During the life cycle of animals and humans, haploid gametes are formed from diploid somatic cells. The diploid parent cell contains two copies of each chromosome, one comes from the mother, the other from the father. The resulting haploid daughter cells only contain one set of chromosomes, the number of chromosomes they contain is half that of the parent cell. During fertilization, the genetic material of the mother and the father is mixed; haploid gametes fuse to form a diploid zygote. The somatic cells of the offspring are produced from the zygote by a series of mitoses. Meiosis plays a key role in sexual reproduction.
Meiosis involves two main stages, which both consist of a prophase, a metaphase, an anaphase and a telophase. During the prophase of the first stage, chromosomes are formed from the chromatin which consists of DNA and proteins. As meiosis is preceded by a duplication of DNA, chromosomes contain two chromatids with identical DNA content. The cell is diploid, the maternal and paternal sets of chromosomes are colored red and blue in the animation. Then the centrosome duplicates and moves towards the opposite poles of the cell.
In the metaphase, the nuclear membrane breaks down, and the spindle apparatus is formed. Then the chromosomes line up along the equatorial plane of the cell and a genetic crossover occurs: genetic material is exchanged randomly between homologous chromosomes. This greatly affects the variety of the resulting cells.
In the anaphase, chromosomes move towards the opposite poles of the cell, then in the telophase the cell is divided and the nuclear membrane is formed again.
That is, by the end of the first stage of meiosis the number of chromosomes is halved, two haploid daughter cells are formed from the diploid parent cell.
The second stage of the meiotic process is an equational division. In the prophase, the centrosome duplicates and moves towards the opposite poles of the cell.
In the metaphase, the nuclear membrane breaks down and a spindle apparatus is formed. Then the chromosomes line up along the equatorial plane of the cell.
In the anaphase chromosomes split into chromatids, forming chromosomes with one chromatid, which then move towards the opposite poles of the cell.
Then in the telophase the cell splits and the nuclear membrane is formed again. That is, during the two stages of meiosis, four haploid cells are formed from one diploid cell.