MEIOSIS

• Homologous chromosome


  A set of one maternal and one paternal chromosome that pair up with each other inside a cell
• A normal diploid human body cell has 46 chromosomes because it contains 2 sets of chromosomes – 23 from the father (paternal) and 23 from the mother (maternal)

  
• 23 pairs of homologous chromosomes, 22 pairs autosomes (somatic) & 1 pair sex chromosome

  
• Karyogram


  A diagram or photograph of the chromosomes of a cell, arranged in homologous pairs and in a numbered sequence

  
• Meiosis


  A type of cell division that reduces the number of chromosomes in the parent cell by half and produces four haploid sex cells/gametes

  
• Oogenesis


  The formation of egg cells

  
• Spermatogenesis


  The formation of sperm cells

  
• Gametogenesis


  The process by which gametes, or germ cells, are produced in an organism

  
• Four functional sperm cells, One functional ovum
• Meiosis I


  1. Prophase I

  2. Metaphase I

  3. Anaphase I

  4. Telophase I

  
• Meiosis II


  1. Prophase II

  2. Metaphase II

  3. Anaphase II

  4. Telophase II

  
• Meiosis I


  Reduces the number of chromosomes from diploid (2n) to haploid (n) number, thus called reductional division

  
• Meiosis II


  Similar to mitosis, thus called equational division

  
• Prophase I


  1. Chromatin condenses to form shorter & thicker chromosomes

  2. Homologous chromosomes pair up, lie side by side called synapsis to form a tetrad

  3. Exchange of genetic material (crossing-over) occur between homologous chromosomes

  4. Crossing-over contributes to genetic variability in the offspprings

  
• Metaphase I


  Paired homologous chromosomes align at the metaphase plate and are attached to spindle fibers
• Anaphase I


  Homologous chromosomes separate and move toward opposite poles of the cell by the spindle fibers

  
• Telophase I


  1. Chromosomes reach opposite poles

  2. In most organisms, the nuclear membrane forms, followed by cytokinesis, producing two haploid daughter cells (genetically different chromosomes)

  
• Prophase II


  1. Nuclear membrane breaks

  2. New spindle fibers form
• Metaphase II


  Chromosomes align at the metaphase plate and are attached to their centromeres to the spindle fibers
• Anaphase II


  1. Each chromosome is divided into two sister chromatids

  2. The chromatids (now called chromosomes) move toward the opposite poles

  
• Telophase II


  1. Nuclear membrane is formed around each set of chromosomes

  2. Spindle fibers break

  3. Cytokinesis splits the chromosome sets into new cells, forming the final products of meiosis: four haploid cells

  
• Meiosis produces genetic variations among sexually producing organisms

  
• Genetic variability among organisms is essential for reproduction and survival in a changing environment
• Meiosis allows sexual reproduction of diploid organisms
• Meiosis maintains the chromosome numbers constant from generation to generation