Meiosis Intro.notebook January 12, 2016 Meiosis Nov 126:42 PM • Meiosis is the reduction division of a diploid nucleus (two copies of each chromosome) to form two haploid nuclei (one copy of each chromosome in each nuclei) • The purpose of this is to create gametes (sex cells) with genetically unique combinations of chromosomes Nov 126:42 PM Meiosis Intro.notebook January 12, 2016 Ploidy Haploid One set of chromosomes Diploid Two sets of chromosomes Polyploid More than two sets of chromosomes Organism Diploid Haploid Homo sapiens 46 chromosomes 23 chromosomes Drosophila Melanogaster 8 chromosomes 4 chromosomes Nov 126:42 PM Process of Meiosis • In meiosis, homologous chromosomes (identical pairs of chromosomes – one from each parent ‐ that contain the same genetic information) are separated into different daughter cells • Meiosis has two stages: meiosis I and meiosis II • Similar to mitosis, meiosis I and II each include prophase, metaphase, anaphase, and telophase Nov 126:42 PM Meiosis Intro.notebook January 12, 2016 • Prophase I includes synapsis and crossing over • Synapsis is the association of four chromatids (two homologous chromosomes from each parent) • The resulting complex is called a tetrad • In humans there are 23 tetrads and 92 chromatids in this phase Nov 126:42 PM • The synaptonemal complex forms between the members of the tetrad and genetic information in the form of alleles is exchanged by crossing over • Crossing over results in great genetic variation • Crossing over occurs between adjacent non‐sister chromatids • These chromatids are the same length and have centromeres in the same position and contain the same genes Nov 126:42 PM Meiosis Intro.notebook January 12, 2016 • Homologous chromosomes are held together at chiasmata, which are sites of crossing over(usually shown as a single crossing over, but in reality many can from along the 4 chromatids) • Other typical events of prophase occur • During meiosis I, homologous chromosomes separate • Tetrads align at the equator in metaphase I • The sister kinetechores of each homologue are attached to spindle fibers attached to only one of the poles Nov 126:42 PM • Each pole receives a mixture of maternal and paternal chromosomes (creates a great amount of genetic variation) • In telophase I chromosomes decondense; the nuclear membrane may reform and cytokinesis usually occurs • Interkinesis separates meiosis I and II; no DNA synthesis occurs • It is a very short period and may be absent in some organisms Nov 126:42 PM Meiosis Intro.notebook January 12, 2016 • Chromatids separate in Meiosis II • Prophase II is brief, involves recondensation of the chromosomes, and events are very similar to those of prophase in mitosis (as are most stages of Meiosis II) • Chromosomes line up at the equator in metaphase II • The chromatids separate in anaphase II (they are now called chromosomes) • In telophase II there is one copy of each homologous chromosome at each pole • The end product is typically 4 haploid cells Nov 126:42 PM Nov 126:42 PM Meiosis Intro.notebook January 12, 2016 Mar 119:48 AM
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