Chapter 20 Germ Cells and Fertilization

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Chapter 20 Germ Cells and Fertilization

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Asexual Reproduction

• Identical genetically
• Plants
• Forming multicellular offshoots and detach
• Animals
• Budding (Hydra)
• Splitting and regenerating (anemones, marine
  worms)
• Reproducing without mating (some lizards)

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Sexual Reproduction

• Different genetically
• By mixing parental genomes in most animals and
  plants

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Both

• Normally reproduce asexully
• Occasionally sexually

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Parent cells (diploid) ?Meiosis Gametes cells
(haploid) ? Fertilization Offspring cells
(diploid) ?
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Genetic Reassortment

• Combination among chromosomes
• n haploid number of chromosomes, 23 in human
• 2n number of genetically different gametes,
• 223 8.4 X 106
• Genetical recombination through chromosomal
  recombination
• Chromosomal crossing-over breaking of double
  helix of DNA and reciprocally exchange between
  replicated homologue pair with a chiasma
  physically connecting two nonsister chromatids

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Duplicated Homologous Chromosomes

• Homologs paternal and maternal chromosomes which
  are similar but not identical
• Sister chromatids fully replicated chromosomes
  closely associated
• Paring initial association of homologs, may be
  mediated by complementary DNA base-pair
  interactions
• Bivalent duplicated homolog pair with four
  chromatids

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Structure of Chromosome
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• Structure of
• Chromosome

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• Structure of
• Chromosome

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Sister Chromatids
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Mitosis
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Meiosis

• Prophase I
• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis
• Prometaphase I
• Metaphase I
• Anaphase I
• Telophase I

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Bivalent
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Chiasmata

• Chiasmata holding paternal and maternal homologs
  together until spindle separates them in anaphase
  
• Two major ways to segregate chromosomes
• in mitosis splitting between two sister
  chromatids.
• in meiosis I separation between two homologous
  chromosomes.

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Pairing of Sex Chromosomes

• During prophase I of primary spermatocytes the
  small regions of homology at chromosomes X and Y
  are possible crossover sites.
• Segregation of X and Y into two daughter cells
  (secondary spermatocytes) produces two types of
  sperms.

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Cell Cycle
Synthesis
Gap 2

• Meiosis
• Prophase
• Leptotene
• Zygotene
• Pachytene
• Diplotene
• Diakinesis
• Prometaphase
• Metaphase
• Anaphase
• Telophase

Gap 1
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Synaptonemal Complex

• Synaptonemal complex (100 nm wide)
• First appears between two nonsister chromatids of
  a bivalent at zygotene
• Completes in pachytene
• Separates in the diplotene to form chiasmata.

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Synaptonemal Complex
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Recombination Nodules

• A protein with 90 nm in diameter containing Rad51
  (in eukaryotes) and RecA (in E. coli) catalyzing
  recombination.
• Recombination hot spots induced by meiotic
  endonuclease, Spo 11
• Synaptonemal complex may mediate the prevention
  of the adjacency of two crossover sites

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Genetic Maps

• Two genes close together are less likely, and two
  genes located far away are more likely, to be
  separated by a crossover.
• The distance of each pair of genes is displayed
  as the percentage of recombination between them.
• Centimorgan (cM) standard unit of genetic
  distance a 1 probability that two genes will be
  separated by crossover event during meiosis.

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Polymorphism

• DNA markers short DNA sequences
• To estimate the distance by measuring the
  coinheritance of DNA markers that differ between
  the individuals in the population (polymorphic).
• Genetic distance in centimorgans
• True physical distances

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Meiotic Cell Divisions

• Division I with replication and crossing-over,
  then separate two homologue chromosomes
• Division II without replication and
  crossing-over, simply split between two new
  sister chromatids at centromere
• Nondisjunction homologs fail to separate about
  10 in human oocytes, inducing miscarriages or
  Down's syndrome

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Meiosis
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• Normal
• Gametogenesis

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• Nondisjunction

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• Nondisjunction

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Location of Primordial Germ Cells
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Sry Gene on Y Chromosome

• The Sry gene (sex-determining region of Y) in Y
  chromosome has testis determining function.
• Sry is expressed in a subset of cells
  differentiating into Sertoli cells.

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Functions of Sertoli cell

• Stimulating primordial germ cells into sperm
• Secreting anti-Müllerian hormone
• Stimulating the migration of connective tissue
• Stimulating the development of Leydig cells

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• Seminiferous Tubule

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• Secondary Follicle

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• Secondary Follicle

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Oocyte in Follicle
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OvulatedOocyte
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• Fertilization

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Fertilization
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A Model of Fertilization
Prevention of Polyspermy

• Change of membrane potential
• Formation of fertilization envelope

• Strongylocentrotus purpuratus
• Arbacia punctulata

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Ovulating Sea Urchin
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TEM of the sea urchin egg

• Cell membrane with microvilli
• Coated with vitelline layer
• Underlined with cortical granules and vesicles

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Vitelline Layer and Cortical Granule
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Elevation of Vitelline Layer
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Acrosomal Reaction
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Fertilin
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Cortical Reaction
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• Fertilization

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• Class Dismissed!