Medical Genetics I: Introduction




Meiosis is the phase of the cell cycle where replicated chromosomes are separated into two daughter nuclei(at the end of Meiosis I) and four daughter nuclei(at the end of Meiosis II). It occurs in germ cells (gametes) and gives rise to genetically different cells in which the number of chromosomes is halved, providing genetic variation.  The resulting daughter cells are haploid (n).

Stages of Meiosis (Credit: Rdbickel, CC BY-SA 4.0)


Meiosis is first divided into 2 successive cell divisions; Meiosis I and Meiosis II and both of them are further divided into 5 stages each.

  1. Prophase I and II
  2. Prometaphase I and II
  3. Metaphase I and II
  4. Anaphase I and II
  5. Telophase I and II

In meiosis I, the chromosomal number is halved from 46 to 23, while during meiosis II is identical to mitosis (discussed in the previous lesson). The cell goes directly from Meiosis I to Meiosis II. There is no interphase between the 2 divisions.

Prophase I

  • The chromosomes condense and become visible
  • Synapsis: Intimate pairing between homologous chromosomes, occurs, leading to the formation of a tetrad or bivalent. A tetrad/bivalent has 2 homologous chromosomes and 4 sister chromatids. Synapsis leads to the formation of the synaptonemal complex
  • Recombination nodules on the synaptonemal complex lead to crossover between the non-sister chromatids in the tetrad. The point of cross over is known as a chiasma. Crossing over is one of the two factors that increase genetic variability. 
    A tetrad has 2 homologous chromosomes A and B and each homologous chromosome has sister chromatids 1 and 2. Now we have 4 chromatids; A1, A2, B1 and B2. Crossing over will happen between non-sister chromatids such as A1 and B2 or A2 and B2 but never between A1 and A2 or B1 and B2.
  • Synaptonemal complex disintegrates.
  • Non-sister chromatids of homologous chromosomes that have crossed over are joined together at the chiasma.


  • Disruption of the nuclear membrane that surrounds the nucleus.
  • Kinetochores start to appear
  • spindle formation 


  • kinetochore microtubules align the paired homologous chromosomes(with 4 sister chromatids) at the center of the cell at the metaphase/equatorial plate


  • The chiasma disappears and the homologous chromosomes are separated
  • The moving to opposite sides of the cell is done by shortening of kinetochore microtubules and elongation of polar microtubules
  • Independent assortment: the random movement of the chromosomes( with 2 sister chromatids) to the opposite poles of the cells and the second reason for genetic variability will occur here. 


Daughter chromosomes (each containing 2 sister chromatids each) are at opposite sides of the cell and the nuclear envelope reforms around each set of 23 chromosomes.

Stages of Meiosis (Credit: Ali Zifan, CC BY-SA 4.0)

What comes after meiosis?

  • Cytokinesis: When the cytoplasm of the cell divides by formation of a cleavage furrow(actin ring) that deepens till it separates the daughter cells.
  • After cytokinesis, the cells proceed to the Meiosis II division which is followed by cytokinesis. After that, the cell can go back to G1 or G0 phase.
  • The chromosome number in Meiosis I is halved from 46 to 23 while the chromosome number in Meiosis II  stays the same. The offspring are genetically different, resulting in 4 daughter cells with 23 number of chromosomes each
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