How many divisions occur in meiosis
Meiosis can be divided into two phases, meiosis I and meiosis II.
Both phases can in turn be divided into different sub-phases. The division of the chromosomes is the same in the germ cell formation of men and women, but there are differences in the division of cell organelles and cytoplasm. While in spermatogenesis (formation of sperm) four identical and equally large germ cells are created, in oogenesis (formation of egg cells) the germ cells divide irregularly, so that in the end a large egg cell with all cell organelles and almost the entire cytoplasm is created. The other three resulting, very small cells are called polar bodies. They are dismantled again after a short time.
The chromosomes condense. The homologous chromosomes are attached to each other and the crossing over takes place. Here, the homologous chromosomes exchange gene segments.
The homologous chromosomes arrange themselves on the equatorial plane and the spindle apparatus is formed.
The spindle fibers separate the homologous chromosomes from one another - a set of chromosomes migrates to each side of the cell.
The cells divide and two cells are created, each with a haploid (and different!) Set of chromosomes.
A second cell division is initiated by the centromeres building up the spindle apparatus.
The chromosomes arrange themselves on the equatorial plane and the spindle apparatus is formed again. The spindle fibers now attach themselves to the centromeres of the chromatids.
The spindle fibers shorten, separate the chromatids from one another and each draw one of the chromatids to the poles.
During spermatogenesis (the formation of germ cells in males), the cytoplasm divides evenly and four sperm are formed.
In oogenesis, on the other hand (the formation of germ cells in female organisms), almost the entire cytoplasm is assigned to one of the four cells. Here an egg cell and three small polar bodies are created that have no function as germ cells. At the end of the telophase, new cell walls are formed.
In addition to the topic of meiosis, you will find a learning video here that explains everything to you again!
Basics of meiosis
All people are different from one another. Even though there are only 46 chromosomes that can carry different information! This is due to the fact that all germ cells of a person (i.e. egg cells and sperm) differ from one another and, in addition, during fertilization, i.e. the fusion of egg cell and sperm, the information from father and mother is mixed.
Human germ cells contain only one set of chromosomes (haploid, n). This means that the chromosomes do not exist as homologous pairs, but only simply. If germ cells were diploid, like all other cells in the body, a person's chromosome set would not be present twice but four times after fertilization. With each further fertilization, the set of chromosomes would double again. The chromosome set of germ cells has to be halved during meiosis. Therefore, germ cell formation differs from normal cell division in several ways.
In meiosis, it is also important to re-mix the existing characteristics, i.e. genetic information. This happens in two places: During the crossing-over, homologous chromosomes exchange their information with one another, so that chromosomes with new combinations of characteristics are created. During the germ cell formation, the homologous chromosomes are also distributed completely randomly to different cells. These genetic processes that lead to a new combination of traits are called recombination. Additional variation arises from the fact that maternal and paternal germ cells merge with each other during fertilization and thus new combinations of characteristics can arise. These only exist in sexual reproduction. On the other hand, there is asexual reproduction, parthenogenesis. Here reproduction takes place simply via mitotic cell division.
In prokaryotes there is no sexual reproduction and therefore no recombination in meiosis. Here recombination occurs through what is known as horizontal gene transfer.
There are three different types of gene exchange between bacteria:
- Transformation: Free DNA is taken from the environment by the bacterium and built into its own genetic material.
- Transduction: A gene packaged in a bacteriophage is injected into a bacterium.
- Conjugation: With the help of cytoplasmic bridges, two bacterial cells make direct contact with one another so that DNA plasmids can be exchanged.
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