What is Meiosis? Meiosis is a cell division process that results in the formation of sex cells. -gametes in animals -spores in plants Meiosis involves two divisions producing a total of four daughter cells. Summary of the Phases of Meiosis A cell undergoing meiosis will divide two times; the first division is meiosis 1 and the second is meiosis 2. Each has four phases which will be covered in this presentation. meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1 meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2 Location: Purpose: Meiosis only occurs in sexually reproducing organisms such as eukaryotic plant and animal diploid cells. Diploid cells are cells which have two sets of chromosomes. The purpose of meiosis to reduce the normal diploid cells (2n) to haploid cells (n), “n” being the chromosome number. Prophase I: • • • • • • • Already copied chromosomes start to thicken and shorten. Chromosomes pair together. Crossing-over occurs. Two homologous pairs join together to form tetrads. Nucleolus disappears. Meiotic spindle forms between two pairs of centrioles. Spindle enters nucleus. Metaphase I: • Centrioles at opposite poles of cell. • Tetrads become situated on metaphase plate. • Spindle fibers from one pole attach to one chromosome of each pair; spindle fibers from other pole attach to other chromosomes. Metaphase Plate Meiotic Spindle Anaphase I: • Two chromosomes of each tetrad separate, move toward opposite poles. • Note: Sister chromatids remain attached at centromeres, move together toward poles. Telophase I: • Homologous chromosome pairs complete migration to poles; now haploid set of chromosomes at each pole. • Nuclear envelope forms around each chromosome set. • Spindle disappears. • Cytokinesis occurs. Begins with the two sister cells formed in Meiosis I The four phases of Meiosis II are: 1. Prophase 2 2. Metaphase 2 3. Anaphase2 4. Telophase2 The results are four haploid daughter cells, each with half the chromosomes as the original diploid cell Step 1: Prophase 2 - Begins without duplicating DNA. -Nucleolus and nuclear envelope disappear -Centriole duplicates and they move to opposite poles of the cell Step 2: Metaphase 2 -The spindle fibers from the centrioles attach to the chromosomes - Chromosomes line up on the equatorial plate. Step 3: Anaphase 2 - Centromeres divide and separated sister chromatids move to each pole. Step 4: Telophase 2 - Four haploid daughter cells are produced, and cell division is complete. What is special about the end product? The daughter cells are genetically different from the parent cell, and have half the number of chromosomes. Prophase 1 •Nucleolus disappears, and meiotic spindle forms between 2 centrioles •Nuclear envelope disappears, meiotic spindle enters Metaphase1 •Centrioles move to opposite poles of cell •The chromosomes are arranged on the metaphase plate •Spindle fibers from each pole attach to a chromosome from each pair of sister chromatids Anaphase 1 •Chromosomes of each bivalent separate and are pulled by the spindle to opposite poles Telophase 1 •The chromosomes continue to move until there is a haploid set of chromosomes at each pole •A nuclear envelope forms around each chromosome set •The spindle disappears and cytokinesis follows Prophase 2 •Nucleolus disappears and meiotic spindle forms between 2 centrioles •The nuclear envelope disappears and the spindle apparatus forms Metaphase 2 •Centrioles move to opposite poles of cell •Chromosomes are arranged on the metaphase plate Anaphase 2 •Centromeres are pulled apart •Separated chromatids are moved to opposite poles of the cell by the meiotic spindle Telophase 2 •Nuclear envelope forms around chromosomes •Cytokinesis produces four daughter cells, each having a set of haploid chromosomes "BioCoach Activity." Meiosis. 2006. Pearson Education, Inc.. 11 Oct 2006 <http://www.phschool.com/science/biology_place/biocoach/mei osis/teloii.html>. Carter, J. Stein. "Meiosis." Meiosis. 02 Nov. 2004. Sci Links. 11 Oct 2006 <http://biology.clc.uc.edu/courses/bio104/meiosis.htm>. Foster, Gerry. "Section 3: Cells and the Cell Cycle - Part c." Meiosis. 2006. 11 Oct 2006 <http://www.dvbiology.org/biologyweb/bcell2.htm#part%20d>. "The Biology Project: Cell Biology." Meiosis I & II. Aug. 2004. Department of Biochemistry and Molecular Biophysics . 11 Oct 2006 <http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3. html>.