1 / 16

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

truly
Download Presentation

Unit 3.4: Cell Division: Meiosis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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:

More Related