Mitosis and meiosis

• Mitosis

  Cell division where the daughter cells have the same number of chromosomes as the parent cell
• Phases in Mitosis:

  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Prophase

  • Chromosomes condense and become visible
  • Nucleoli and Nuclear membrane disappear
  • Centrioles move to the poles of the cell and form spindle fibres
  • Spindle fibres attach to the centromeres of chromosomes
• Metaphase

  • Chromosomes move towards the middle of the cell
  • Chromosomes line up on the equator of the cell
• Anaphase

  • Tension in the chromosome causes centromere to split
  • One chromatid moves to each pole of the cell as the spindle fibre contracts
• Telophase

  • Single stranded chromatids reach the poles of the cell
• Interphase

  • Longest stage of the cell cycle
  • Split into 3 stages: G1, S, G2
• G1 / Gap 1

  Organelles are replicated and the cell prepares for DNA replication
• S / Synthesis

  • DNA replication occurs
  • The amount of DNA doubles
  • After DNA is replicated it now consists of two identical strands joined at a centromere
  • Each strand is a Chron
• G2 / Gap 2

  • Organelles needed for division are replicated
  • Energy reserves are built up as ATP is needed in division
• Meiosis

  • Occurs in the making of gametes
  • It is a halving division e.g. Humans have the diploid number of 46, after meiosis the cell would have been halved to a haploid number of 23
  • 4 daughter cells are produced
  • Daughter cells are not genetically identical
• Role of Meiosis

  • Consists of 2 stages of division
  • Meiosis 1 - Parent cell divides having 2 daughter cells
  • Meiosis 2 - Both cells divide again
• stages of Meiosis 1

  • Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I
• Stages of meiosis II

  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II
• Prophase I

  • Homologous chromosomes pair up to form bivalents
  • Everything in normal prophase
• Metaphase I

  In a homologous pair, one chromosome is from the mother and the other is from the father. They lie at the equator randomly, with either one facing either pole
• Independent assortment

  • During meiosis different combinations of maternal and paternal go into each cell
  • This produces new genetic combinations with genes from both parents going into both daughter cells
• Anaphase I

  • Spindle fibres shorten, pulling the pairs apart, one pair is pulled to one pole and the other to the opposite pole
  • The homologous chromosome of each bivalent separate
  • They are pulled to opposite poles
• Telophase I

  • Chromosomes reach poles and may de-condense, Now with half the number of chromosomes
• Meiosis II

  • Second stage of division
  • No pairing of homologous chromosomes
  • Chromatids separate at anaphase
• Prophase II

  • Same as Prophase I
• Metaphase II

  • Chromosomes move to the middle of the spindle, with each chromosome attached to the spindle fibre by their centromere
  • Independent assortment happens as the chromatids of the chromosomes can face either pole
  • They line up on the equator
• Anaphase II

  • The chromosomes divide and the spindle fibres shorten
  • Chromatids are now pulled to opposite poles
• Telophase II

  • Chromatids reach the poles
  • The spindle disintegrates
  • The nucleoli and nuclear envelop reform
  • The chromatids become long and thin
• Significance of mitosis

  • Keeps the chromosome number constant from one generation to the next
  • Generates genetic variation in the gametes
  • genetic variation happens in the crossing over during prophase I and in independent assortment at metaphase I and metaphase II as the daughter cells contain different combinations of maternal and parental chromosomes
• Homologous pair

  Chromosomes that are have the same size and shape, have their centromeres in the same place and have the same genes