What is meiosis, when does it happen, what is it for and differences with mitosis. Simple explanation and summary for all schools.
Meiosis is a fundamental process in biology that leads to the formation of sex cells, or gametes , and is essential for sexual reproduction in organisms. Unlike mitosis , which produces genetically identical cells, meiosis allows genetic variation to be generated through two successive cell divisions, reducing the number of chromosomes in the parent cell by half. This mechanism is crucial for ensuring genetic diversity in populations and for the correct development of the organism. In this article, we will explore the stages of meiosis and their role in the formation of gametes.
Meiosis is a process of cell division in which a single parent cell produces four daughter cells , all different from each other and with half the genetic makeup of the parent cell .
Let’s discover together the two types of meiosis , the differences with mitosis and how the process works.
How Meiosis 1 Works
Functions of meiosis:
- reproduction ⇒ meiosis plays a central role in gametogenesis: in fact, since in sexual reproduction a fusion between the male and female gametes occurs, it is necessary that the gametes (with a number n of chromosomes ⇒ haploid) have half the chromosomal set of a somatic cell (with 2n chromosomes ⇒ diploid), so that by fusing they form a zygote with the complete genetic heritage (n+n=2n), half of which is of paternal origin and the other half is maternal.
In which types of cells does meiosis occur? Meiosis only affects cells intended for reproduction . Meiosis is made up of 2 successive cell divisions, without DNA duplication occurring between the two : the first is called reductive, because it reduces the number of chromosomes in the cell from 2n (diploid) to n (haploid), and the second equational, because it leaves the number of chromosomes unchanged.
- diploid cell ⇒ n pairs of homologous chromosomes, that is, which carry the same characteristics
- haploid cell ⇒ n chromosomes
How Meiosis 2 Works
Here are the stages of meiosis 2:
- Prophase : much longer and more complex than that of mitosis. The chromatin condenses to form metaphase chromosomes, then the homologous chromosomes pair up along their entire length to form structures called tetrads. They then begin to move apart, remaining joined at points called chiasmata. At these chiasmata, crossing-over occurs ⇒ exchange of genetic material (of a chemical nature) between homologous chromosomes. Crossing-over greatly increases genetic variability, randomly recombining genetic material.
Also in this phase, the nuclear membrane dissolves, the centrioles migrate to the poles and begin to form the spindle. - Metaphase : the tetrads align longitudinally in the equatorial plate ⇒ the centromeres attach to the spindle and the entire chromosomes migrate to the poles of the cell.
- Anaphase : the migration of chromosomes is completed.
- Telophase : the division of the 2 cells and the reformation of the nuclear membrane occurs.
At the end of meiosis I, 2 haploid cells have been formed.
Meiosis II is essentially the same as mitosis , but with half the number of chromosomes. Prophase is also much faster because the chromatin is already spiralized.
At the end of meiosis II, 4 daughter cells are formed , all different from each other and haploid, that is, with half the genetic heritage of the mother cell.
Therefore, in meiosis, starting from a diploid cell, 4 haploid cells are formed, all different from each other.
Meiosis and mitosis: the differences
Mitosis occurs in the somatic cells of the organism, while meiosis is limited to cells intended for reproduction. In mitosis, the two daughter cells have the same number of chromosomes, while in meiosis ,
haploid cells are produced.
In mitosis, chromosomes behave independently of each other.
In meiosis,
a relationship of mutual mechanical dependence occurs in homologous chromosomes: crossing over.
In mitosis , daughter cells are identical in terms of their genetic makeup, while
meiosis produces genetically different cells.
This variability is due to the random migration of chromosomes and crossing-over.
Mitosis takes place in a relatively short time, lasting about one to two hours, while meiosis is a longer process .
In the human male, for example, the completion of the meiotic cycle can take 24 days, and in the female several years.