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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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vocabulary
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.

chromosome number
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 (!)
slide4

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
slide5

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
slide6

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
ii phases of meiosis
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)
slide8

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 **
slide9

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
slide10

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
iii purpose of meiosis
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.,
slide12

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
iv gametogenesis creation of gametes
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
v comparing mitosis and meiosis
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