Meiosis and Sexual Life Cycles

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Meiosis and Sexual Life Cycles

• Chapter 13

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Important Definitions

• Heredity
• Variation
• Genetics
• DNA

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Genes

• units of hereditary information
• Made of DNA
• LOCATED ON CHROMOSOMES
• Specific sequence of nucleotides
• Code for specific protein
• Action of proteins produce inherited traits
• Locus specific location on chromosome

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Compare asexual with sexual reproduction

• Parents?
• Genes passed to offspring?
• Appearance of offspring?
• Is self pollination in plants result of sexual or
  asexual reproduction?

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Big Picture

• Human (2N46)
• Meiosis reproductive organs ONLY
• Gametes (1N23)
• Fertilization
• Zygote (2N46)
• Mitosis
• Adult human

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Karyotype

• Photomicrograph of somatic cell chromosomes

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Autosome

• Chromosome that is not a sex chromosome
• 22 pairs of autosomes
• Exists as homologous chromosomes
• A pair of chromosomes that have the same size and
  centromere position
• Same genetic loci
• Inherit one homologue from each parent

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Sex chromosomes

• X and y
• Female ? XX
• Male ?Xy

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Gametes

• Only cells NOT produced by mitosis
• Produced in reproductive organs
• Ovaries and testes
• Produced by MEIOSIS

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Much variety in sexual cycles

• Adult organism may be
• 2N 1N 1N
• Meiosis mitosis mitosis
• Gametes gametes gametes
• Fertilization fertilization
  fertilization
• Zygote zygote zygote
• Mitosis meiosis mitosis (2n adult)

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Meiosis

• Reduces the chromosome number by ½
• Replication of DNA
• Two consecutive divisions
• Meiosis I and Meiosis II

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Interphase I

• Precedes meiosis
• Same activities as interphase
• in mitosis

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Meiosis I Prophase I

• Chromosomes condense
• Synapsis
• pairing of homologous chromosomes
• forms tetrad
• Crossing over
• occurs between nonsister chromatids
• Chiasmata site of cross over
• Increases variation

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Synapsis and crossing over
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Asci in Sordaria fimicola
Identify the asci exhibiting crossing over!
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Prophase I continues

• Centriole pairs move apart
• Spindle microtubules form
• Nuclear envelope and nucleoli disperse
• Requires 90 of time of meiosis

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Prophase I
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Early to late prophase I
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Metaphase I
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Anaphase ISeparation of homologous pairs
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What Stage is this?
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Telophase I
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Meiosis II
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How are these different from Meiosis I
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What stage?
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Sources of genetic variation

• Independent assortment
• Crossing over
• Random fertilization
• Be able to discuss each of these

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Compare mitosis with meiosis