Meiosis and Sexual Life Cycles

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Chapter 13

• Meiosis and Sexual Life Cycles

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Mitosis vs. Meiosis

• asexual reproduction
• one parent all genes come from one parent
• offspring are identical to parent
• division of somatic cells

• sexual reproduction
• two parents each donates half of total genes
• offspring is unique
• division of germ cells

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Sexual Life Cycles

• There are three basic patterns of sexual life
  cycles
• animal
• fungi and protists
• plants and protists
• In all alternation of meiosis and fertilization
  is common

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Meiosisdiploid and haploid stages

• 
  1n
• 1n
• II
  1n
• 2n
• I
  1n
• 1n
• II
  1n

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Meiosis I

• reduction division
• separates homologues
• 1n
• 2n
• I 1n

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Meiosis I, cont.

• Interphase I (precedes meiosis)
• G1 , S, G2
• Meiosis I
• prophase I, metaphase I, anaphase I, telophase I
• Cytokinesis

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Interphase I

• precedes meiosis
• G1 first growth phase
• S synthesis
• G2 second growth phase

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Prophase I

• longest phase
• synapsis
• tetrad formation
• crossing-over
• chiasma (chiasmata, pl)

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Crossing-over

• chiasma

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Metaphase I

• tetrads align at the metaphase plate

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Anaphase I

• homologous chromosomes pull apart
• separation accomplished by
• disassembly of kinetochore microtubules
• elongation by nonkinetochore microtubules

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Telophase I

• Chromosomes reach the poles
• each pole with haploid number

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Cytokinesis

• Division of cytoplasm
• animal cells form a cleavage furrow
• plant cells form a cell plate

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Meiosis II

• mitotic division
• separates sister chromatids
  
  1n
• 1n
• II
  1n
• 
  1n
• 1n
• II
  1n

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Meiosis II, cont

• Interkinesis (may be absent)
• Meiosis II
• prophase II, metaphase II, anaphase II,
  telophase II
• Cytokinesis

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Meiosis II, cont

• prophase II
• spindle apparatus forms
• metaphase II
• sister chromatids align at equator
• anaphase II
• sister chromatids separate
• telophase II
• each pole with haploid number

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Cytokinesis

• division of cytoplasm
• animal cells - cleavage furrow
• plant cells - cell plate

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Gametogenesisformation of gametes

• Spermatogenesis
• Oogenesis

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Zygote Possibilities

• single birth
• 1 sperm 1 ovum 1 zygote
• monozygotic (identical) twins 1/3
• 1 sperm 1 ovum 1 zygote
• dizygotic (fraternal) twins 2/3
• 2 sperm 2 ova 2 zygotes

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Events that contribute to Variations

• independent assortment chromosomes
• crossing-over
• random fusion of gametes
• mutations

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Possible Genetic Combination (independent
assortmt. random fusion)

• possible combinations 2n
• 223 8 million
• each gamete contains 1/8,000,000 possible
  assortments
• ovum x sperm
• 1/8 million x 1/8 million
• 1/64 trillion

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Chromosomes

• human 46 chromosomes, 23 pairs
• autosomal chromosomes 44, 22 pairs
• sex chromosomes 2, 1 pair
• X y

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Female Sex Chromosomes

• X
• female XX
• X
• all gametes carry X chromosome
• homogametic sex
  

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Male Sex Chromosomes

• X
• male XY
• y
• 1/2 of gametes carry X, 1/2 carry y
• heterogametic sex
• hemizygous

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Chapter 12

• The END