Meiosis
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Meiosis is the splitting of a cell to produce gametes, which are the reproductive cells in the body.
Meiosis is needed for sexual reproduction, because meiosis causes genetic variation by producing cells that are not all genetically identical, and with half the chromosome number. In testicles, meiosis produces sperm, which are the male reproductive cells. In ovaries, meiosis produces ova, which are the female reproductive cells.
Meiosis can be divided into Meiosis I and Meiosis II.
[change] Meiosis I
Just like Mitosis, meiosis includes; Prophase, Metaphase, Anaphase and Telophase. Interphase I is not included as it is the cell in its normal state before its meiotic stages.
Prophase I: The chromosomes become visible, the nuclear envelope disappears and the centrioles (located at the top and bottom of the nucleus) begin forming spindle fibres that envelope the chromosomes. The chromosomes cross over with their homologous (opposite) pairs and certain sections of each chromosome gets transferred over. This ensures that each gamete cell has the right coding in each nucleus after meiosis has finished.
Metaphase I: The chromosomes line up along the equatorial (the central line) of the spindle fibres in homologous pairs.
Anaphase I: The chromosomes are divided so that there are equal amounts on either side of the cell. As there are 46 chromosomes in a human, 23 end up on either side.
(Cytokinesis, the division of cells into two, takes place. The cell divides.)
Telophase I: The two daughter cells are completely divided, a nucleic envelope forms and the chromosomes become less visible. There are 23 chromosomes in each of these cells.
[change] Meiosis II
The two cells prepare to divide again in a stage known as Interkinesis or Interphase II. Both of these cells with go through meiosis II.
Prophase II: The chromosomes become visible, the nuclear envelope disappears and the centrioles form the spindle fibres.
Metaphase II: The chromosomes line up along the middle line on the spindle fibres
Anaphase II: The chromosomes get split into its two chromatids. Chromatids are the two strands of DNA (deoxyribo-nucleic acid) that make up the chromosome. They are joined by a mid-way connection called a centromere.
(Cytokinesis takes place. The cell divides.)
Telophase II: The cells are completely divided. The nucleic envelope reforms and four new cells with the same DNA are created.
In males, all four cells become sperm. In females, only one becomes a mature ova, while the remaining three become re-absorbed into the body.
