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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
Meiosis
  • Overview of meiosis
  • Homologous chromosomes
  • Stages of meiosis
  • Spermatogenesis & oogenesis
a overview of meiosis
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
a overview of meiosis3
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
a overview of meiosis4
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
a overview of meiosis5
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
b homologous chromosomes
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
b homologous chromosomes7
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
c stages of meiosis
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
c stages of meiosis9
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
c stages of meiosis10
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
c stages of meiosis11
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
c stages of meiosis12
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
c stages of meiosis13
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
c stages of meiosis14
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
c stages of meiosis15
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
d gametogenesis
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
d gametogenesis17
D. Gametogenesis
  • Spermatogenesis
    • Stages
      • Spermatogonium
      • Primary spermatocyte
      • Secondary spermatocyte
      • Spermatid
      • Mature spermatozoan
d gametogenesis18
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
d gametogenesis19
D. Gametogenesis
  • Oogenesis
    • Stages
      • Oogonium
      • Primary oocyte
      • Secondary oocyte
      • Ootid
      • Ovum
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