Cell division

• Mitosis


  A form of cell division that produces identical cells, there are four stages: prophase, metaphase, anaphase and telophase
• Cell cycle

  1. Cell forms

  2. Cell grows

  3. Cell divides to form daughter cells
• Cell cycle

  • Controlled by checkpoints
• Cytokinesis


  1. Parent and replicated organelles move to opposite sides of the cell

  2. Cytoplasm divides to produce two daughter cells
• Interphase


  1. Cell grows

  2. Chromosomes and some organelles are replicated

  3. Chromosomes begin to condense
• Prophase


  1. Nuclear envelope breaks down and disappears

  2. Chromosomes condense

  3. Centrioles move to opposite poles for spindle formation
• Metaphase


  Chromosomes move to the equator and attach to spindle fibres via centromeres
• Anaphase


  Sister chromatids are separated
• Telophase


  1. Nuclear envelope reforms

  2. Spindle is broken down and disappears

  3. Chromosomes uncoil
• Meiosis


  A form of cell division that gives rise to genetic variation, produces haploid gametes
• Genetic variation in meiosis

  • Crossing over of chromatids
  • Independent assortment of chromosomes
• Meiosis


  A type of cell division that produces daughter cells which are genetically different
• Meiosis


  1. Parent cell divides once

  2. Parent cell divides twice

  3. Results in four haploid daughter cells
• Haploid cells


  • Have one set of chromosomes
• Diploid cells


  • Have two sets of chromosomes
• Independent segregation


  Homologous chromosomes line up randomly at the equator during meiosis I, resulting in random distribution to daughter cells
• Crossing over


  Homologous chromosomes exchange genetic material during meiosis I, resulting in new combinations of alleles
• Meiosis vs Mitosis


  • Meiosis has two nuclear divisions, mitosis has one
  • Meiosis produces haploid cells, mitosis produces diploid cells
  • Meiosis introduces genetic variation, mitosis produces genetically identical cells
• Meiosis introduces genetic variation through independent segregation and crossing over
• Gametes


  Haploid cells produced by meiosis
• Fusion of a random egg and a random sperm during fertilization results in 2^(2n) possible chromosome combinations in the offspring
• Prophase I


  • Closely resembles the prophase stage of mitosis
  • Synapsis and crossing over of homologous chromosomes (at chiasmata) which allow the genetic exchange to occur
• Metaphase I


  • Each pair of bivalents align at the equator
  • The position of each bivalent is random (Random assortment) and this contributes to genetic variation
• Anaphase I


  Homologous chromosomes separate
• Telophase I


  Nuclear envelope reforms around haploid nuclei containing half the number of chromosomes
• During meiosis II, another round of cell division occurs, leading to formation of four haploid daughter cells, containing single chromosomes
• Anaphase II


  Centromeres split separating chromatids