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)

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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