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Meiosis 2
 

Meiosis 2

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Meiosis 2 Meiosis 2 Presentation Transcript

  • Meiosis
  • Pg.
    • Title: Meiosis
    • Date: 01/27/10
    • Obj: Describe how spermatogenesis & oogenesis occur through meiosis.
    • Ess: Explain how sperm & egg contain only 23 chromosomes each through meiosis.
  • Pg.
    • Title: Meiosis I & II
    • Date: 1/27/10
    • In:
    • Almost all human cells contain 46 chromosomes. If a sperm has 46 chromosomes & an egg has 46 chromosomes then how many chromosomes will the “baby” have?
    • Is that a good thing???
  • M I T O S I S M E I O S I S I M E I O S I S II
  • Asexual vs. Sexual Reproduction
    • Asexual Reproduction:
      • Binary Fission & Mitosis
      • Produces cells that are identical copies of parent cell
    • Sexual Reproduction:
      • Meiosis
      • Combines genetic material from 2 parents (sperm & egg) so offspring are genetically different from parents
  • How many chromosomes in human cells?
    • Humans have 46 (23 pairs) chromosomes
    • 44 chromosomes (22pairs): autosomal
    • 2 chromosomes (1 pair): sex chromosomes (XX, XY)
    2 sets of chromosomes 1 from mom & 1 from dad
  • How many chromosomes in human cells?
    • All human cells have 46 chromosomes except for the egg & sperm
    • Egg = 23
    • Sperm = 23
    • All other cells = 46 (2 sets of chromosomes)
      • (liver, cardiac, blood cells)
    • Diploid (2N) = 2 sets of chromosomes (46)
    • Haploid (1N) = 1 set of chromosomes (23)
    • N = haploid #
    1 set of chromosomes
  • If egg and sperm had same number of chromosomes as other body cells . . . baby would have too many chromosomes! No Good! The Problem!
  • The Solution! Much Better! If egg and sperm had half the number of chromosomes than other body cells . . . baby would have just the right amount!
  • Meiosis
    • Makes 4 cells genetically different from parent cell & from each other
    • Production of gametes = sperm & egg
    • Contain half the number of chromosomes = haploid (1N)
    • Called the “reduction” division =
      • Diploid Haploid
      • 2N 1N
    • Used in sexual reproduction
    46 23
  • Meiosis allows sex cells to join to form a complete set of chromosomes/instructions to make a new organism. Copyright Pearson Prentice Hall Haploid gametes MEIOSIS Diploid zygote MITOSIS Diploid adult Not in notes
  • You tell me!
    • Drosophila (fruit flies)
      • Diploid: 2N = 8
      • Haploid: 1N = ?
    • Lettuce
      • Diploid: 2N = 8
      • Haploid: 1N = ?
    • Goldfish:
      • Diploid: 2N = ?
      • Haploid: 1N = 48
    Not in notes
  • Meiosis is a special kind of cell division that halves the number of chromosomes in gametes. This insures that the correct number of chromosomes are passed on. Copyright Pearson Prentice Hall Not in notes
  • The goal is to make haploid sex cells Copyright Pearson Prentice Hall 1 st division 2 nd division Not in notes
  • Where does it happen?
    • In males- in the testes
    • In females- in the ovaries
  • Homologous Chromosomes Homologous chromosomes sister chromatids paternal sister chromatids maternal Tetrad
  • Check out the tetrads!
  • Homologous Chromosomes
    • Pair of chromosomes - maternal & paternal - similar in shape & size.
    • Homologous pairs – tetrads - carry genes controlling the same inherited traits.
    • Each locus - position of a gene - is in the same position on tetrads.
  • Homologous Chromosomes Paternal Maternal eye color locus eye color locus hair color locus hair color locus
    • Gametes:
    • Sperm or egg
    Copyright Pearson Prentice Hall
  • Gametogenesis
    • Spermatogenesis: production of sperm
  • Gametogenesis
    • Oogenesis: production of egg
    Meiosis I Meiosis II
  •  
  • Remember, humans have 23 pairs of chromosomes…………….
    • Pairs 1-22 are said to be Homologous chromosomes (they go together because they have same traits but are not identical)
    Copyright Pearson Prentice Hall gene for eye color gene for hair color
  • Phases of Meiosis
      • Meiosis I
    Copyright Pearson Prentice Hall Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Interphase I
      • Meiosis I
  • Phases of Meiosis
    • Cells undergo a round of DNA replication, forming duplicate chromosomes.
    Copyright Pearson Prentice Hall Interphase I
  • Phases of Meiosis
    • Each chromosome pairs with its corresponding homologous chromosome to form a tetrad .
    • There are 4 chromatids in a tetrad .
    Copyright Pearson Prentice Hall MEIOSIS I Prophase I
  • Phases of Meiosis
        • Crossing-over: Homologous chromosomes exchange portions of their chromatids in a process
        • Crossing-over produces new combinations of alleles.
    Copyright Pearson Prentice Hall
    • Synapsis
    • and
    • Crossing
    • Over in
    • Action
    Copyright Pearson Prentice Hall
  • Phases of Meiosis
    • Spindle fibers attach to the chromosomes.
    • Tetrads line up in the middle: metaphase plate.
    Copyright Pearson Prentice Hall MEIOSIS I Metaphase I
  • Phases of Meiosis
    • Spindle fibers pull the homologous chromosomes toward opposite ends of the cell.
    • Tetrads split into single chromosomes.
    Copyright Pearson Prentice Hall MEIOSIS I Anaphase I
  • Phases of Meiosis
    • Nuclear membranes form.
    • Cell separates into two cells.
    • The two cells produced have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.
    Copyright Pearson Prentice Hall MEIOSIS I Telophase I and Cytokinesis
  • Phases of Meiosis
      • Meiosis II
        • The two cells produced by meiosis I now enter a second meiotic division.
        • Unlike meiosis I, neither cell goes through chromosome replication.
        • Each of the cell’s chromosomes has 2 chromatids.
    Copyright Pearson Prentice Hall
  • Phases of Meiosis
    • Meiosis II
    Copyright Pearson Prentice Hall Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase I and Cytokinesis I Meiosis II
  • Phases of Meiosis
    • Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
    Copyright Pearson Prentice Hall MEIOSIS II Prophase II
  • Phases of Meiosis
    • The chromosomes line up in the center of cell.
    Copyright Pearson Prentice Hall MEIOSIS II Metaphase II
  • Phases of Meiosis
    • The sister chromatids separate and move toward opposite ends of the cell.
    Copyright Pearson Prentice Hall MEIOSIS II Anaphase II
  • Phases of Meiosis
        • Meiosis II results in four haploid (N) daughter cells.
    Copyright Pearson Prentice Hall MEIOSIS II Telophase II and Cytokinesis
    • Gametes:
    • Sperm or egg
    Copyright Pearson Prentice Hall
  • Gametogenesis
    • Spermatogenesis: production of sperm
  • Gametogenesis
    • Oogenesis: production of egg
    Meiosis I Meiosis II
  •  
  • Gamete Formation
    • Gamete Formation
      • In male animals, meiosis results in four equal-sized gametes called sperm.
    Copyright Pearson Prentice Hall
  • Gamete Formation
    • In many female animals, only one egg results from meiosis. The other three cells, called polar bodies , are usually not involved in reproduction.
    Copyright Pearson Prentice Hall
  • Interphase I
    • DNA replicates forming duplicate homologous chromosomes. (S phase).
    • Centriole pairs also replicate.
    • Nucleus & nucleolus visible.
  • 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 & cytokinesis
    Prophase I Metaphase I Anaphase I Telophase I & Cytokinesis
  • Prophase I
    • Longest and most complex phase (90%).
    • Homologous chromosomes come together to form a tetrad = Synapsis
    • Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids) .
  • Prophase I - Synapsis Homologous chromosomes sister chromatids paternal sister chromatids maternal Tetrad
  • Prophase I centrioles spindle fiber spindle fibers
  • Crossing Over
    • Segments of non-sister chromatids break and reattach to the other chromatid .
    • May occur between non-sister chromatids at the chiasmata .
    • Chiasmata (chiasma) - sites of crossing over .
  • Crossing Over - variation variation nonsister chromatids chiasmata: site of crossing over Tetrad
  • Metaphase I
    • Shortest phase
    • Tetrads align on the metaphase plate .
    • INDEPENDENT ASSORTMENT OCCURS:
      • Orientation of homologous pair to poles is random.
      • Leads to variation
  • Metaphase I metaphase plate OR metaphase plate
  • Anaphase I
    • Homologous chromosomes separate and move towards the poles.
    • Sister chromatids remain attached at their centromeres .
  • Anaphase I
  • Telophase I
    • Each pole now has haploid (1n) set of chromosomes .
    • Cytokinesis occurs and two haploid daughter cells are formed.
  • Telophase I
  • Meiosis II
    • No interphase II
    • four phases :
    • a. prophase II
    • b. metaphase II
    • c. anaphase II
    • d. telophase II & cytokinesis
    • Similar to mitosis
  • Prophase II
    • Centrioles separate
    • Nuclear envelope disappears
  • Prophase II
  • Metaphase II
    • Sister chromatids line up at metaphase plate
    • Spindle fibers attach at centromere
  • Metaphase II metaphase plate metaphase plate
  • Anaphase II
    • sister chromatids separate & move toward opposite ends of the cell
  • Anaphase II
  • Telophase II
    • Nuclei form.
    • Cytokinesis occurs.
    • Four haploid daughter cells produced.
      • gametes = sperm or egg
  • Telophase II
  •  
  • Mitosis vs. Meiosis
    • Mitosis
    • Meiosis
    Diploid (2n) Parent Cells Replication of Chromosomes – double the # End of Mitosis & Meiosis I Results in 2 diploid (2n) daughter cells End of Meiosis II Results in 4 haploid (n) daughter cells
  • Pg. 143 Meiosis Vocab Word Description Picture Homologous chromosomes Tetrads Crossing over Haploid Diploid Locus Spermatogenesis Oogenesis