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Meiosis and Chromosome Assortment Powerpoint Lecture

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					Meiosis and
Chromosome
 Assortment
  Introduction to Biology
  Chromosomes in Human
          Cells
• Somatic cells include all cells in the human
  body except sperm and eggs.
  • Gametes are human sperm and egg cells.
• Each human somatic cell has 23 pairs of
  chromosomes, 46 total.
  • Each pair of chromosomes are called homologous
    chromosomes.
• Each homologous chromosome carries a
  copy of the same genes, either from the
  father or mother.
   LE 13-3
             Pair of homologous   5 µm
             chromosomes




 Centromere

         Sister
         chromatids
• This is called a
  karyotype. All
  23 pairs of
  homologous
  chromosomes
  are lined up.
  LE 13-4

             Key

               Maternal set of
               chromosomes (n = 3)
2n = 6
               Paternal set of
               chromosomes (n = 3)


Two sister chromatids
of one replicated
chromosomes
                                     Centromere




Two nonsister
                                     Pair of homologous
chromatids in
                                     chromosomes
a homologous pair
                                     (one from each set)
• The sex chromosomes are called X
  and Y
  o Human females have two X chromosomes.
  o Human males have one X and one Y
    chromosome
• The 22 pairs of chromosomes that do
  not determine sex are called
  autosomes.
      Inheritance of Genes
• A gene is a unit of heredity that carries the
  information for a specific trait or body
  function.
  o A gene is made of a segment of DNA.
  o Each gene is located on a specific chromosome.
  o Everyone has two copies of each gene (one on each
    homologous chromosome).
• A cell with a full pair of each chromosome is
  called diploid.
  o Diploid is written shorthand as 2n.
  o All somatic cells are diploid (46 chromosomes).
• A cell with only one of each homologous
  chromosome is called haploid.
  o Haploid is written shorthand as n.
  o All gametes are haploid and have 23 total chromosomes.
• Gametes are haploid cells, containing only
  one set of chromosomes
• For humans, this means 23 total chromosomes
  (no pairs)
  o This includes 22 autosomes and a single sex chromosome
  o In an unfertilized egg (ovum), the sex chromosome is always X
  o In a sperm cell, the sex chromosome may be either X or Y
     Chromosomes and the
       Human Sex Cycle
• At sexual maturity, the ovaries and testes begin
  producing sperm and eggs through meiosis.
  o Gametes are the only types of human cells produced by meiosis,
    rather than mitosis
• Meiosis is a form of cell division that results in one
  set of chromosomes in each gamete instead of
  two.
  o The resulting daughter cells are haploid.
• When fertilization occurs, the haploid sperm and
  haploid egg fuse together to form a diploid
  embryo.
                  Interphase
• At the end of interphase, each cell has grown
  into its full size, produced a full set of
  organelles, and duplicated its DNA.
  o The cell is diploid at this point.
     • The nucleus contains 23 homologous chromosome pairs.
     • Each chromosome is made of two sister chromatids
       (copies).
                   Prophase I

• The cells begin to divide, and the chromosomes pair
  up, forming a structure called a tetrad, which contains
  four chromatids.
                 Prophase I
• As homologous chromosomes pair up and form tetrads,
  they undergo a process called crossing-over.
   • First, the chromatids of the homologous chromosomes
     overlap each other.
   • Then, the crossed sections of the chromatids are
     exchanged.
   • Crossing-over is important because it produces new
     combinations of genes in the cell.
               Metaphase I

• As prophase I ends, a spindle forms and attaches to
  each tetrad.
• During metaphase I of meiosis, paired homologous
  chromosomes line up across the center of the cell.
                 Anaphase I
• During anaphase I, spindle fibers pull each
  homologous chromosome pair toward opposite
  ends of the cell.
• When anaphase I is complete, the separated
  chromosomes cluster at opposite ends of the cell.
  Telophase I and Cytokinesis
• During telophase I, a nuclear membrane forms
  around each cluster of chromosomes.
• Cytokinesis follows telophase I, forming two new cells.
        Summary of Meiosis I
• Two new haploid cells have been produced.
• Each haploid cell contains one chromosome out of
  the original pair.
   • Each chromosome still contains two sister
     chromatids.
               Prophase II
• As the cells enter prophase
  II, their chromosomes—
  each consisting of two
  chromatids—become
  visible.
• The chromosomes do not
  pair to form tetrads,
  because the homologous
  pairs were already
  separated during meiosis I.
               Metaphase II

• During metaphase of meiosis II,
  chromosomes line up in the center
  of each cell.
                Anaphase II
• As the cell enters anaphase, the
  paired chromatids separate.
 Telophase II and Cytokinesis
• The two daughter cells from Meiosis I divide, resulting
  in four daughter cells, each with two chromatids.
• These four daughter cells now contain the haploid
  number (N)—just two chromosomes each.
        Summary of Meiosis II
• A total of four cells have been produced.
• Each cell is haploid and only contains one out of the
  original pairs of homologous chromosomes.
• Each chromosome only contains a single chromatid.
   A Comparison of Mitosis
        and Meiosis
• Mitosis produces cells that are genetically identical to
  the parent cell.
• Meiosis reduces the number of chromosomes sets
  from two (diploid) to one (haploid).
• Meiosis allows crossing over of chromosomes.
   o This produces cells that are genetically different from the parents and each
     other.
• Three events are unique to meiosis, and all
  three occur in meiosis l:
  o Synapsis and crossing over in prophase I:
    Homologous chromosomes physically connect and
    exchange genetic information
  o At the metaphase plate, there are paired
    homologous chromosomes (tetrads), instead of
    individual replicated chromosomes
  o At anaphase I, it is homologous chromosomes,
    instead of sister chromatids that separate and are
    carried to opposite poles of the cell
LE 13-9
                            MITOSIS                                                             MEIOSIS

                                                          Parent cell                Chiasma (site of
                                                                                                                         MEIOSIS I
                                               (before chromosome replication)       crossing over)



  Propase                                                                                                                Prophase I
                                           Chromosome                    Chromosome
                                            replication                   replication                      Tetrad formed by
  Duplicated chromosome                                                                                    synapsis of homologous
                                                            2n = 6
  (two sister chromatids)                                                                                  chromosomes




                                            Chromosomes              Tetrads
  Metaphase                                 positioned at the        positioned at the                               Metaphase I
                                            metaphase plate          metaphase plate




  Anaphase                                  Sister chromatids    Homologues                                              Anaphase I
  Telophase                                 separate during      separate                                                Telophase I
                                            anaphase             during
                                                                                                                     Haploid
                                                                 anaphase I;
                                                                                                                      n=3
                                                                 sister
                                                                 chromatids
                                                                                             Daughter
                                                                 remain together
                                                                                              cells of
                                                                                             meiosis I

                     2n                             2n                                                                    MEIOSIS II
                          Daughter cells
                            of mitosis
                                                                                     n        n            n         n
                                                                                    Daughter cells of meiosis II

                                                                     Sister chromatids separate during anaphase II
                   Mitosis         Meiosis

    DNA        During           During
 replication   interphase       interphase
  Divisions    One              Two

Synapsis and Do not occur       Form tetrads in
crossing over                   prophase I
  Daughter     Two diploid,     Four haploid,
cells, genetic identical to
               parent cell
                                different from
                                parent cell and
 composition                    each other
Role in animal Produces cells   Produces
    body       for growth and   gametes
               tissue repair
 Genetic Variation Among
        Offspring
• The behavior of chromosomes during meiosis
  and fertilization is responsible for most of the
  variation that arises in each generation
• Three mechanisms contribute to genetic
  variation:
  o Independent assortment of chromosomes
  o Crossing over
  o Random fertilization
Independent Assortment of
      Chromosomes
• In independent assortment, each pair of
  chromosomes sorts maternal and paternal
  homologous chromosomes into daughter cells
  independently of the other pairs.
• Example:
  o One human sperm cell could contain 15 chromosomes from
    his father, and 8 from his mother
  o Another contains 20 from the mother, 3 from the father.
LE 13-10


       Key

 Maternal set of
 chromosomes       Possibility 1                                Possibility 2
 Paternal set of
 chromosomes

                                     Two equally probable
                                       arrangements of
                                       chromosomes at
                                         metaphase I




                                           Metaphase II




                                            Daughter
                                              cells

       Combination 1       Combination 2               Combination 3    Combination 4
  Independent Assortment
     of Chromosomes
• The number of combinations possible when
  chromosomes assort independently into
  gametes is calculated by 2n, where n is the
  haploid number
• For humans (n = 23):
  o 223 = 8,388,608 possible combinations!
             Crossing Over
• Crossing over produces new chromosomes with a
  mixture of genes from each parent.
• Instead of a chromosome that is 100% from the
  person’s father or mother, it might now be 95% from
  the father, 5% from the mother.
LE 13-11
           Prophase I                        Nonsister
           of meiosis                        chromatids



                          Tetrad


                                             Chiasma,
                                             site of
                                             crossing
                                             over


           Metaphase I




           Metaphase II




           Daughter
           cells



                               Recombinant
                               chromosomes
     Random Fertilization
• Random fertilization adds to genetic variation
  because any sperm can fuse with any egg.
            Genetic Diversity
• How many possible combinations of genes are there
  from two parents?
• Independent assortment:
       223 = 8,388,608 combinations of chromosomes in each
       sperm or egg cell.
• Random assortment:
      8.4 million possible sperm combinations
  +   8.4 million possible egg combinations
                = 16.8 trillion possible embryos
           Genetic Diversity
• How many possible combinations of genes are there
  from two parents?
• Crossing over
  o Average of 1,000 genes in each chromosome
  o At the most, about half of the chromosome can cross over to
    its homologous partner.
  o This results in 3.3 novemquardragintillion (1 followed by 150
    zeros) gene combinations for each chromosome pair crossing
    over.
          Genetic Diversity
• How many possible combinations of genes are there
  from two parents?
• Total
   3.3 novemquardragintillion possible chromosome
   combinations
      x 23 chromosomes
      x 16.8 trillion possible sperm-egg combinations
   =1.3 quinquinquagintillion (1 followed by 168 zeros)
   possible different genetic combinations for two
   people.

				
DOCUMENT INFO
Description: A Powerpoint lecture covering the different stages of meiosis, specifically how it relates to the human sex cycle and gametogenesis.