MEIOSIS

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MEIOSIS

• Objectives
• meiosis is an early step in sexual reproduction
  in which the pairs of chromosomes separate and
  segregate randomly during cell division to
  produce gametes containing one chromosome of each
  type (2a)
• only certain cells in a multi-cellular organism
  undergo meiosis (2b)

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Diploid/Haploid Cells

• Diploid cells (body cells) have 2 of each type of
  chromosome-alleles (human 23 prs), 2n in number
• Haploid (sex cells-gametes) have only one kind of
  each chromosome, n in number
• Each organism species have a characteristic of
  chromosomes

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• Pair of chromosomes are called HOMOLOGOUS
  chromosomes-have genes for the same traits, in
  same order, but may not be identical
• Meiosis allows offspring to have same of
  chromosomes as parents (half from each parent)

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Meiosis

• Has 2 separate divisions
• Meiosis I and II
• Begins with one 2n, diploid cell, end of meiosis
  II with 4 haploid ,n, cells
• Males ? sperm (gametes)
• Females? egg (gamete)
• Fertilization results in a zygote cell, 2n.

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Phases

• Interphase
• Chromosomes replicate
• Identical chromatid sisters held by centromere
• Prophase I
• Chromosomes coil up and spindle forms
• Each pair of homologous chromosomes form a
  TETRAD
• Crossing over may occcur (exchange of genetic
  material), resulting in newcombinations of alleles

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• Metaphase I
• Centromere attaches to spindle pulling tetrads
  into the middle
• homologous chromosomes line up side by side as
  tetrads (mitosis-line up independently)

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• Anaphase I
• Homologous chromosomes separate, sisters held
  together by centromere
• This is step that makes sure each cell will only
  receive 1chromosome from each homologous pair

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• Telophase I
• Spindle dissolves, cytoplasm divides into 2 cells
• Each cell only has half the genetic information

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Meiosis II

• Prophase II
• Spindle forms in each of the 2 cells
• Chromatid sisters attach themselves via
  centromeres
• Metaphase II
• Chromosomes allign themselves In the middle
  randomly

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• Anaphase II
• Centromere splits , sister chromatids separate to
  opposite poles
• Telophase II
• Nucleus forms, spindle dissolves, cytoplasm
  divides.
• 4 cells are formed , each only containing one
  chromosome from each homologous pair from parent
  cell

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• Some cells go from Anaphase I to Metaphase II,
  still producing 4 haploid gametes.
• Provides genetic variation within a species

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Meiosis
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Mistakes

• Nondisjunction
• Homologous pairs do not separate correctly
• TRISOMY
• Fusion of gametes form a zygote with an extra
  chromosome (21Downs Syndrome)
• MONOSOMY
• Missing a chromosome
• In humans most zygotes do not survive with this
  condition
• Turner Syndrome female with only one X
  chromosome
• Sterile
• Polyploidy
• Organism has more than the usual of chromosome
  sets
• Animals rarely survive
• Plants usually produce larger flowers or fruits,
  healthier