Lecture 2 - Meiosis

• Important things to remember about meiosis:

  1. There is only 1 interphase, similar to mitosis
  2. The first division randomly separates homologous chromosomes. This does not happen in mitosis.
  3. The second division results in the splitting of sister chromatids, similar to mitosis.
  4. The end result is genetically unique daughter cells, unlike mitosis.
  5. Each genetically unique daughter cell is haploid, unlike mitosis
• Mitosis v.s Meiosis

  Mitosis:

  • occurs in the bodies stem cells (e.g - colon)
  • results in two diploid daughter cells
  • each daughter cell is genetically identical
  • can continue in perpetuity

  Meiosis:

  • occurs in the germ (sex) cells
  • results in four haploid daughter cells
  • each daughter contains half the number of chromosomes as the parent
  • Each daughter cell is genetically unique
  • Is a terminal event
• Prophase I (Meiosis I):

  1. Chromosomes first become visible as thin threads. DNA replication is already complete.
  2. Homologous chromosomes continue to condense and undergo synapsis (gene-for-gene pairing)
  3. When synapsis is complete, each pair of homologous chromosomes forms a bivalent. Each chromosome consists of two sister chromatids.
  4. The chromosomes continue to shorten and thicken and the chiasmata between non-sister chromatids become apparent.
  5. The nuclear envelope begins to break down.
• Prometaphase I: Spindles attach to kinetochores on chromosomes.
• Metaphase I: Homologous pairs line up in the center of cell, with bivalents oriented randomly with respect to each other.
• In mitosis spindle fibres attach to kinetochores that oppose each other.
• In meiosis the kinetochores orient side by side (mono-orientation).
• Mitosis (metaphase):

  1. All 46 chromosomes aligned in one plane
  2. Mitotic spindles attached to kinetochores orientated opposite each other on both sister chromatids
  3. No recombination

  Meiosis I (metaphase I):

  1. 23 pairs of homologous chromosomes aligned in one plane
  2. Mitotic spindle attached to mono-oriented kinetochores on the sister chromatids
  3. Recombination occurring between homologous chromosomes
• Anaphase I: Homologous chromosomes separate, but sister chromatids do not separate.
• Telophase I and cytokinesis: Daughter cells are ready to move into prophase II.
• End of Meiosis I: Amount of DNA has been replaced by 1/2 BUT so has the type of DNA been reduced by 1/2.
• Meiosis II: almost identical to mitosis, involves the separation of the daughter cells.
• Prophase II: The nuclear envelope breaks down and the chromosomes condense.
• Prometaphase II: Spindles attach to kinetochores on chromosomes.
• Metaphase II: Chromosomes align in the centre of the cell.
• Anaphase II: Sister chromatids separate.
• Telophase II and cytokinesis: The nuclear envelope reforms and the cytoplasm divides. It results in 4 genetically unique daughter cells.
• Similarities between Metaphase (II) in mitosis and meiosis II:

  1. Mitotic spindles attach to both sister chromatids
  2. Separation of sister chromatids
  3. Alignment of all chromosomes along equatorial plate
• Differences between Metaphase (II) in mitosis and meiosis II:

  1. Only 1 of the homologues in metaphase II
  2. only recombined chromosomes in meiosis
  3. Half the number of chromosome in metaphase II