Meiosis and Sexual Reproduction

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Meiosis and Sexual Reproduction

• Chapter 10

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Meiosis

• Meiosis - nuclear division that reduces
  chromosome number from diploid to haploid (in
  sexually reproducing organisms)
• Gametes reproductive cells
• Sexual reproduction gamete formation and fusion
  of gametes to form a cell
• Zygote product of gamete fusion, always has
  diploid number of chromosomes

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Homologous chromosomes

• Homologous chromosomes chromosome pairs that
  look alike
• Similar length
• Centromere position
• Similar banding patterns
• Genes controlling same traits at same position
• One from father, one from mother
• Distinction between homologous and identical
  chromosomes

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Human Karyotype
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Homologous chromosomes
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Homologous Chromosomes

• Homologous chromosomes have genes controlling the
  same trait at the same position
• Each gene occurs in duplicate
• A maternal copy from the mother, a paternal copy
  from the father
• Many genes exist in several variant forms in a
  large population
• Homologous copies of a gene may encode identical
  or differing genetic information
• The variants that exist for a gene are called
  alleles
• An individual may have
• Identical alleles for a specific gene on both
  homologs (homozygous for the trait), or
• A maternal allele that differs from the
  corresponding paternal allele (heterozygous for
  the trait)

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Overview of Meiosis

• Meiosis requires 2 nuclear divisions produces 4
  haploid daughter cells
• Daughter cells receive one of each kind of
  parental chromosome, but in different
  combinations
• Synapsis pairing of homologous chromosomes
• Results in a bivalent
• Homologues align and separate
• Meiosis I

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Overview of Meiosis

• Daughter chromosomes, derived from sister
  chromatids, move to opposite poles
• Chromosomes in the four daughter cells have only
  one chromatid
• Meiosis II

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Overview of Meiosis
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Genetic Variation

• Genetic recombination process in which new
  genetic information is incorporated into a
  chromosome or DNA fragment
• Environmental changes, asexual reproduction might
  continue to produce offspring with a parents
  disadvantageous gene combination
• Sexual reproduction might give offspring better
  chance of survival
• Allows for adaptations

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Genetic Recombination

• 3 means of genetic recombination in sexual
  reproduction
• Crossing-over
• Independent assortment
• Fertilization

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Crossing-over

• Crossing-over exchange of genetic material
  between nonsister chromatids of a bivalent during
  meiosis I
• Homologues are held together by chiasmata,
  regions where the nonsister chromatids are
  attached due to cross-over

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Crossing-over
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Independent Assortment

• Independent assortment homologous chromosomes
  separate independently or randomly during meiosis
• Possible combinations 223 or 8,388,608

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Fertilization

• Gametes produced by one person are expected to be
  genetically different from the gametes produced
  by another person
• Fusion of gametes causes combination of parental
  genes
• Possible combinations (223)2 or
  70,368,744,000,000

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Phases of Meiosis

• Prophase I
• Homologous chromosomes pair during synapsis
  (crossing-over)
• Nuclear envelope fragments
• Metaphase I
• Homologous pairs align at the metaphase plate
• Kinetochores visible

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Meiosis I
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Phases of Meiosis

• Anaphase I
• Homologous chromosomes separate, and move towards
  poles
• Telophase I
• Daughter cells have one chromosome from each
  homologous pair
• Nuclear envelope reforms
• Interkinesis
• Chromosomes still contain two chromatids

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Meiosis I
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Meiosis II

• Duplicated, haploid chromosomes align at
  metaphase plate
• Sister chromatids separate at the centromere,
  giving rise to daughter chromosomes
• At end of telophase II and cytokinesis, there are
  four haploid cells

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Meiosis II
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Meiosis II
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Meiosis versus Mitosis

• Meiosis
• Requires two nuclear divisions
• Chromosomes synapse and cross over
• Centromeres survive Anaphase I
• Halves chromosome number
• Produces four daughter nuclei
• Produces daughter cells genetically different
  from parent and each other
• Used only for sexual reproduction

• Mitosis
• Requires one nuclear division
• Chromosomes do not synapse nor cross over
• Centromeres dissolve in mitotic anaphase
• Preserves chromosome number
• Produces two daughter nuclei
• Produces daughter cells genetically identical to
  parent and to each other
• Used for asexual reproduction and growth

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Meiosis vs. Mitosis
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Meiosis I vs. Mitosis
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Meiosis II vs. Mitosis
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Human Life Cycle

• Life cycle all reproductive events that occur
  from one generation to the next
• In animals, meiosis occurs only during gamete
  production
• In males, meiosis is a part of spermatogenesis
• In females, meiosis is a part of oogenesis
• A sperm and egg fuse at fertilization
• Results in a zygote - one-celled stage of an
  individual of the next generation
• Undergoes mitosis
• Results in multicellular embryo that gradually
  takes on features determined when zygote was
  formed
• All growth occurs as mitotic division
• As a result of mitosis, each somatic cell in body
• Has same number of chromosomes as zygote
• Has genetic makeup determined when zygote was
  formed

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Spermatogenesis
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Oogenesis
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