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Mitosis Meiosis
Making Copies and Making Babies
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Meiosis

• What would happen if sexually reproducing
  organisms didnt reduce their chromosome number
  prior to conception?
• Parent with 1 chromosome would have offspring
  with 2 chromosomes
• Next generation would have 4 chromosomes, next
  would have 8, then 16, then 32, 64, 128, 256, 512

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Meiosis

• Gamete cells (sperm egg) have only half the
  chromosomes of their parent
• Somatic cells (not gametes) are diploid
• 2 copies of each chromosome (one from each
  parent)
• called homologous chromosomes

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Homologous Chromosomes

• have genes for the same traits in the same
  location
• The version of those genes may be different (e.g.
  red hair from mom and brown from dad) but still
  determine the same trait (hair colour)

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Homologous Chromosomes
homologous chromosomes
centromere
sister chromatids
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Homologous Chromosomes

• position of a gene on a chromosome is called its
  locus (plural loci)
• chromosomes of a homologous pair carry genes for
  the same trait at the same locus

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Genes Alleles

• genes for a specific trait on each homologous
  chromosome might not be the same
• different forms of the same gene are called
  alleles

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Diploid vs. Haploid

• 2 copies of each chromosome (one from mom and one
  from dad)

• Only 1 copy of each chromosome i.e. gametes
• Meiosis creates haploid gametes

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Stages of Meiosis

• similar to mitosis but some significant
  differences
• broken down into Meiosis I and Meiosis II
• In Meiosis I, cells become haploid
• In Meiosis II the sister chromatids separate into
  haploid daughter cells

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Before Meiosis I

• During interphase the cell replicates its genetic
  information (just like in mitosis)
• This is referred to as the pre-meiotic S phase
• replicated chromosomes are joined by a centromere
  to which the spindle fibers attach

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Meiosis I

• Meiosis I is broken down into the following
  stages
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I Cytokinesis

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Meiosis II

• Meiosis II is broken down into the following
  stages
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II Cytokinesis

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Prophase I

• Similarity to mitosis
• the chromosomes condense the nuclear membrane
  dissolves

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Prophase I

• Different from mitosis
• homologous chromosomes pair up
• known as synapsis
• forms a tetrad
• Non-sister chromatids exchange pieces of
  themselves in a process called crossing-over or
  genetic recombination
• allows for genetic variation to exist in a species

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Crossing Over
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Metaphase I

• homologous chromosomes are attached
• this pair moves together to the metaphase plate
  at the equator of the cell

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Anaphase I

• homologous chromosomes are each pulled by their
  centromere to opposite poles of the cell
• sister chromatids remain attached

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Telophase I Cytokinesis

• Homologs are now separate
• Each pole is now haploid
• Each chromosome still has 2 chromatids
• Cytokinesis occurs simultaneously
• result is two haploid daughter cells, each with
  half the number of chromosomes of the parent cell

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Interphase?

• Some cells may have no interphase after meiosis I
• Some cells may have a short one
• Some cells may have a very long period of
  dormancy before meiosis II occurs
• In any case, there is NO replication of DNA! (No
  S-phase)

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Prophase II

• spindle begins to appear again
• Spindle fibers attach to chromosomes at their
  centromere and begin to move to the metaphase
  plate

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Metaphase II

• chromosomes line up on the metaphase plate or
  equator(Just like mitosis!)

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Anaphase II

• Centromeres of sister chromatids finally separate
• sister chromatids of each pair are now
  independent chromosomes
• sister chromatids move toward opposite poles

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Telophase II Cytokinesis

• Nuclei begin to form at opposite poles of the
  cells
• Chromosomes unwind and elongate
• Cytokinesis occurs simultaneously
• End result of the entire process is 4 daughter
  cells with half (haploid) number of chromosomes

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Meiosis Animation

• http//www.youtube.com/watch?vD1_-mQS_FZ0feature
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