chapter 4

• Meiosis


  The form of eukaryotic cell division that produces haploid sex cells or gametes (which contain a single copy of each chromosome) from diploid cells (which contain two copies of each chromosome)
• Meiosis


  1. DNA replication

  2. Meiosis I

  3. Meiosis II
• Meiosis I


  • Separates the pairs of homologous chromosomes
  • Reduces the cell from diploid to haploid
• Prophase I


  1. Leptotene: chromosomes start to condense

  2. Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads)

  3. Pachytene: crossing over between pairs of homologous chromosomes to form chiasmata

  4. Diplotene: homologous chromosomes start to separate but remain attached by chiasmata

  5. Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes
• Prometaphase I


  Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores
• Metaphase I


  1. Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate

  2. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate
• Anaphase I


  The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell
• Telophase I


  The chromosomes become diffuse and the nuclear membrane reforms
• Cytokinesis


  The final cellular division to form two new cells, followed by Meiosis II
• Meiosis I is a reduction division: the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome
• meiosis I in Male

  prophase 1- chromosome began to condense

  -homologous chromosome pair crossing over occurs

  -recombinant chromosomes

  Metaphase I-spindle fibers attach to chromosomes

  -chromosomes line up in center of cell

  Anaphase I-chromosomes starts to move to opposite ends of cell as spindle fibers shorten

  Telophase I-chromosomes reach opposite ends nuclear membrane forms

  Cytokinesis-cell division occurs
• Meiosis I in Male

  1. Prophase 1

  2. Metaphase I

  3. Anaphase I

  4. Telophase I

  5. Cytokinesis
• Prophase 1


  • Chromosome began to condense
  • Homologous chromosome pair crossing over occurs
  • Recombinant chromosomes
• Metaphase I


  • Spindle fibers attach to chromosomes
  • Chromosomes line up in center of cell
• Anaphase I


  • Chromosomes start to move to opposite ends of cell as spindle fibers shorten
• Telophase I


  • Chromosomes reach opposite ends, nuclear membrane forms
• Cytokinesis


  • Cell division occurs
• Meiosis in females

  Meiosis I in Females

  Prophase I

  -chromosomes begin to condense

  -homologous chromosomes pair crossing over occurs

  -recombinant chromosomes

  Metaphase I

  -spindle fibers attach to chromosomes

  -chromosomes line up in center of cell

  Anaphase I

  chromosomes reach opposite ends

  -nuclear membrane forms

  Cytokinesis

  cell division occurs
• Meiosis in females


  1. Meiosis I in Females

  2. Prophase I

  3. Metaphase I

  4. Anaphase I

  5. Cytokinesis
• Prophase I


  • Chromosomes begin to condense
  • Homologous chromosomes pair
  • Crossing over occurs
  • Recombinant chromosomes
• Metaphase I


  • Spindle fibers attach to chromosomes
  • Chromosomes line up in center of cell
• Anaphase I


  • Chromosomes reach opposite ends
  • Nuclear membrane forms
• Cytokinesis


  • Cell division occurs