Meiosis (Chapter 13)

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Meiosis (Chapter 13)
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Why meiosis?

• Produces haploid cells.
• Genetic diversity
• Evolution

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Mitosis

• Two identical daughter cells
• Interphase
• Cell growth, preparing for cell division
• Prophase, Metaphase, Anaphase, Telophase
• Cytokinesis

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Fig. 12-4
0.5 µm
Chromosomes
DNA molecules
Chromo- some arm
Chromosome duplication (including DNA synthesis)
Centromere
Sister chromatids
Separation of sister chromatids
Centromere
Sister chromatids
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Fig. 13-3b
5 µm
Pair of homologous replicated chromosomes
Centromere
Sister chromatids
Metaphase chromosome
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Meiosis

• 2 successive rounds of cell division
• No replication of DNA between rounds
• Meiosis I
• Half the number of chromosomes
• Meiosis II
• 4 haploid cells

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Meiosis

• Each round of division has a
• Prophase
• Metaphase
• Anaphase
• Telophase

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Fig. 13-7-3
Interphase
Homologous pair of chromosomes in diploid parent
cell
Chromosomes replicate
Homologous pair of replicated chromosomes
Sister chromatids
Diploid cell with replicated chromosomes
Meiosis I
Homologous chromosomes separate
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Haploid cells with replicated chromosomes
Meiosis II
Sister chromatids separate
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Haploid cells with unreplicated chromosomes
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Meiosis

• Synapsis
• Homologous chromosomes
• Closely associated
• Synaptonemal complex
• Homologous chromosomes are paired
• Protein complex between them
• Occurs in prophase I

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Meiosis

• Crossing-over
• Homologues exchange chromosomal information
• Genetic recombination
• Chromosomes from one parent carry info from both
• Chiasma cross (pluralChiasmata)
• Site where crossing-over happens

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Cross-over

• Between non-sister chromatids
• Stabilized by sister chromatids

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Prophase I

• DNA coils tighter
• DNA already duplicated
• Sister chromatids joined at centromeres

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Prophase I

• Sister chromatid cohesion
• Sister chromatids closely associate
• Homologous chromosomes line up next to each other
• Crossing over happens
• Non-sister chromatids

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Prophase I

• Crossing over ends
• 4 chromatids (2 homologs) stay close due to
• 1.Sister chromatid cohesion
• 2.Chiasmata where crossover occurs

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Metaphase I

• Homologous pairs align beside each other
• Metaphase plate (center)
• Chiasmata holds homologous chromosomes together
• Maternal homologue orients towards one pole
• Paternal homologue orients towards other pole

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Metaphase I
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Fig. 13-8b
Prophase I
Metaphase I
Centrosome (with centriole pair)
Centromere (with kinetochore)
Sister chromatids
Chiasmata
Spindle
Metaphase plate
Homologous chromosomes
Fragments of nuclear envelope
Microtubule attached to kinetochore
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Anaphase I

• 90 meiosis is spent in Prophase Metaphase
• Spindle fibers begin to shorten
• Pull apart homologous chromosomes
• Go to separate poles
• Sister chromatids remain together
• Mitosis-sister chromatids separate

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Anaphase I

• Each pole has a complete haploid set of
  chromosomes
• Each pole has one member of the homologous pair
• Either a maternal or paternal homologue

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Anaphase I
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Telophase I

• Homologues cluster at the poles
• Nuclear membrane reforms
• Each daughter cell contains half the of
  chromosomes
• Sister chromatids
• Different due to crossover

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Telophase I

• Cytokinesis may occur
• Second division occurs after variable length

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Meiosis I
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Prophase II

• Nuclear membrane breaks down
• New spindles form

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Metaphase II

• Spindle fibers bind to both sides of the
  centromere

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Anaphase II

• Spindle fibers contract
• Sister chromatid cohesion is released
• Splits the sister chromatids
• Move to opposite poles

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Telophase II

• Nuclear envelope reforms
• 4 haploid cells

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Meiosis II
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Meiosis
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Sexual reproduction

• Gametes
• Egg sperm
• Half the number of chromosomes
• Zygote
• Egg and sperm combine
• Fertilization or syngamy
• Fusion of gametes to form a new cell

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Sexual reproduction

• Life cycles alternate
• Diploid haploid chromosome numbers
• Alternates between meiosis fertilization
• Offspring inherit chromosomes from both parents
• Variations occur producing 3 types of sexual life
  cycles

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1. Animals

• Majority of time as diploids
• Haploids do not under go mitosis
• Germ-line cells
• Cells that will under go meiosis
• Produce gametes

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2. Fungi and some algae

• Spend majority of time as haploid
• Zygote undergoes meiosis
• Then mitosis

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3. Plants

• Alternate between multicellular haploid
• Multicellular diploid phase

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Evolution

• Asexual reproduction
• Inherit chromosomes from one parent
• Identical to parent
• Protists reproduce asexually
• Plants reproduce asexually

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Sexual reproduction

• Generates genetic diversity
• Evolutionary adaptation depends on a populations
  genetic variation

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Genetic diversity

• 1. Independent assortment
• 2. Crossover
• 3. Random fertilization

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Independent assortment

• Genes on different chromosomes
• Orient independently
• Homologous pairs line up as a matter of chance

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Independent assortment
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Crossover

• Recombinant chromosomes
• Carry information from 2 different parents

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Random fertilization
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