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Meiosis Division of Sex Cells Meiosis A 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. Diploid – 2 sets of chromosomes (2N) Haploid – 1 set of chromosomes (N) Homologous pair – chromosomes that each have a corresponding chromosome from the opposite sex parent Karyotyping Meiosis KM 3 Homologues Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues. Meiosis KM 4 Meiosis KM 5 Meiosis KM 6 Meiosis creates genetic variation During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. No daughter cells formed during meiosis are genetically identical to either mother or father During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. Meiosis KM 7 Stages of Meiosis Meiosis usually involves 2 distinct stages Meiosis I (PI, MI, AI, TI) Meiosis II (PII, MII, AII, TII) Interphase Meiosis in preceded by a preparatory phase known as INTERPHASE (G1,S,G2) (G1-first growth,S-DNA is copied,G2-growth) *cell grows (matures) *chromosomes replicate before Meiosis begins, directly before Prophase I Prophase I Chromatin condenses so that chromosomes become visible Nuclear envelope disassembles Centrosome (with centrioles) move to opposite ends of the cell Spindle fibers (cytoplasmic fibers) begin to appear Each chromosome pairs with its corresponding homologous chromosome, through a process called synapsis, to form a tetrad. (This step does not occur in Mitosis) Crossing-over (when chromosomes overlap and exchange portions of their chromatids) may occur, which leads to genetic recombination, or genetic variation. Crossing over Chiasmata (chiasma) – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes. Meiosis KM 11 Metaphase I Homologous pairs of chromosomes (tetrads) line up along the midline of the cell, often called the metaphase plate Spindle fibers attach to the centromere (kinetochore) of chromosomes (some say this happens in “prometaphase” Anaphase I The homologous chromosomes move toward opposite ends (poles) of the cell. Telophase I & Cytokinesis I Chromosomes move to opposite ends of cell Nuclear membranes form for 2 new nuclei Spindle apparatus disappears The cell separates into 2 “daughter” progeny cells through cytokinesis Cytokinesis: division of the cytoplasm of the cell that finalized the division of the entire cell. Meiosis I results in two haploid (N) cells, having half the genetic information as the parent cell. Prophase II In prophase II, Spindle fibers form; chromosomes begin to move to the midline of the cell Metaphase II The chromosomes line up at the metaphase plate w/ each sister chromatid facing opposite poles of the cell. Anaphase II Sister chromatids separate and move to opposite ends of the cell. Telophase II & Cytokinesis II Nuclear envelope forms around each of the 4 new cells; cytokinesis II occurs, resulting in 4 haploid cells. Gamete Formation In males, meiosis results in 4 sperm cells In females, meiosis results in 1 egg cell and three polar bodies, which are not used in reproduction. Mitosis vs Meiosis Results in Cells are Occurs in Mitosis 2 Diploid Cells (2N) Genetically Identical Somatic (Body) Cells Meiosis 4 Haploid Cells (N) Genetically Different Sex Cells Mitosis vs. meiosis Meiosis KM 21 Meiosis KM 24 Mitosis Interphase (G1,S,G2) G1-first growth period S phase, new DNA is synthesized by the cell resulting in each chromosome with two molecules of DNA. These are attached together at a structure called the centromere. G2-second growth period Remember that interphase is not part of mitosis! Prophase *chromosomes becoming visible *nuclear membrane disintegrates *paired centrioles(centrosomes) move to opposite ends of the cell *mitotic spindle forms *Late prophase is sometimes given a special name Prometaphase because of the two distinct events that happen then, namely, the disintegration of the nuclear membrane and, the attachment of the chromosomes to the spindle. Some would argue that the attachment of the chromosomes to the spindle happens during metaphase. Metaphase *chromosomes are lined up at the midline *fibers are attached to kinetochore (according to some) Remember that the chromosomes are still duplicated chromosomes during metaphase Anaphase *chromatids of each duplicated chromosome separate *chromosomes travel toward the centrosomes When the chromosomes reach the end of the spindles, telophase begins Telophase *membranes for the two new nuclei form *spindles are broken down by the cell *unduplicated chromosomes begin to unravel and stretch out At the end of telophase the nuclei have taken on the appearance of interphase nuclei, and the cell cycle is ready to begin again in each of the two cells, assuming cytokinesis has occurred. The basic point of cytokinesis is to divide the cytoplasm of the original cell such that each of the nuclei. from mitosis gets roughly equal amounts of cytoplasm and the organelles each new cell is going to need. Mitosis & Meiosis links http://staff.jccc.net/pdecell/celldivision/mito Mitosis Stages sis1.html#prophase (good description) http://www.cellsalive.com/mitosis.htm?iframe=true&width=90%&height=90% Mitosis and Meiosis Animation Mitosis Animation Mitosis Video Mitosis Stages (basic) http://www.johnkyrk.com/mitosis.html http://www.youtube.com/watch?v=VlN7K1-9QB0 http://library.thinkquest.org/C0118084/Gene/Chromoso mal_Inheritance/StagesMitosis.htm