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Unit 3.4: Cell Division: Meiosis

Unit 3.4: Cell Division: Meiosis. Vocabulary:. Diploid : di - = two ; having 2 full sets of chromosomes; denoted as 2n Haploid : having one full set of chromosomes; denoted as n . Gametes : sex cells; egg, sperm, spores

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Unit 3.4: Cell Division: Meiosis

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  1. Unit 3.4: Cell Division: Meiosis

  2. Vocabulary: Diploid: di- = two; having 2 full sets of chromosomes; denoted as 2n Haploid: having one full set of chromosomes; denoted as n. Gametes: sex cells; egg, sperm, spores Homologous chromosomes: a pair of chromosomes that carry similar genetic information; one of the pair comes from the mother, the other comes from the father. Karyotype: a “picture” of all the chromosomes in a single nucleus of an organism Tetrad: “tetra-” = four; the four chromatids that make up a pair of homologous chromosomes. Crossing-over: exchange of genetic material between pairs of homologous chromosomes.

  3. Chromosome Number: • Number of chromosomes in an organism does not correspond to the complexity of the organism • Fruit fly: 8 chromosomes • Lettuce: 18 chromosomes • Humans: 46 chromosomes • Potatoes: 48 chromosomes • Chicken: 78 chromosomes • King Crab: 208 chromosomes • Ferns: 1260 chromosomes (!)

  4. Fruit flies have 8 chromosomes (2n=8): • 4 from the mother: one green, one red, one orange, one little dot • 4 from the father: other green, red, orange, and little dot. • Each pair of chromosomes is a homologous pair • Diploid (2n): having two full sets of chromosomes • One set is “homologous” to the other set: they are the same size and shape, but may have different forms of a gene (long fur/short fur; small eyes/large eyes; etc…) • One set of chromosomes comes from the mother. • Other set of chromosomes comes from the father. • (Different forms of a trait are called “alleles”) • Body cells are diploid

  5. Diploid: 2 copies of each chromosome Haploid: 1 copy of each chromosome • Haploid (n): having only one set of chromosomes • One of each homologous pair: • If diploid cell has 46 chromosomes (2 copies of #1, 2 copies of #2, etc…), then a haploid cell has 23 chromosomes (1 copy of #1, 1 copy of #2, etc…) • Only sex cells (gametes) are haploid: • Egg, sperm, spores

  6. Karyotype of human male • Autosomes vs. sex chromosomes: • Autosomes: chromosomes that determine traits not associated with the sex (gender) of an organism • In humans: 44 of the 46 chromosomes of a diploid set are autosomes • Sex chromosomes: two chromosomes that determine the sex of the organism • In humans: 2 of the 46 chromosomes of a diploid set are sex-determining chromosomes: • XX = female; XY = male

  7. II. Phases of Meiosis • Meiosis often called “reduction-division” • Two stages: • Meiosis I: reduces chromosome number by half (2n  n) • Meiosis II: like cell division(mitosis)

  8. Phases of Meiosis 1: • Prophase 1: • Synapsis occurs: pairing up of homologous chromosomes to form a tetrad • “tetra-” = 4; 4 sister chromatids • Crossing-over can occur: exchange of complementary genetic material between homologous chromosomes. • ** Increases genetic variety of species **

  9. 2n = 4 2n = 4 n = 2 • Metaphase 1: • Tetrads line up in “middle” of spindle • In mitosis, homologues are not necessarily next to each other, but in meiosis, they always pair together! • Anaphase 1: • Homologous chromosomes separate to opposite sides of cell • Randomly! – some from mom and some from dad • This is how traits can “skip” generations – you carry genes from all your grandparents… • Telophase 1: • Each nuclei is haploid

  10. Phases of Meiosis 2: • **No duplication of chromosomes, no cytokinesis** • Prophase 2: • New spindles form; chromosomes move to spindle – like mitosis • Metaphase 2: • Chromosomes line up in middle – like mitosis • Anaphase 2: • Sister chromatids are pulled apart at the centromere – like mitosis • Telophase 2: • Chromosomes unravel into chromatin • Nuclear membranes form around chromatin • …like mitosis

  11. III. Purpose of Meiosis • **Maintains constant chromosome number: meiosis reduces chromosome # from diploid to haploid (2n  n) • If no meiosis, then too much DNA • Ex: if 46 chromosomes in sperm + 46 chromosomes in egg = 92 chromosomes; then next generation will have 184 chromosomes, etc.,

  12. Creates greater genetic variety: • Gametes are genetically different from parent cell and from each other • Crossing-over creates new combinations of genes • Random segregation of chromosomes means some from your mom and some from your dad will be present in your offspring. • Independent Assortment

  13. IV. Gametogenesis: creation of gametes • Occurs in reproductive organs • Spermatogenesis: creation of sperm • Yields 4 mature sperm • Oogenesis: creation of ova, (eggs) • Unequal division of cytoplasm yields one mature ovum and 3 polar bodies. • Extra cytoplasm = more organelles for future zygote • Polar bodies are not viable

  14. V. Comparing Mitosis and Meiosis Mitosis Meiosis 4 haploid daughter cells Genetically different from parent cell and each other Maintains constant chromosome number for a species Sex cells only Sexual reproduction • 2 diploid daughter cells • Genetically identical to parent cell • For growth and repair of an organisms • Body cells that are not sex cells • Asexual reproduction

  15. Warm-up answers for this unit:

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