Meiosis

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Meiosis

• Discovery
• Meaning diminuation
• 1838 Edouard van Beneden
• Chromosome number changes from diploid(2N) to
  haploid(1N)

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Meiosis cell cycle

• Interphase mammalian cell
• G1 cell growth 10 hrs
• S DNA synthesis/ chromosome duplicate 9 hrs
• G2 prepares for meiosis 4 hrs
• M mitosis/meiosis 1 hr

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Chromosome Structure

• One DNA molecule
• 2 telomeres
• 1 centromere
• Following S phase replication
• Two DNA molecules
• Chromatids

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Meiosis 1

• Prophase 1
• Leptonema
• Replicated chromosomes become visible
• Zygonema
• Homologous chromosomes pair synapsis
• Bivalents
• Tetrads four chromatids
• Pachynema
• Homologous chromosomes fully paired
• Crossing-over occurs
• Diplonema
• Homologous chromosomes begin to repel
• Revealing chromatids
• Separation reveals chiasmata areas of
  crossing-over
• Diakinesis
• Chromosomes shorten and thicken
• Nucleolus and nuclear membranes disappear
• Microtubule(spindles) appear attached to
  kinetochore

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Meiosis 1

• Metaphase 1
• Spindle assembly completes
• Homologs align across center plane
• Anaphase 1
• Homologous chromosomes separate
• Migrate toward opposite poles
• Telophase 1
• Homologous chromosomes(with 2 chromatids)
  complete separation
• Nuclear membrane sometimes reforms
• Cytokinesis produces two separate daughter cells

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Meiosis 2

• Prophase 2
• Chromosomes(with 2 chromatids) condense again
• Metaphase 2
• Chromosomes move the center plane
• Kinetochores attach to spindle fibers
• Centromere divides
• Anaphase 2
• Chromatids separate
• Daughter Chromosomes move to opposite poles
• Telophase 2
• Nuclear membrane forms around chromosomes
• Nucleolus reforms
• Cytokinesis produces four haploid daughter cells

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Meiosis

• Important points
• Chromosomal material replicates once
• Cell divides twice
• Reduction-division
• Equatorial division
• Reduces chromosome number by one-half
• Reduction-division
• Occurs during gametogenesis
• Ensures constant chromosome number

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Gametogenesis spermatogenesis

• Primordial germs cells in the male fetus line the
  seminiferous tubules in the developing testes and
  become spermatogonia.
• Spermatogonia divide for awhile by mitosis, then
  differentiate into primary spermatocytes
• Primary spermatocytes undergo meiosis 1 to
  produce secondary spermatocytes
• Secondary spermatocytes undergo meosis 2 to
  produce four haploid spermatids
• Spermatids differentiate into spermatozoa
• Spermatozoa are small cells which store little
  nutrients, lack many normal cellular components,
  but retain many mitochondria which supply energy
  for motility.
• The head of the sperm cell contains an acrosomal
  vesicle which house many enzymes that assist the
  cell in penetration of an egg.

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Gametogenesis - Oogenesis

• Primordial germ cells are the only cells that
  will eventually divide by meiosis
• The process by which eggs are produced in the
  female
• Primordial germ cells become oogonia in the ovary
• In the fetus, multiply first by mitosis, then
  become primary oocytes
• Primary oocytes enter the diplotene stage of
  meiosis prophase 1, then stop.
• With sexual maturity, the oocyte completes
  meiosis 1 giving two haploid nuclei
• But the cell divides asymmetrically producing a
  secondary ooctye(with most of the cytoplasm, and
  a smaller polar body
• Both cells then undergo meiosis 2 producing four
  haploid nuclei
• The secondary oocyte again divides asymmetrically
  giving and Ovum and another polar body.
• The Ovum becomes the Mature Egg, Polar bodies
  degenerate

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Oogenesis

• These events occur in the ovary where the oogonia
  are surrounded by cells that form the Graafian
  follicle.
• Hormonal stimulation causes the Ovum to complete
  meiosis 1, and rupture of the follicle releasing
  the Ovum into the fallopian tubes Ovulation
• If the Ovum is fertilized in the tubules, it will
  complete meiosis 2. If not fertilized it will
  also degenerate with the polar bodies.

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