Meiosis

• Meiosis occurs in reproductive cells
• Meiosis consists of "two" cell divisions
• • Meiosis in males produces sperm cells and in females produces eggs
  • Starting cell has 46 chromosomes; ending cells only have 23
• The first division is called meiosis I, the second division is called meiosis II.
• Prophase I:

  • Chromosomes condense
  • Nucleolus disperses
  • Homologous pairs formed
  • Spindle attaches to chromosomes at centromeres
  • Nuclear envelope breaks up
  • Spindle begins to form around centrosomes
• Crossing over happens when genetic material is swapped between two chromosomes
• Metaphase I:

  • M for "middle"
  • Chromosomes are in pairs in the middle of the cell
  • Chromosomes line up at metaphase plate
• Homologous Pairs:

  • of chromosomes line up at the metaphase plate
• Anaphase I:

  • A for "away"
  • homologous pairs split, whole chromosomes pulled to opposite ends
  • cell elongates
• Telophase I:

  • T for "two"
  • chromosomes disperse
  • nucleoli form
  • nuclear envelopes assemble spindle breaks up
  • cytokinesis begins
• The number of chromosomes halves from diploid (2n) to haploid (n)
• • Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells.
  • This process is required to produce egg and sperm cells for sexual reproduction.
• A cell needs to undergo interphase before entering meiosis
• Crossing over only occurs in meiosis I
• Prophase II:

  • Spindles start to form
• Metaphase II:

  • "M" for middle
  • Chromosomes line up in the middle, but not in pairs like metaphase I
• Anaphase II:

  • "A" for away
  • Chromatids are pulled away by spindle fibers
• • Cytokinesis occurs after telophase I and II
  • Cytokinesis is the division of cytoplasm
• Nondisjunction: chromatids are not pulled apart properly
• How many cells are produced in meiosis?

  Four daughter cells
• The four daughter cells resulting from meiosis are haploid and genetically distinct.