be minutes to hours)

Meiosis II (Looks just like mitosis but there are two cells) Prophase II Spindle fibers reattach to the centromeres If nuclear membrane reformed, it dissolves again Cenrioles move to opposite sides of the cell Metaphase II Individual chromosomes line up on the metaphase plate or equator of each cell

Anaphase II Spindle fibers pull the chromosomes apart causing sister chromatids to go to opposite sides of the cell Telophase II Nuclear membrane and nucleolus reappears Spindle fibers dissolve Chromatids uncoil into chromatin Cytokinesis occurs (Cleavage furrow forms in animal cells; cell plate forms in plant cells) Produces 4 different haploid cells

Meiosis provides variation in sexual reproduction increasing species ability to evolve and survive (Natural selection; survival of the fittest) Caused by independent assortment during the formation of a tetrad Caused by crossing-over within homologous chromosomes

Genetic Recombination nonsister chromatids chiasmata: site of crossing over variation Tetrad 9