Meiosis Notes

Meiosis Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell; happens in gametes (sex cells) – sperm & egg Homologous chromosomes – chromosomes that have a corresponding chromosome from the opposite-sex parent (2 sets of chromosomes, one from each parent)

Diploid vs. Haploid Diploid – a cell that contains both sets of homologous chromosomes (two sets); represented by the symbol 2N Found in somatic or body cells (ex. Skin, digestive tract) Example : Humans – 2N = 46 Haploid – a cell that contains only a single set of chromosomes (one set); represented by the symbol N or 1N Found in gametes or sex cells – sperm & egg Example: Humans – N = 23

Steps of Meiosis Meiosis usually involves two distinct divisions, called Meiosis I and Meiosis II By the end of Meiosis II, the one diploid cell that entered meiosis has become 4 haploid cells

Meiosis I Chromatin Nuclear Envelope Centrioles Interphase I Cell undergoes a round of DNA replication forming duplicate chromosomes

Meiosis I TETRAD CROSSING OVER Homologous chromosomes Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad

Meiosis I As the homologous chromosomes pair up and form tetrads in Meiosis I, they exchange portions of their chromatids in a process called crossing-over This produces chromosomes with different combinations than the parents

Meiosis I Metaphase I Chromosomes attach to the spindle fibers and move to the Middle of the cell

Meiosis I Anaphase I The spindle fibers pull the homologous chromosomes toward opposite ends of the cell

Meiosis I Telophase I & Cytokinesis Nuclear envelopes form & the cell separates into two cells

Meiosis II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell The two cells produced by meiosis I now enter a second meiotic division, but unlike the first meiotic division, neither cell goes through a round of DNA replication before entering Meiosis II

Meiosis II Prophase II Spindle fiber forms from the centrioles and the chromosomes attach themselves

Meiosis II Metaphase II Chromosomes line up in the center of each cell (similar to mitosis)

Meiosis II Anaphase II The sister chromatids separate and move toward opposite ends of the cell

Meiosis II Telophase II & Cytokinesis Nuclear membranes form and the cells separate into 4 haploid (N) daughter cells

Gamete (Sex Cell) Formation In male animals (including humans), the haploid gametes produced by meiosis are called sperm 4 sperm are produced from one meiotic division

Gamete (Sex Cell) Formation In female animals (including humans), the haploid gametes produced by meiosis are called eggs The cell divisions at the end of meiosis I & II are uneven, so that 1 large egg is produced and other 3 cells produced, called polar bodies, are not involved in reproduction

Meiosis Animation Meiosis I Animation http://wps.prenhall.com/wps/media/objects/487/498728/CDA9_1/CDA9_1b/CDA9_1b.htm Meiosis II Animation http://wps.prenhall.com/wps/media/objects/487/498728/CDA9_1/CDA9_1c/CDA9_1c.htm