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Meiosis Overview of meiosis Homologous chromosomes Stages of meiosis Spermatogenesis & oogenesis A. Overview of Meiosis Meiosis: Specialized nuclear division in which the number of chromosomes is reduced by half Purpose of meiosis: the formation of gametes Occurs only in germ-line tissue

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Meiosis

  • Overview of meiosis

  • Homologous chromosomes

  • Stages of meiosis

  • Spermatogenesis & oogenesis


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A. Overview of Meiosis

  • Meiosis:

    • Specialized nuclear division in which the number of chromosomes is reduced by half

    • Purpose of meiosis: the formation of gametes

    • Occurs only in germ-line tissue

    • Diploid number: the number of chromosomes in a germ-line cell

    • Haploid number: the number of chromosomes in a gamete; ½ The diploid number


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A. Overview of Meiosis

  • Zygote:

    • During the process of fertilization, a haploid gamete from one parent fuses with the haploid gamete from the other parent

    • The resulting diploid cell is called a zygote


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A. Overview of Meiosis

  • Gametes:

    • In most sexually reproducing species, there are two distinct types of gametes

    • Spermatozoa (or pollen in plants) are compact, highly motile gametes that contribute their chromosomes to the zygote; “Male” gametes

    • Ova (or ovules in plants) are much larger and contribute both chromosomes and cytoplasm (the bulk of the cell mass) to the zygote; “Female” gametes


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A. Overview of Meiosis

  • Gonads

    • In most multicellular species, germ-line tissue is found in organs called gonads

    • Spermatozoa are produced in gonads called testes

    • Ova are produced in gonads called ovaries

    • Sexually dimorphic species: two separate genders, with each individual having either male or female gonads

    • Sexually monomorphic species (hermaphroditic species): each individual contains both male and female gonads


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B. Homologous Chromosomes

  • Homologous chromosome pairs:

    • For each chromosome in a diploid nucleus, there is another very similar chromosome in the same nucleus

    • This pair of very similar chromosomes is called a homologous chromosome pair

    • One chromosome in each pair comes from one parent, and the other chromosome comes from the other parent


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B. Homologous Chromosomes

  • Homologous chromosomes are similar in:

    • Size

    • Position of the centromere

    • Banding patterns in staining procedures

    • The type of genetic information they contain

  • During meiosis, the homologous chromosomes are separated, so that a gamete receives only one member of each homologous chromosome pair


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C. Stages of Meiosis

  • Prior to meiosis:

    • The diploid germ-line cell goes through a complete interphase, including an S phase

    • Therefore, at the start of meiosis, each of the chromosomes is in a replicated state (consisting of sister chromatids connected at the centromere)

  • Meiosis is accomplished in two divisions: meiosis I and meiosis II


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C. Stages of Meiosis

  • Meiosis I:

    • At the start of meiosis I, the two chromosomes in each homologous chromosome pair line up along their lengths

    • During meiosis I the homologous chromosomes separate and move to opposite sides of the cell. (Note that the chromatids stay together at this point.)

    • At the end of meiosis I, the cell divides into two

    • Note that each daughter cell formed by meiosis I is haploid, but the chromosomes are still in their replicated state


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C. Stages of Meiosis

  • Meiosis II:

    • Each of the cells from meiosis I can undergo meiosis II

    • During meiosis II, the centromeres split, the sister chromatids separate and become daughter chromosomes, and the daughter chromosomes move to opposite sides of the cell

    • New nuclei form, and the cell divides

    • Therefore, meiosis (I and II together) has the potential of forming four haploid cells, with the chromosomes in an unreplicated state at the end of the process


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C. Stages of Meiosis

  • Meiosis I is divided into four stages: prophase I, metaphase I, anaphase I, and telophase I

  • Prophase I: during prophase I, the nuclear membrane & nucleolus disperse, and a spindle forms. The homologous chromosomes condense and pair in five steps:

    • Leptonema: the chromosomes begin to condense and have the appearance of slender threads

    • Zygonema: the homologous chromosomes align completely along their lengths, forming paired chromosomes called bivalents. The connection between the chromosomes is called the synaptonemal complex


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C. Stages of Meiosis

  • Prophase I (continued):

    • Pachynema: the bivalent chromosomes continue to condense, becoming very short & thick

    • Diplonema: the chromosomes in each bivalent begin to partially separate. The two chromosomes in the bivalent remain connected at X-shaped regions called chiasmata (singular: chiasma)

    • Diakinesis: the chiasmata migrate to the ends of the bivalents


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C. Stages of Meiosis

  • Metaphase I:

    • The bivalents align at the equator of the spindle

  • Anaphase I:

    • The homologous chromosomes separate and move to opposite poles of the spindle

    • Note that the the chromatids do not separate at this time

  • Telophase I:

    • The chromosomes at each pole may decondense, and new nuclei form

    • Cytokinesis takes place, resulting in two cells


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C. Stages of Meiosis

  • Meiosis II is divided into four stages: prophase II, metaphase II, anaphase II, and telophase II

  • Prophase II

    • Chromosomes condense; Membrane disperses; Spindle forms


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C. Stages of Meiosis

  • Metaphase II

    • Chromosomes align at equator of spindle

  • Anaphase II

    • The sister chromatids separate and move to opposite poles of the spindle

  • Telophase II

    • Chromosomes decondense; New nuclei form

    • Cytokinesis takes place


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D. Gametogenesis

  • Spermatogenesis

    • The cytokinesis divisions (cell divisions) after meiosis I and meiosis II are equal

    • This means that one germ line cell in the testes divides by meiosis to produce four cells of equal size

    • Each of these four cells develops into a spermatozoan


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D. Gametogenesis

  • Spermatogenesis

    • Stages

      • Spermatogonium

      • Primary spermatocyte

      • Secondary spermatocyte

      • Spermatid

      • Mature spermatozoan


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D. Gametogenesis

  • Oogenesis

    • When the primary oocyte undergoes meiosis I, the cytokinesis is unequal, resulting in one very large cell (the secondary oocyte) and one much smaller cell (the first polar body)

    • When the secondary oocyte divides in meiosis II, again the division is unequal. The result is one very large gamete (the ovum) and a second polar body

    • Therefore, a single diploid germ-line cell in an ovary will produce only one gamete


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D. Gametogenesis

  • Oogenesis

    • Stages

      • Oogonium

      • Primary oocyte

      • Secondary oocyte

      • Ootid

      • Ovum


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