Meiosis is a process in which the diploid (double) set of chromosomes in primordial germ cells becomes a haploid (single) set of chromosomes. Thus, meiosis is an important prerequisite for sexual reproduction in which male and female gametes fuse together.
as gametes refers to the sex cells of a living being. In humans, these are the sperm and egg cells.
function of meiosis
In meiosis, the germ cells are created, which fuse with each other during fertilization during sexual reproduction. The resulting fertilized egg cell is called zygote designated. It has a diploid set of chromosomes, which means it has 44 autosomes and 2 gonosomes.
If the two merging germ cells each had a diploid set of chromosomes, the zygote would have a fourfold set. To prevent this, meiosis must take place.
If a cell has more than two sets of chromosomes, it is called polyploidy. Animals with a polyploid set of chromosomes are usually not viable. In the case of plants, however, individuals with a multiple set of chromosomes are sometimes deliberately bred in order to increase yield.
Meiosis plays a central role in genetic diversity. It randomly distributes the maternal and paternal chromosomes to the developing germ cells.
As a result, the genetic material is recombined in different versions – germ cells are created that inherit different characteristics.
the genetic recombinationn in meiosis is an important factor in evolution. On the one hand, genetic diversity is increased, on the other hand, new characteristics can arise. This allows a species, for example, to better adapt to life in a particular environment.
process of meiosis
Meiosis takes place in the gonads in both male and female organisms. This affects the male and female primordial germ cells, which have a diploid chromosome set consisting of two chromatid chromosomes.
at Women meiosis occurs in several phases with longer breaks. The first phase occurs until the second year of life and pauses in the prophase of the 1st meiotic division, which you will learn more about in the following section. With the onset of ovulation, the development of the sex cell continues. The entire process is only completed after fertilization by a sperm.
The development process of the ovum is called oogenesis designated.
at men on the other hand, meiosis takes place constantly from sexual maturity. You produce sperm for the rest of your life and are therefore able to reproduce until the end of your life.
The process by which sperm are formed is called spermatogenesis.
Meiosis consists of two steps: reduction division (1st meiosis) and equivalency division (2nd division of meiosis). In the following figure you will find an overview of the individual phases of meiosis. Steps 1 to 4 are assigned to the reduction division, steps 5 to 8 to the equivalence division.
reduction division (1st division)
the reduction division describes the first phase of meiosis. Here the separation takes place homologous chromosomesi.e. the similar chromosome pairs that are in a diploid chromosome set.
The first meiotic division is in four phases subdivided:
Prophase I
The genetic material condenses into chromosomes. The resulting homologous two-chromatid chromosomes are stored parallel to each other. This is called chromosome pairing designated. Each pair represents four parallel strands – tetrads called – dar.
In genetics, the term denotes condensation the compression of DNA into a compact structure.
Then it comes to the so-called crossing over.
At the crossing over Pieces of chromatids, i.e. genetic information, are exchanged between the chromosomes of homologous pairs. This expands the possibilities of recombination of the genetic material.
This concept is visualized in the following figure:
Figure 2: Crossing over
Finally, the cell’s nuclear membrane dissolves and spindle apparatus arises. This plays a central role in the following phase of meiosis.
In some texts, the formation of the spindle apparatus and the dissolution of the cell nucleus are described as a further phase of meiosis: the prometaphase.
Metaphase I
The homologous pairs of chromosomes line up at the equatorial plane. One chromosome of each pair is connected to one of the spindle poles by a spindle fiber. Thus, maternal and paternal chromosomes are randomly distributed, which is the genetic recombination allows.
Anaphase I
The homologous pairs of chromosomes are separated. One chromosome per pair migrates to the respective spindle poles. Maternal and paternal chromosomes are thus randomly distributed to the resulting daughter cells. The genome of the cell was thus halved.
Telophase I
The cell divides into two daughter cells, each with 23 two-chromatid chromosomes. In addition, the genetic material in both cells uncoils.
Equation division (2nd division)
The second meiotic division takes place according to the principle of mitosis. Above pro, meta, analog and Telophase II germ cells are formed that contain monochromatid chromosomes.
prophase II
In prophase II, the uncoiled genome condenses back into 23 two-chromatid chromosomes. In addition, the nuclear membrane dissolves again. Finally, the spindle apparatus is formed.
Metaphase II
In metaphase II, the two-chromatid chromosomes arrange themselves in the equatorial plane. The spindle fibers attach to the centromeres of the chromosomes.
Anaphase II
In anaphase II, the chromosomes divide at the centromere into two one-chromatid chromosomes, which then migrate to opposite poles.
Herein lies the central difference between the two phases of meiosis: In anaphase I will be whole chromosomes drawn towards the spindle poles.
Telophase II
Finally, in telophase II, four haploid cells with one chromatid chromosomes arise. The cell nucleus is newly formed, the DNA uncoils.
That result of meiosis varies by gender:
- Mmale sex: In animal organisms, four haploid spermatozoa develop from a diploid primordial sperm cell. In higher plants, pollen cells develop instead.
- Female gender: Only one forms from a diploid urinary cell egg cell. It is rich in plasma and capable of fertilization. The other three cells are low in plasma and atrophy as so-called pole or direction bodies.
Difference between meiosis and mitosis
Both meiosis and mitosis are processes of cell division. Their goal is the duplication of genetic material and cells. Nevertheless, they differ in a few points. The table gives you an overview.
MeiosisMitosisGoals
- formation of gametes
- Genetic recombination➝ Genetic diversity
Proliferation of body cells for regeneration and growthResultFour genetically identical haploid cells with 23 chromosomesTwo genetically identical diploid cells with 46 chromosomesLocationGond (ovaries and testicles)Somatic cellsProcedureTwo-step procedure:One-step procedure:Pro-, meta-, ana- and telophase
Meiosis – The most important thing
- Meiosis is the prerequisite for fertilization, since the diploid chromosome set is reduced and haploid germ cells are formed as a result.
- During fertilization, the haploid germ cells fuse to form a zygote with a diploid set of chromosomes.
- During meiosis, maternal and paternal chromosomes are randomly distributed.
- The process enables a recombination of the genetic material, for example by crossing over.
- It consists of the reduction division (1st division) and the equating division (2nd division).
- These two steps in turn consist of pro-, meta-, ana- and prophase respectively.
- Equation division takes place according to the principle of mitosis.