Sexual Reproduction and Meiosis

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Sexual Reproduction and Meiosis

• Chapter 12

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Asexual reproduction

• mitosis
• clonal
• BUDDING
• stay in colony
• detach
• Parthenogenesis from ovum

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Sexual Life Cycle
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Reduction Division

• In sexual reproduction, gametes fuse
  (fertilization) to produce a zygote.
• Gamete formation involves a mechanism (meiosis)
  that reduces the number of chromosomes to half
  that found in other cells.
• Adult body cells are diploid.
• Gamete cells are haploid.
• alternation of generations

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Meiosis

• 23 pairs (mom/dad) of chromosomes can make 223
  8,388,608 types of gametes

• 2 unrelated people can make 70,368,744,177,700
  unique offspring

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Meiosis

• Synapsis
• Homologues pair along their length.
• Homologous recombination
• Genetic exchange (crossing over) occurs between
  homologous chromosomes.
• Reduction division
• Meiosis involves two successive divisions, with
  no replication of genetic material between them.

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Crossing over

• Separates alleles
• Lesser chance of separation when close

5 chiasmata
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Unique Features of Meiosis
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Prophase I

• Homologous chromosomes become closely associated
  in synapsis, exchange DNA segments via crossing
  over, and then separate. No two chromosomes are
  same after this process.
• Presence of a chiasma indicates crossing over has
  occurred.

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Metaphase I

• Terminal chiasmata holds homologous pair
  together.
• Spindle microtubules attach to kinetochore
  proteins on the outside of each centromere.
• Joined pairs of homologues lines up on metaphase
  plate.
• orientation of each pair is random

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Completing Meiosis

• Anaphase I (Independent assortment and reduction
  division)
• Spindle fibers begin to shorten and pull whole
  centromeres toward poles.
• Each pole receives a member of each homologous
  pair.
• complete set of haploid chromosomes
• random orientation results in independent
  assortment

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Independent assortment
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Completing Meiosis

• Telophase I
• Chromosomes are segregated into two clusters one
  at each pole.
• Nuclear membrane re-forms around each daughter
  cell.
• Sister chromatids are no longer identical due to
  crossing over.

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Second Meiotic Division

• Meiosis II resembles normal mitotic division.
• prophase II - nuclear envelope breaks down and
  second meiotic division begins
• metaphase II - spindle fibers bind to both sides
  of centromere
• anaphase II - spindle fibers contract and sister
  chromatids move to opposite poles
• telophase II - nuclear envelope re-forms
• Final result - four haploid cells

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Mitosis v. Meiosis
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Mitosis v. Meiosis
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Independent Assortment
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Combine gametes How many unique zygotes?
female types male types zygote types
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Variability

• Different types of eggs or sperms possible
• Crossing over and exchange of genetic material
  during prophase I
• Independent assortment during anaphase I
• 2n types of eggs or sperms from each parent.
• Sexual reproduction
• Random Fertilization 2n x 2n

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Sex

• Asexual reproduction - individual inherits all
  its chromosomes from a single parent
• parthenogenesis - development of an adult from an
  unfertilized egg
• Sexual reproduction - produces genetic
  variability.
• Segregation of chromosomes tends to disrupt
  advantageous combinations.
• Only some progeny maintain advantages.

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Evolutionary Consequences of Sex

• Evolutionary process is revolutionary and
  conservative.
• pace of evolutionary change is accelerated by
  genetic recombination
• evolutionary change not always favored by
  selection
• may act to preserve existing gene combinations

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