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Part II: Genetic Basis of Life


Part II: Genetic Basis of Life Meiosis and sexual reproduction Meiosis occurs at different times in the life cycle of plants, animals, and fungi, but its phases are ... – PowerPoint PPT presentation

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Title: Part II: Genetic Basis of Life

Part II Genetic Basis of Life
  • Meiosis and sexual reproduction
  • Meiosis occurs at different times in the life
    cycle of plants, animals, and fungi, but its
    phases are the same.
  • In humans, meiosis is a part of spermatogenesis
    and oogenesis.

Halving the Chromosome Number Sexual Reproduction
  • Meiosis is nuclear division reducing chromosome
    number from diploid (2n) to haploid (n) number.
  • Haploid (n) number is half the diploid number of
  • Requires gamete formation and then fusion of
    gametes to form a zygote.
  • A zygote always has a full (2n) number of

Halving the Chromosome Number Homologous Pairs
  • Diploid cells chromosomes in pairs.
  • Each set of chromosomes is a homologous pair
    each member is a homologous chromosome or
  • A location on one homologue contains same type of
    gene at the same locus on other homologue.

Halving the Chromosome Number Homologous Pairs
  • Chromosomes duplicate just before nuclear
  • Duplication produces two identical parts called
    sister chromatids, held together at the
  • Non-sister chromatids do not share the same
  • One member of each homologous pair is inherited
    from either male or female parent.

  • Meiosis involves two nuclear divisions and
    produces four haploid daughter cells.
  • Each daughter cell has half the number of
    chromosomes in the diploid parent nucleus.

  • Prior to Meiosis I, DNA replication occurs and
    each chromosome has two sister chromatids.
  • During Meiosis I, homologous chromosomes come
    together and line up in synapsis.
  • During synapsis, the two sets of paired
    chromosomes lay alongside each other as bivalents
    or tetrad.

  • Crossing over is an exchange of homologous
    segments between non-sister chromatids of
    bivalent during Meiosis I.
  • This results in genetic recombinations.
  • After crossing over occurs, sister chromatids of
    a chromosome are no longer identical.

Halving the Chromosome Number Meiosis II
  • No replication of DNA is needed between Meiosis I
    and Meiosis II b/c chromosomes were already
  • During Meiosis II, centromeres divide sister
    chromatids separate.
  • Chromosomes in the four daughter cells have only
    one chromatid.
  • Each daughter cell is haploid, and in animal life
    cycle, becomes gametes.

Genetic Recombination
  • Due to genetic recombination, offspring have a
    different combination of genes than their
  • Without recombination, asexual organisms must
    rely on mutations to generate variation among

Genetic Recombination Crossing-Over
  • Crossing over of nonsister chromatids results in
    exchange of genetic material between non-sister
    chromatids introduces variation.
  • At synapsis, homologous proteins are held in
    position by a nucleoprotein lattice (the
    synaptonemal complex).

Genetic Recombination Crossing-Over
  • As lattice of synaptonemal complex breaks down at
    the beginning of anaphase I, homologues are
    temporarily held together by chiasmata, regions
    where non-sister chromatids are attached due to
  • Due to crossing over, chromatids are no longer

Genetic Recombination Fertilization
  • Meiosis increases variation.
  • When gametes fuse at fertilization, chromosomes
    donated by parents combine.

Phases of Meiosis
  • Prophase I
  • Nuclear division is about to occur nucleolus
    disappears nuclear envelope fragments
    centrosomes migrate away from each other spindle
    fibers assemble.
  • Homologous chromosomes undergo synapsis forming
    bivalents crossing over may occur at this time.
  • Chromatin condenses.

  • Metaphase I
  • During prometaphase I, bivalents held together by
    chiasmata have moved toward the metaphase plate.
  • Kinetochores are regions just outside
    centromeres they attach to spindle fibers called
    kinetochore spindle fibers.
  • Bivalents independently align themselves at the
    metaphase plate of the spindle.

Phases of Meiosis
  • Anaphase I
  • The homologues of each bivalent separate and move
    toward opposite poles.
  • Each chromosome still has two chromatids.

Phases of Meiosis
  • Telophase I
  • Only occurs in some species.
  • When it occurs, the nuclear envelope reforms and
    nucleoli reappear.
  • Interkinesis
  • This period between meiosis I and II is similar
    to interphase in mitosis.
  • However, no DNA replication occurs.

  • Meiosis II
  • During metaphase II, the haploid number of
    chromosomes align at metaphase plate.
  • During anaphase II, centromeres divide and
    daughter chromosomes move toward the poles.
  • At the end of telophase II and cytokinesis, there
    are four haploid cells.

  • Mitosis II continued
  • Due to crossing-over, each gamete can contain
    chromosomes with different genes.
  • In animals, the haploid cells mature and develop
    into gametes.
  • In plants, the daughter cells become spores and
    divide to produce a haploid adult generation.

Comparison of Meiosis and Mitosis
  • Meiosis I differs from Mitosis
  • DNA is replicated only once before both mitosis
    and meiosis in mitosis there is only one nuclear
    division, in meiosis there are 2.
  • During prophase I of meiosis, homologous
    chromosomes pair and undergo crossing-over.

Comparison of Meiosis to Mitosis
  • During metaphase I, paired homologous chromos
    align at the metaphase plate in mitosis
    individual chromos align.
  • During anaphase I, homologous chromos with
    centromeres intact separate and move to opposite
    poles in mitosis at this stage, sister
    chromatids separate and move to poles.

Comparison of Meiosis to Mitosis
  • Meiosis II differs from Mitosis
  • Events in Meiosis II are same stages as in
  • However, the nuclei contain the haploid number of
    chromosomes in meiosis.
  • Mitosis produces two daughter cells meiosis
    produces four daughter cells.

The Human Life Cycle
  • Life cycle refers to all reproductive events
    between one generation and next.
  • In animals, adult is diploid and gametes are
  • Mosses are haploid most of their cycle, oak trees
    are diploid.
  • In fungi and algae, organism you see is haploid
    and produces haploid gametes.

  • In males, meiosis is part of spermatogenesis or
    production of sperm.
  • In the testes of males, primary spermatogenesis
    with 46 chromos divide meiotically to form two
    secondary spermatocytes, each with 23 duplicated
  • Secondary spermatocytes divide to produce four
    spermatids (23 chromos).

  • Spermatogenesis continued
  • Spermatids then differentiate into sperm.
  • Meiotic cell division in males always results in
    four cells that become sperm.

  • Oogenesis is in the ovaries of human females,
    primary oocytes with 46 chromosomes divide
    meiotically to form two cells, each with 23
    duplicated chromosomes.
  • One of the cells, secondary oocyte, receives most
  • Other cell, a polar body, disintegrates or
    divides again.

  • Secondary oocyte begins in meiosis II and then
    stops at metaphase II.
  • At ovulation, secondary oocyte leaves ovary and
    enters an oviduct where it may meet sperm.
  • If sperm enters oocyte, it is activated to
    continue meiosis II, resulting in mature egg and
    another polar body, each with 23 chromosomes.

The Human Life Cycle Oogenesis
  • Polar bodies serve to discard unnecessary
    chromosomes and retain most of the cytoplasm in
    the egg.
  • The cytoplasm serves as a source of nutrients for
    the developing embryo.
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