Meiosis is the cell division that occurs in the formation of gametes, reducing the number of chromosomes in a species by half.
Thus, a diploid parent cell gives rise to 4 haploid daughter cells.
The process occurs through two stages of successive cell divisions, giving rise to four cells:
- Meiosis I: Reduction step, as the number of chromosomes is reduced by half.
- Meiosis II: Equational step, the number of chromosomes in cells that divide remains the same in cells that form.
Meiosis occurs when the cell enters the reproduction phase, being the essential process for the formation of gametes, spores and in the divisions of the zygote.
stages of meiosis
Meiosis I
At interphase, the chromosomes are thin and long. DNA and chromosomes duplicate to form chromatids.
After duplication, cell division begins .
Prophase I
Prophase I is a very complex phase, being divided into five consecutive sub-phases:
- Leptotene: each chromosome is made up of two chromatids. Note the presence of small condensations, the chromomers.
- Zygotene: the pairing of homologous chromosomes begins, called synapse, which is completed in pachytene.
- Pachytene: each pair of homologous chromosomes has four chromatids, constituting a bivalent or tetrad, formed by sister chromatids: those originating from the same chromosome and homologous chromatids: those originating from homologous chromosomes. These can be broken at the same time, and the two pieces can switch places, performing a permutation or crossing over . As chromosomes carry genes , gene recombination occurs .
- Diplotene: the homologous chromosomes begin to move apart, but remain linked by the regions where the permutation occurred. Such regions constitute the chiasms .
- Diakinesis: Condensation and separation of homologous chromosomes continues to occur. As a result, the chiasmata slide towards the tips of the chromatids, a process called chiasmal termination . As the phases evolve, the nucleolus and nuclear envelope disappear.
Metaphase I
In metaphase I, the cell membrane disappears. Pairs of homologous chromosomes arrange themselves in the equatorial plane of the cell.
Homologous chromosome centromeres attach to fibers emerging from opposite centrioles . Thus each component of the pair will be pulled in opposite directions.
Anaphase I
In anaphase I, the centromeres do not divide. Each component of the homologous pair migrates towards one of the poles of the cell.
Telophase I
In telophase, the chromosomes uncoil, the nuclear envelope and nucleolus reorganize, and cytokinesis, division of the cytoplasm, occurs. Thus, two new haploid cells appear.
Meiosis II
Meiosis II is extremely similar to mitosis. The formation of haploid cells from other haploids is only possible because it occurs during meiosis II, the separation of the chromatids that form the dyads.
Each chromatid of a dyad goes to a different pole and can now be called a sister chromosome. The stages of meiosis II are as follows:
Prophase II
Chromosomes condense and centrioles duplicate. The nucleolus and nucleus disappear again.
Metaphase II
The centrioles are ready to be duplicated and the chromosomes arrange themselves in the equatorial region.
Anaphase II
The sister chromatids separate and migrate to each pole of the cell, pulled by the spindle fibers.
Telophase II
The spindle fibers disappear and the chromosomes are already at the poles of the cell. The nuclear envelope reappears and the nucleolus reorganizes. Finally, cytokinesis occurs and the appearance of 4 haploid daughter cells.
What are the differences between mitosis and meiosis?
Mitosis and meiosis correspond to the two types of cell division . However, some characteristics differentiate the two processes:
- Mitosis gives rise to two daughter cells identical to the parent cell. Meanwhile, in meiosis, 4 daughter cells are generated with genetic material different from that of the parent cell. Furthermore, the daughter cells still have half the number of chromosomes as the parent cell.
- Meiosis halves the number of chromosomes in the daughter cells. In mitosis, the number of chromosomes is maintained between the parent cell and the daughter cells.
- Mitosis occurs in most somatic cells in the body. Meiosis occurs only in germ cells and spores.
