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Meiosis. Summary of process:. Process to reduce number of chromosomes in sex cells (diploid germ-line cells produces haploid gametes). Meiosis:. Two nuclear divisions: reduction division (Meiosis I) and mitotic-like division (Meiosis II). Produces 4 genetically different haploid gametes.

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meiosis
Meiosis
  • Summary of process:
  • Process to reduce number of chromosomes in sex cells (diploid germ-line cells produces haploid gametes)
  • Meiosis:
  • Two nuclear divisions: reduction division (Meiosis I) and mitotic-like division (Meiosis II)
  • Produces 4 genetically different haploid gametes
  • Part of sexual reproduction

How are new organisms produced?

meiosis2

From Mom

From Dad

Meiosis
  • Unique features of meiosis:
  • Synapsis:
  • Meiosis:
  • Prophase I
  • Homologous chromosomes (= ‘matching’ chromosomes) pair together lengthwise

How are new organisms produced?

meiosis3

synapsis

Meiosis
  • Unique features of meiosis:
  • Crossingover:
  • Meiosis:
  • Prophase I
  • Homologous chromosomes ‘exchange’ sections of their chromosomes
  • Location of crossing over = chiasma (plural, chiasmata)

How are new organisms produced?

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Meiosis
  • Unique features of meiosis:
  • Reductiondivision:
  • Meiosis:
  • Anaphase I
  • Homologous chromosomes separate and move into different cells; no further chromosome replication occurs
  • Results in ‘daughter cells’ with half the number of chromosomes (one chromosome of each homologous pair)

How are new organisms produced?

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Meiosis
  • Interphase I
  • Chromosomes (chromatin) replicate forming sister chromatids as in mitosis
  • Stages of meiosis:
  • Chromosomes begin to condense
  • Centrioles replicate (in animal cells)
  • All activity occurs within nucleus of germ-line cell

How are new organisms produced?

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Meiosis
  • Prophase I:
  • Chromosomes condense
  • Stages of meiosis:
  • Centrioles move apart; spindle apparatus forms
  • Nuclear membrane breaks down
  • Synapsis (homologous chromosomes pair)
  • Crossing over between homologous chromosomes

How are new organisms produced?

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Meiosis
  • Metaphase I:
  • Homologous pairs randomly line up at metaphase plate (chromosome combination in gametes depends on line up in metaphase -- reason for independent assortment)
  • Stages of meiosis:
  • Kinetochore microtubules attach to one side of each homologous chromosome

How are new organisms produced?

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Meiosis
  • Anaphase I:
  • Kinetochore microtubules shorten; homologous chromosomes pulled to opposite poles (sister chromatids NOT pulled apart)
  • Stages of meiosis:
  • Each pole gets a haploid set of chromosomes (one or the other homologue)

How are new organisms produced?

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Meiosis
  • Telophase I:
  • Nuclear membrane reforms around each haploid set of chromosomes
  • Stages of meiosis:
  • Cytokinesis usually occurs

How are new organisms produced?

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Meiosis
  • Meiosis II:
  • Identical to mitosis except no chromosome replication during brief (if any) interphase II
  • Stages of meiosis:

How are new organisms produced?

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Meiosis
  • Genetic recombination results from:
  • Arrangement of homologous chromosomes during metaphase I
  • Results of sexual reproduction:
  • Crossing over
  • Random fertilization
  • Genetic recombination produces genetic variation among individuals
  • Genetic variation necessary for adaptations by natural selection
  • Other source of genetic variation is mutation

How are new organisms produced?