Meiosis

• Meiosis

  • is the process that results in the formation of sperm cells and egg cells
  • Cells that go through meiosis are typically found in the testies and ovaries
  • These cells are germline cells that are diploid having two sets of chromosomes
  • Diploid cells undergo meiosis to produce haploid cells having one set of chromosomes
  • during fertilization the haploid cells fuse to form a diploid offspring
• Interphase

  • DNA replicates and each chromosome becomes doubled
  • doubled chromosome consists of 2 identical strands of DNA
  • Meiosis has two divisions
  • 1st division is meiosis 1 = half's the number of chromosomes
  • 2nd division is meiosis 2 = sister chromatids are split
  • the end result of the two divisions = 4 haploid cells
• Meiosis 1 (Prophase 1)

  Prophase 1 - individual replicated chromosomes become visible each chromosome consists of identical sister chromatids joined at their centromeres

  • spindle fibers start to form and the nuclear envelope disintegrates
  • Homologous chromosomes line up next to each other and an event unique to meiosis occurs
  • Unique event = Crossing Over
  • During Crossing over homologous chromosomes (lined up next to each other) exchange DNA between adjacent homologous chromatids
  • After crossing over has occurred the sister chromatids of one chromosome are no longer identical to one another
• Metaphase 1

  • homologous chromosomes line up along the equator of the cell
  • for each pair of homologs the orientation on the equator is random
  • Each different orientation results in gametes with different combinations of parental chromosomes
  • This process is called independent assortment
  • Once the chromosomes are lined up the ends of the spindle fibers attach to each centromere
• Anaphase 1

  • the chromosomes move apart from one another along the spindle fiber to the opposite ends of the cell
  • At this point each chromosome is still double stranded and has two sister chromatids
  • We are separating homologous chromosomes into two different cells so that each new cell will have only one member of the homologous pair and will be haploid
• Telophase 1

  • the spindle fiber disintegrates and cytokinesis cell division begins
  • two cells are created and each cell has half the number of chromosomes compared to the original cell
  • After cytokinesis is complete a second cell division or Meiosis two starts
  • This division is identical to mitosis and the resulting cells have the exact same number of chromosomes as the original cell at the end of meiosis one
• Prophase 2

  • we have two cells each with two chromosomes
  • spindle fibers start to form at the poles of the cell
• Metaphase 2

  • chromosomes line up along the equator (different from Metaphase 1)
  • Each cell only has one of each homologous chromosome
• Anaphase 2

  • sister chromatids move away from each other along the spindle fiber
• Telophase 2

  • cytokinesis and formation of four genetically different haploid cells