Definition
Meiosis is a special form of nuclear division and is also called maturity division. It contains two divisions, which turns a diploid mother cell into four haploid daughter cells. These daughter cells each contain a 1-chromatide chromosome and are not identical. These daughter cells are needed for sexual reproduction.
Introduction
In men, the germ cells are the spermatozoa formed in the testicles. The equivalent in a woman are her eggs, which she has from birth. One haploid germ cell from each parent fuses to form a double set of chromosomes, which can be found in all other cells of the body. If one of the two divisions is defective during meiosis, numerical chromosomal aberrations may occur, such as trisomy 21 (known as Down syndrome).
What is the function of meiosis?
The function of meiosis is the production of germ cells in both female and male organisms. These are needed for sexual reproduction and are found accordingly in mammals and humans. After meiosis, one cell with a double (diploid) set of chromosomes turns into four cells with a single (haploid) set of chromosomes.
This reduction of the chromosome set is very important, because otherwise two germ cells with a double chromosome set would fuse together during fertilization. The result would be a living being with a quadruple (tetraploid) chromosome set. This chromosomal aberration accounts for about 5% of all miscarriages.
Besides the reduction of the chromosome set and the production of germ cells, meiosis has another function. Meiosis ensures genetic diversity by randomly distributing the chromatids among the four daughter cells. In addition to the random distribution of the genetic material, there is also the exchange of genetic information between maternal and paternal chromosomes. This process is called crossing-over and further increases genetic recombination and diversity. This topic could also be of interest to you: Tasks of the cell nucleus
What is the process of meiosis?
The course of meiosis is always the same and can be roughly divided into two parts. These in turn consist of several phases, which are, however, identical in both divisions. First division of meiosis The meiosis starts with the doubling of the two chromatids, so that the cell has a double set of chromosomes with four chromatids.
This is followed by the first division of meiosis, in which the two pairs of chromosomes are separated from each other. The resulting two cells each have one chromosome with two chromatids. This division is called reduction division, because the double set of chromosomes is halved.
It proceeds in several phases, which have the same names as in mitosis: In addition, in this part of meiosis the genetic material is recombined within the chromosomes. It is an exchange of certain DNA segments between both chromosomes, which is called crossing-over. Second division of meiosis The second part of meiosis consists of the so-called equation division.
Here, the two sister chromatids are separated from each other. A total of four germ cells are formed, which contain only one chromatid as the genetic genome. As in the first meiotic division, the four phases (prophase, metaphase, anaphase, telophase) can also be found here.
The separation of the sister chromatids in the second part of meiosis can be compared to mitosis, since there too the chromatids are separated and drawn to opposite cell poles.
- Prophase
- Metaphase
- Anaphase
- Telophase.
Meiosis is important for germ cell development and can be divided into different stages. First of all, one must distinguish between meiosis I and meiosis II.
This classification is useful because two cell divisions occur during meiosis. The first division is called reduction division, because the two homologous chromosomes are separated from each other. Thus, a double set of chromosomes is transformed into a single set of chromosomes.
This first meiosis can be divided into four phases: The original cell has two chromosomes, which are doubled by replication. The result is a cell with four chromatids. In the prophase, the chromosomes are condensed and approach each other.
This spatial proximity of both chromosomes is important for the following crossing over.Here, both chromosomes exchange genetic material, which increases genetic diversity. Next comes the metaphase, in which the two homologous chromosomes are arranged in the equatorial plane. At the same time, the spindle apparatus is formed.
In the anaphase, the pairs of chromosomes are separated from each other and drawn to the opposite cell poles. In the last phase, the telophase, the cell membrane constricts itself so that two daughter cells are formed. These have a simple set of chromosomes, but consist of two chromatids.
Next comes the second division of meiosis. This is called equation division and affects both haploid daughter cells. During this division, the sister chromatids are separated from each other, resulting in a total of four cells with one chromatid.
Meiosis II is very similar to mitosis and can also be divided into the same phases: In the prophase, the sister chromatids condense and the spindle apparatus begins to form. In the metaphase, the chromatids arrange themselves in the equatorial plane so that both chromatids have approximately the same distance from the cell pole. In the anaphase, the sister chromatids are separated from each other and migrate towards the cell pole.
In the telophase the cell membrane constricts again and new nuclear shells are formed. Thus, a total of four daughter cells are formed, which contain a simple set of chromosomes in the form of a chromatid as genetic material. These germ cells, gametes, or gametes, are different in both sexes.
In women, the eggs are present from birth, but are in a kind of dormant mode until puberty. With the beginning of sexual maturity, one egg matures every month, which can then be fertilised. In men, the production of sperm in the testicles does not begin until the beginning of puberty. In contrast to women, men are still able to produce germ cells until old age.
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
