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Meiosis and genetic variation Diploid cells have two copies of each chromosome • Gametes are produced through meiosis • Stages of meiosis • Homologous chromosomes contain copies of the same genes • Meiosis produces haploid cells that are genetically unique via – Independent assortment – Crossing over http://scienceprofonline.googlepages.com/meiosis.jpg/meiosis-full;init:.jpg Jan 20, 2017 • One maternal, one paternal Diploid cells have two copies of each chromosome Siblings share 1/2 of their genes, on average • Alleles are different forms of the same gene. Each get one chromosome from mother and one from father, but they might not both get the same chromosome from a parent. Meiosis 1 Meiosis- key concepts INTERPHASE • What is the purpose of meiosis? • What happens during meiosis? • How does meiosis generate diversity? MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I Sister chromatids remain attached Sister chromatids Chiasmata Spindle Tetrad 5 Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Homologous chromosomes separate Pairs of homologous Tertads line up chromosomes split up 1 Meiosis 2 MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming Sister chromatids separate • Mitosis produces an exact copy of the parent cell. – Used for growth and asexual reproduction. • Meiosis produces reduced (haploid) gametes, which are genetically unique. – Necessary only for sexual reproduction. chromosomes are still double Figure 13.8 Mitosis vs Meiosis Not paired Homologs pair in meiosis 1 Mitosis vs. Meiosis diploid haploid Sister chromatids separate in meiosis 2 Meiosis produces genetic variation through Independent Assortment Crossing over • Mitosis • Meiosis • Conserves the number of chromosome sets • Reduces the number of chromosome sets from two to one • Daughter cells genetically identical to their parent cell and to each other • Daughter cells genetically distinct from parent cell and from each other Independent Assortment • Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs 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 Figure 13.10 Combination 1 Combination 2 Combination 3 Combination 4 2 Crossing Over Crossovers are random Prophase I of meiosis Nonsister chromatids • Produces Recombinant Chromosomes that carry genes derived from two different parents Tetrad Chiasma, site of crossing over Metaphase I This tetrad has 3 crossovers: 2&3 1&3 2&4 Metaphase II Daughter cells Recombinant chromosomes Figure 13.11 Spindle fibers from one pole attach to BOTH sister chromatids Spindle fibers from the other pole attach to the other homolog Crossovers are essential for correct alignment at metaphase 1 Chiasmata hold the pair together while Tug of war Aligns tetrads The human life cycle Key Haploid gametes (n = 23) Haploid (n) Diploid (2n) Gametes are the only haploid cells Ovum (n) Sperm Cell (n) Meiosis occurs during gamete formation FERTILIZATION MEIOSIS Ovary Diploid zygote (2n = 46) Testis Mitosis and development Figure 13.5 • Alternation of generations Multicellular diploid adults (2n = 46) • Mosses, fungi, and some protists have a dominant haploid generation. – multicellular diploid and haploid stages Haploid multicellular organism Haploid multicellular organism (gametophyte) n Mitosis n n Mitosis Mitosis n n n Spores Mitosis n n n Gametes Gametes MEIOSIS Diploid multicellular organism (sporophyte) Figure 13.6 B MEIOSIS 2n (b) Plants and some algae n FERTILIZATION 2n Mitosis FERTILIZATION Zygote 2n Zygote 3