MEIOSIS rev2

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    MeiosisMeiosis

    The form ofcell divisioncell division by which gametes,gametes, with

    halfhalfthe number ofchromosomes,chromosomes, are

    produced.

    Diploid (2n)Diploid (2n) pp haploid (n)haploid (n)

    MeiosisMeiosis is required forsexual reproduction.sexual reproduction.

    Two divisionsTwo divisions

    meiosis Imeiosis I and meiosis IImeiosis II.

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    MeiosisMeiosis

    Sex cells (also known germ cells) divide toproduce gametes (sperm or egg).

    Gametes have halfthe # ofchromosomes.

    Occurs only in gonads (testes or ovaries).

    Male: spermatogenesis

    Female: oogenesis

    Meiosis is similar to mitosis with some key

    chromosomal differences.

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    MeiosisMeiosis

    2n=4

    germ cell

    diploid (2n)diploid (2n)

    n=2

    n=2

    meiosis I

    n=2

    n=2

    n=2

    n=2

    sex cells

    (sperm,eggs, ovules,

    pollen)

    haploid (n)haploid (n)

    meiosis II

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    InterphaseInterphase II Similar to mitosismitosis interphase.

    ChromosomesChromosomes replicate (S phase).(S phase).

    Each duplicated chromosomechromosome consists of

    two identical sisterchromatidschromatids attached at

    theircentromerescentromeres.

    CentrioleCentriole pairs also replicate.

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    Interphase IInterphase I NucleusNucleus and nucleolusnucleolus visible.

    nuclear

    membrane

    nucleolus

    cell membrane

    chromatin

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    Homologous ChromosomesHomologous Chromosomes

    Pair ofchromosomeschromosomes (maternalmaternal and paternalpaternal) that are

    similar in shapeshape andand sizesize.

    Homologous pairs (tetrads)(tetrads) carry genes controlling

    the same inherited traitssame inherited traits.

    Each locuslocus (position of a gene)(position of a gene) is in the same

    position on homologues.

    Humans have 23 pairs ofhomologous chromosomes.homologous chromosomes.a. 22 pairs ofautosomesautosomes

    b. 1 pair ofsex chromosomessex chromosomes

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    Homologous ChromosomesH

    omologous Chromosomes

    Paternal Maternal

    eye color

    locus

    eye color

    locus

    hair color

    locushair color

    locus

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    Sex ChromosomesSex Chromosomes

    XX chromosome - female XY chromosome - male

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    Meiosis I (four phases)Meiosis I (four phases) Cell divisionCell division that reduces the chromosomechromosome

    number by oneone--half.half.

    four phasesfour phases:

    a.a. prophase Iprophase I

    b.b. metaphase Imetaphase I

    c.c. anaphase Ianaphase I

    d.d. telophase Itelophase I

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    Prophase IProphase I Longest and most complex phase (90%).Longest and most complex phase (90%).

    ChromosomesChromosomes condense. SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes

    come together to form a tetradtetrad.

    TetradTetrad is two chromosomeschromosomes or fourchromatidschromatids (sister and nonsister chromatids).

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    Prophase IProphase I -- SynapsisSynapsisHomologous chromosomes

    sister chromatids sister chromatids

    Tetrad

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    Crossing OverCrossing Over

    segments of nonsisterchromatidschromatids break and

    reattach to the otherchromatidchromatid.

    This is an important source ofgenetic variation.

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    Crossing OverCrossing Over -- variationvariation

    nonsister chromatids

    chiasmata: siteof crossing over variation

    Tetrad

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    Crossing over

    Now, chromatidsPaternal AA and

    Maternal aa are

    changed from their

    original form.

    This is a source of

    genetic variationgenetic variation.

    The DNA is a

    mixture of genes

    that has neverexisted before.

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    Prophase IProphase I

    centriolesspindle fiber

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    Metaphase IMetaphase I

    OR

    Shortest phaseShortest phase

    TetradsTetrads align on the metaphase platemetaphase plate.

    Independent

    assortment

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    Anaphase IAnaphase I

    Homologous chromosomesHomologous chromosomes separate andmove towards the poles.

    SisterSister chromatidschromatids remain attached at their

    centromerescentromeres.

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    Telophase ITelophase I Each pole now has haploidhaploid set of

    chromosomeschromosomes.

    CytokinesisCytokinesis occurs and two haploid daughter

    cells are formed.

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    Telophase ITelophase I

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    Meiosis IIMeiosis II

    NoNo interphaseinterphase IIII(or very short - no more DNA replicationDNA replication)

    Prophase IIProphase II

    Metaphase IIMetaphase II

    Anaphase IIAnaphase II

    TelophaseTelophase IIIIRemember:Remember:

    Meiosis IIMeiosis II is very similar to mitosismitosis

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    Metaphase IIMetaphase II same as metaphasemetaphase in mitosismitosis

    metaphase platemetaphase plate

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    Anaphase IIAnaphase II same as anaphaseanaphase in mitosismitosis

    sister chromatids separatesister chromatids separate

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    Telophase IITelophase II Same as telophasetelophase in mitosismitosis.

    Nuclei form. CytokinesisCytokinesis occurs.

    Remember:Remember: four haploid daughter cellsfour haploid daughter cells

    produced.produced.

    gametes = sperm or egggametes = sperm or egg

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    Telophase IITelophase II

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    SpermatogenesisSpermatogenesis

    2n=46

    Male germ cell

    diploid (2n)diploid (2n)

    n=23

    n=23

    meiosis I

    n=23

    n=23

    n=23

    n=23

    haploid (n)haploid (n)

    meiosis II

    sperm

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    OogenesisOogenesis

    2n=46

    Female germ cell

    diploid (2n)diploid (2n)

    n=23

    n=23

    meiosis I

    n=23

    n=23 egg

    haploid (n)haploid (n)

    meiosis II

    n=23

    n=23polar

    bodies

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    FertilizationFertilization

    The fusion of a spermsperm and eggegg to form a zygotezygote.

    A zygote is a fertilized egg

    n=23

    egg

    spermn=23

    2n=46zygote

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    Genetic VariationGenetic Variation Important to population as the rawImportant to population as the raw

    material formaterial fornatural selectionnatural selection..

    Question:Question:

    What are the three sources ofWhat are the three sources of

    genetic variation during sexualgenetic variation during sexualreproduction?reproduction?

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    Answer:Answer:1. crossing over (prophase I)1. crossing over (prophase I)

    2. independent assortment (metaphase I)2. independent assortment (metaphase I)

    3. random fertilization3. random fertilization

    Remember: variation is good!

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    Question:Question:

    A cell containing 20 chromosomes20 chromosomes(diploid)(diploid) at the beginning of meiosis

    would, at its completion, produce

    cells containing how many

    chromosomeschromosomes?

    Answer:Answer:10 chromosomes (haploid)10 chromosomes (haploid)

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    KaryotypeKaryotype A method of organizing theA method of organizing the chromosomeschromosomes of aof a

    cell in relation to number, size, and type.cell in relation to number, size, and type.

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    KaryotypeKaryotype

    Question

    What is theWhat is the

    sex of thissex of this

    person?person?

    Answer

    MALEMALE

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    Question:Question:

    A cell containing 4040 chromatidschromatids atthe beginning of meiosis would, at its

    completion, produce cells containinghow many chromosomeschromosomes?

    Answer:Answer:

    10 chromosomes10 chromosomes