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MEIOSIS Meiosis cell division that occurs as a part of sexual reproduction in which the daughter cells receive the haploid number of chromosomes in varied combination Diploid The full number of chromosomes Haploid Half the diploid number of chromosomes Chromosome gene carrying structure in the nucleus Homologous Chromosome Similarly made chromosomes with the same shape and genes Sister Chromatid one of two genetically identically chromosomal units that are the result of DNA replication (they are attached together) Tetrad two homologous chromosomes which have paired during meiosis Synapsis pairing of homologous chromosomes during prophase I of meiosis Crossing Over exchange of segments of genetic material between non-sister chromatids of homologous chromosomes during synapsis of meiosis I Gamete Haploid sex cell (the egg or sperm) Zygote Diploid cell formed by the union of sperm and egg (the product of fertilization) Mitosis Cell division where the daughter cells are genetically identical to the parental cells in chromosome number and structure (occurs during growth and repair) FIGURE 13.4 DESCRIBING CHROMOSOMES Key Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set) OVERVIEW OF MEIOSIS: HOW MEIOSIS REDUCES CHROMOSOME NUMBER (END RESULTS) Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes FIGURE 13.8 THE MEIOTIC DIVISION OF AN ANIMAL CELL MEIOSIS I: Separates homologous chromosomes INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Centromere (with kinetochore) Centrosomes (with centriole pairs) Sister chromatids Nuclear envelope Chiamata Spindle Metaphase plate Homologous Microtubule chromosomes attached to Chromatin separate kinetochore Tetrads line up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Tetrad Pairs of homologous chromosomes split up FIGURE 13.8 THE MEIOTIC DIVISION II: Separates sister chromatids OF AN ANIMAL MEIOSIS CELL TELOPHASE I AND CYTOKINESIS PROPHASE II Cleavage furrow Two haploid cells form; chromosomes are still double METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming Sister chromatids Separate (not identical) During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes FIGURE 13.9 A COMPARISON OF MITOSIS AND MEIOSIS MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2n = 6 Chromosomes positioned at the metaphase plate Metaphase Sister chromatids separate during anaphase Anaphase Telophase 2n Tetrads positioned at the metaphase plate Metaphase I Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n=3 Daughter cells of meiosis I 2n MEIOSIS II Daughter cells of mitosis n n n Daughter cells of meiosis II Sister chromatids separate during anaphase II n FIGURE 13.10 THE INDEPENDENT ASSORTMENT OF HOMOLOGOUS CHROMOSOMES IN MEIOSIS Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 23 = 8 223 = 8 million FIGURE 13.11 THE RESULTS OF CROSSING OVER DURING MEIOSIS Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I an average of one to three times per chromosome pair. A big contribution to diversity? Metaphase II 3. Random fertilization. Daughter cells No chance of siblings to be identical? Recombinant chromosomes