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MEIOSIS Production of Gametes 1 Meiosis Facts 2 Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced • Diploid (2n) haploid (n) • Called a reduction division (because # of chromosomes is reduced) • Meiosis is sexual reproduction • Two divisions (meiosis I and meiosis II) 3 Meiosis • Germ Line cells (aka sex cells) divide to produce gametes (sperm or egg) • Gametes have half the number of chromosomes • Meiosis is similar to mitosis with some chromosomal differences 4 Meiosis • Occurs only in gonads (testes or ovaries) • Male: Spermatogenesis • Female: Oogenesis 5 Spermatogenesis n=23 Human sex cell Sperm n=23 n=23 2n=46 n=23 Diploid (2n) Haploid (n) n=23 n=23 Meiosis I Meiosis II 6 Interphase I • Similar to mitosis interphase • Chromosomes replicate (S phase) • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres • Centriole pairs also replicate. 7 Interphase I • Nucleus and nucleolus visible. Chromatin Nuclear membrane Cell membrane Nucleolus 8 Meiosis I 9 Stages of Meiosis I • Cell division that reduces the chromosome number by one-half • Four phases: a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I 10 Prophase I • Longest and most complex phase (90%) • Chromosomes condense • Synapsis occurs: homologous chromosomes come together to form a tetrad • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids) 11 Prophase I - Synapsis Homologous chromosomes Sister chromatids Tetrad Sister chromatids 12 Homologous Chromosomes • Pair of chromosomes (maternal and paternal) that are similar in shape and size • Homologous pairs (tetrads) carry genes controlling the same inherited traits • Each locus (position of a gene) is in the same position on homologues 13 Homologous Chromosomes • Humans have 23 pairs of homologous chromosomes • First 22 pairs of chromosomes called autosomes • Last pair called sex chromosomes • XX female or XY male 14 Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal 15 Gene Loci 16 Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata • Crossing over: segments of nonsister chromatids break and reattach to the other chromatid • Chiasmata (chiasma) are the sites of crossing over 17 Crossing Over - Variation nonsister chromatids chiasmata: site of crossing over Tetrad 18 variation Sex Chromosomes XX chromosome - female XY chromosome - male 19 Prophase I Spindle fiber Centrioles Aster fibers 20 Metaphase I • Shortest phase • Tetrads align on the metaphase plate • Independent assortment occurs • Chromosomes separate randomly to the poles of the cells 21 Metaphase I • Independent assortment causes variation in the forming cells • Orientation of homologous pair to poles is random • Formula for determining variation: Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations 22 Metaphase I OR Metaphase plate Metaphase plate 23 Question: • In terms of Independent Assortment, how many different combinations of sperm could a human male produce? 24 Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 223 = ~8 million combinations 25 Anaphase I • Homologous chromosomes separate and move towards the poles • Sister chromatids remain attached at their centromeres 26 Anaphase I 27 Telophase I • Each pole now has haploid set of chromosomes • 1n = 23 (human) • Cytokinesis occurs and two haploid daughter cells are formed 28 Telophase I 29 30 Meiosis II 31 Meiosis II • Short or No interphase II • DNA NOT replicated again • Remember: Meiosis II is similar to mitosis • Prophase I, Metaphase II, Anaphase II, and Telophase II 32 Prophase II • Same as prophase in mitosis • Nuclear envelope breaks down and the spindle apparatus forms 33 Metaphase II • Same as metaphase in mitosis • Chromatids lined up at equator Metaphase plate Metaphase plate 34 Anaphase II • Same as anaphase in mitosis • Sister chromatids separate 35 Telophase II • Same as telophase in mitosis • Nuclei reform • Cytokinesis occurs • Remember: Four haploid daughter cells produced • Gametes = sperm or egg 36 Telophase II 37 Meiosis sex cell n=2 Sperm n=2 n=2 2n=4 Haploid (n) n=2 diploid (2n) n=2 n=2 Meiosis I Meiosis II 38 39 Genetic Variation 40 Variation Important to the survival of populations Aids in natural selection Strongest individuals are able to survive and reproduce 41 Question • What are the three sources of genetic variation in sexual reproduction? 42 ***Answer: 1. Crossing Over (Prophase I) 2. Independent Assortment (Metaphase I) 3. Random Fertilization (sperm joins with egg) 43 Remember: Variation is helpful to the survival of a species! 44 Question: • A diploid cell containing 20 chromosomes (2n = 20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 45 Answer: • 10 chromosomes (haploid) • 1n = 10 46 Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type. 47 48 Trisomy • Genetic disorders such as Down’s Syndrome (Trisomy 21) are the result of nondisjunction 49 Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote 50 Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 51 Answer: • Four cells with 10 chromosomes each 52