Our gametes are haploid cells produced from diploid cells by meiosis, a special type of cell division.
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General life cycle of animals
- father´s diploid cell
- haploid sperm cell
- mother´s diploid cell - These cells contain two copies of each chromosome, one comes from the mother, the other from the father. Our somatic cells are diploid, while gametes are haploid.
- haploid egg - Gametes are haploid, they contain one set of chromosomes. These are transferred to the offspring, that is, they form the maternal set of chromosomes. Gametes are produced from diploid cells by meiosis, the second stage of the meiotic process, also known as reductional division.
- fertilisation - The fusing of two haploid gametes forms one diploid zygote.
- diploid zygote - The somatic cells of the offspring are produced from this by a series of mitoses.
- offspring - Its body consists of diploid cells which are formed from the zygote by a series of mitoses. Certain somatic cells produce haploid gametes by meiosis.
- meiozis - Reductional cell division. That is, diploid cells containing 2n chromosomes divide into cells containing n chromosomes. The number ´n´ is a characteristic property of species, for humans n = 23. Our diploid somatic cells contain 2n = 46 pieces (23 pairs) of chromosomes, while haploid gametes contain n = 23.
The process of meiosis
- centrosome - It controls the movement of chromosomes during cell division. It consists of two orthogonally arranged cylindrical protein complexes, short microtubules.
- chromatin - It consists of DNA and proteins, chromosomes are formed (condensed) from it.
- spindle apparatus - It consists of microtubules which link the centrosome and the centromeres of the chromosomes.
- spindle apparatus
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 fertilisation, 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 coloured 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.
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