Meiosis

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Meiosis

1. Overview of meiosis
2. Homologous chromosomes
3. Stages of meiosis
4. 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