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Meiosis

Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These are our sex cells; sperm in males, and eggs in females.

Stages of Meiosis (9)

Meiosis I (Stages 1 – 5)

1. Interphase

  • The DNA of the cell is copied resulting in two identical full sets of chromosomes
  • Outside of the nucleus are two centrosomes, each containing a pair of centrioles. These structures are critical for the process of cell division
  • During interphase, microtubules extend from these centrosomes

2. Prophase I

  • The copied chromosomes condense into x-shaped structures that can be easily seen under a microscope
  • Each chromosome is composed of two sister chromatids containing identical genetic information
  • The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on
  • The pairs of chromosomes may then exchange bits of DNA in a process called recombination, or crossing over
  • At the end of prophase 1, the membrane around the nucleus in the cell dissolves away, releasing the chromosomes
  • The meiotic spindle, containing microtubules, and other proteins, extends across the cell between the centrioles

3. Metaphase I

  • The chromosome pairs line up next to each other along the centre (equator) of the cell
  • The centrioles are now at the opposite poles of the cell, with the meiotic spindles extending from them
  • The meiotic spindle fibers attach to one chromosome of each pair

4. Anaphase I

  • The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell, and the other chromosome to the opposite pole
  • In meiosis I, the sister chromatids stay together. This is different to what happens in mitosis, and meiosis II

5. Telophase I and Cytokinesis

  • The chromosomes complete their move to the opposite poles of the cell
  • At each pole of the cell, a full set of chromosomes gather
  • A membrane forms around each set of chromosomes to form two new nuclei
  • The single cell then pinches in the centre to form two new daughter cells, each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis


Meiosis II (Stages 6-9)

6. Prophase II

  • Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids)
  • In each of the two daughter cells, the chromosomes condense again into visible x-shaped structures that are microscopically visible
  • The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes
  • The centrioles duplicate
  • The meiotic spindle forms again

7. Metaphase II

  • In each of the two daughter cells, the chromosomes (pairs of sister chromatids) line up end to end along the equator of the cell
  • The centrioles are now at opposite poles in each of the daughter cell
  • Meiotic spindle fibers at each pole of the cell attach to each of the sister chromatids

8. Anaphase II

  • The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle
  • The separated chromatids are now individual chromosomes

9. Telophase II and Cytokinesis

  • The chromosomes complete their move to opposite poles of the cell
  • At each pole of the cell, a full set of chromosomes gather
  • A membrane forms around each set of chromosomes to form two new cell nuclei
  • This is the last phase of meiosis, however cell division is not complete without another round of cytokinesis
  • Once cytokinesis is complete, there are 4 granddaughter cells, each with half a set of chromosomes (haploid)
  • In males, these are the sperm cells
  • In females, one of the cells is an egg cell, while the other three are polar bodies (small cells that do not develop into eggs)

meiosis

Image: By Ali Zifan – Own work; Used information from Campbell Biology (10th Edition) by: Jane B. Reece & Steven A. Wasserman., CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49630204



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