Meiosis - PDF

12.MEIOSIS  Overview Sex cells Two divisions (Meiosis I & Meiosis II) Four daughter (haploid) cells, different to the mother cell (diploid). Sexual reproduction To produce offspring with a different combination of traits Reduce the number of chromosomes Fertilization: 2 haploids into 1 diploid ...

12.MEIOSIS  Overview Sex cells Two divisions (Meiosis I & Meiosis II) Four daughter (haploid) cells, different to the mother cell (diploid). Sexual reproduction To produce offspring with a different combination of traits Reduce the number of chromosomes Fertilization: 2 haploids into 1 diploid ❖SYNAPSIS occurs, nuclear membrane breaks down ❖HOMOLOGOUS CHROMOSOMES (each still ❖HOMOLOGUES LINE UP side by side and CROSSING consisting of 2 chromatids) travel into opposite poles OVER OCCURS. ❖Homologous PAIRS LINE UP AT METAPHASE PLATE such ❖CYTOKINESIS PRODUCES 2 that maternal or paternal member may be oriented toward DAUGHTER CELLS WHICH ARE either pole. INDEPENDENT ASSORTMENT happens. HAPLOID.  CROSSING OVER OCCURS BETWEEN NON-SISTER CHROMATIDS OF HOMOLOGOUS CHROMOSOMES DURING Prophase I. CROSSING OVER means that the chromatids held together by a centromere are no longeridentical. INDEPENDENT ASSORTMENT Independent assortment of paired chromosomes occurs during Metaphase I. Occurs because there are various ways for chromosomes to line up. The order they lie in determines what genetic information is present in each of the 4 gametes produced. ⚠ THIS (ALSO) INCREASES GENETIC VARIATION. WITH 23 CHROMOSOMES IN A HUMAN GAMETE, 8,388,608 COMBINATIONS  Both CROSSING OVER and INDEPENDENT ASSORTMENT assure that GAMETES will contain different combinations of chromosomes, and that the resulting OFFSPRING WILL BE GENETICALLY UNIQUE.  MEIOSIS II Prophase II Cells have one member of each homologous pair. Metaphase II Chromosomes line up at the metaphase plate. Anaphase II Centromeres divide and daughter chromosomes migrate. Telophase II Nuclei form, Cytokinesis happens. NONDISJUNCTION IS THE FAILURE OF PAIRED CHROMOSOMES OR CHROMATIDS TO SEPARATE DURING CELL DIVISION IT RESULTS IN CELLS WITH AN ABNORMAL NUMBER OF CHROMOSOMES  MEIOSIS I MEIOSIS II DNA replication occurs before No replication of DNA occurs. this division Centromeres divide and sister Homologues line up side by side chromatids migrate to opposite at equator (SYNAPSIS) poles to become individual When pairs separate, each chromosomes daughter cell receives one Each of the four daughter cells member of the pair. produced has the HAPLOID Cells are now HAPLOID and each chromosome number and each chromosome is composed of TWO chromosome is composed of ONE SISTER CHROMATIDS CHROMATID MITOSIS VS MEIOSIS  Mitosis Meiosis One cell division Two succesive cel divisions Two identical daughter cells (diploid) Four daughter cells (haploid) genetically different and gentically identical Occurs only in animals, plants and fungi Occurs in all organisms except viruses Creates germ cells Creates all body cells (somatic) Prophase takes much longer Prophase shorter Involves crossing over No crossing over In Metaphase I, pairs of chromosomes line In Metaphase individual chromosomes up along the equator line up along the equator During Anaphase I, the sister chromatides During Anaphase the sister move together to the same poles chromatides are separated to opposite During Anaphase II the sister chromatides poles are separated to opposite poles  Mitosis Meiosis Diploid parent cell Diploid parent cell Interphase, Prophase, Metaphase, Interphase, Prophase, Metaphase, Anaphase and Telophase Anaphase and Telophase (but twice!) In Metaphase individual chromosomes In Metaphase II individual chromosomes line up along the equator line up along the equator During Anaphase the sister During Anaphase II the sister chromatides chromatides are separated to opposite are separated to opposite poles poles Ends with cytokinesis Ends with cytokinesis  HUMAN LIFE CYCLE REQUIRES BOTH MITOSIS AND MEIOSIS. Meiosis in the Female is called OOGENESIS Meiosis in the Male is called SPERMATOGENESIS At fertilization, the resulting Zygote DIVIDES BY MITOSIS FOR THE PROCESSES OF GROWTH AND DEVELOPMENT. MITOSIS IS USED FOR REPAIR THROUGHOUT LIFE. Special Sex Organs: Gonads: Testes ♂ and Ovaries ♀. Female Gamete (n) + Male Gamete (n) = Zygote (2n) - VARIATION- EVOLUTION