Part II: Genetic Basis of Life

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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.

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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
  chromosomes.
• Requires gamete formation and then fusion of
  gametes to form a zygote.
• A zygote always has a full (2n) number of
  chromosomes.

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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
  homologue.
• A location on one homologue contains same type of
  gene at the same locus on other homologue.

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Halving the Chromosome Number Homologous Pairs

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

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• 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.

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• 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.

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• 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.

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Halving the Chromosome Number Meiosis II

• No replication of DNA is needed between Meiosis I
  and Meiosis II b/c chromosomes were already
  doubled.
• 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.

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Genetic Recombination

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

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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).

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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
  crossing-over.
• Due to crossing over, chromatids are no longer
  identical.

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Genetic Recombination Fertilization

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

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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.

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• 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.

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Phases of Meiosis

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

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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.

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• 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.

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• 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.

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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.

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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.

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Comparison of Meiosis to Mitosis

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

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The Human Life Cycle

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

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Spermatogenesis.

• 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
  chromos.
• Secondary spermatocytes divide to produce four
  spermatids (23 chromos).

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Spermatogenesis

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

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Oogenesis

• 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
  cytoplasm.
• Other cell, a polar body, disintegrates or
  divides again.

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Oogenesis

• 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.

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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.