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Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females.
Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II): 1. Interphase: 2. Prophase I: 3. Metaphase I:
Meiosis I
- The chromosome pairs line up next to each other along the centre (equator) of the cell.
- The centrioles are now at opposites poles of the cell with the meiotic spindles extending from them.
- The meiotic spindle fibres 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 together.
- A membrane forms around each set of chromosomes to create two new nuclei.
- The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.
Meiosis II
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 can be easily seen under a microscope.
- 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 (pair of sister chromatids) line up end-to-end along the equator of the cell.
- The centrioles are now at opposites poles in each of the daughter cells.
- Meiotic spindle fibres 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 the opposite poles of the cell.
- At each pole of the cell a full set of chromosomes gather together.
- A membrane forms around each set of chromosomes to create 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 four granddaughter cells, each with half a set of chromosomes (haploid):
- in males, these four cells are all 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).
Illustration showing the nine stages of meiosis.
Image credit: Genome Research Limited
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