Meiosis refers to a form of cell division in which, in addition to cell division, the diploid chromosome set is reduced to a haploid chromosome set so that the newly formed cells each have only one set of chromosomes. In the human organism, meiosis serves to generate haploid germ cells, which have a single set of chromosomes, from diploid primordial germ cells (eggs in the ovary and sperm in the testes).
What is meiosis?
Meiosis is the name given to a form of cell division in which, in addition to cell division, the diploid set of chromosomes is reduced to a haploid set of chromosomes. Meiosis, also called maturation division or reduction division, first occurs in girls and boys during sexual maturity, between the ages of 12 and 14. The diploid primordial germ cells already created in the embryonic stage give rise to haploid gametes, germ cells each with one set of chromosomes. During meiosis, four haploid daughter cells are formed from one diploid cell. This is because the reduction division of the cell with the division of the two sets of chromosomes to both daughter cells is immediately followed by the second meiotic division, which is comparable to a normal cell division (mitosis). The two daughter cells divide again, resulting in a total of four haploid daughter cells. In the male, four sperm cells of equal value but with differently recombined genes are formed from one primordial germ cell during meiosis. In the female, the balance after meiosis is a little different. In the first phase, which occurs just before ovulation, there is a large cell that receives almost all of its cytoplasm and a haploid set of chromosomes, and a small cell (polar corpuscle) that also has a set of chromosomes and very little cytoplasm. In the subsequent mitosis, which occurs only after ovulation, both cells divide again, taking their haploid chromosome set with them. However, the division of the large cell again proceeds asymmetrically. The large egg and another small polar corpuscle are formed, so that in the overall balance, meiosis has set one fertilizable egg on its way and three small, non-survivable, polar corpuscles.
Function and task
Because of sexual reproduction, meiosis or reduction division is very important for humans for two reasons. In both males and females, the primordial germ cells carry a diploid set of chromosomes, which would double to a quadruple set of chromosomes if the cells were united for the purpose of reproduction. Therefore, a reduction division must occur in the cells intended for reproduction in order to obtain a diploid zygote after the union of the two cells. The zygote can be considered the primordial mother of stem cells, from which the entire new individual arises through a variety of mitoses and special differentiations of the cells. Although sexual reproduction carries risks, mainly in spontaneous mutations during recombination of genes, and in the inheritance of genetic defects that have occurred, it offers two advantages in particular over asexual cloning, a simple duplication that could be followed by nature with much less risk. Because of sexual reproduction, a population is created with different individuals that can perform differently and successfully in a changing environment. From generation to generation, the predispositions that offer the best conditions for survival in each case can thus prevail. On the other hand, among the abundance of unfavorable gene mutations, isolated mutations also occur that enable new, previously unknown abilities that result in progress or better adaptation to changing environmental conditions for the population as a whole. Meiosis plays an essential role in these processes of adaptation and development. During the complex 1st phase of meiosis, which can be divided into several steps, crossing-over often occurs when chromosome pairs separate from each other. This means that the random combination of the mother and father chromosomes is supplemented by a random exchange of individual homologous chromatids within the chromosomes, multiplying the recombination possibilities.
Diseases and disorders
During the complex individual steps of meiosis, errors can occur, usually with serious consequences. Maldivision of chromosomes may occur or replication failure may occur.Genes can be assigned to the “wrong” chromosome (translocation) or homologous chromosomes cannot be separated (non-disjunction). Likewise, entire chromosomes may be missing or supernumerary in the gametes. In principle, all chromosomes can be affected, but certain chromosomal disorders are far more common and well known than others. Most chromosomal disorders cause premature miscarriages or do not result in pregnancy. The best-known chromosomal disorder is trisomy 21, also known as Down syndrome. Chromosome 21 or parts of it are present in triplicate. Externally, the disease is recognizable by slanted eyelid wrinkles and usually there are deformities in the nasopharynx as well as malformations of organs. The mental development of the children, who usually show an open and very warm nature, varies. School-leaving qualifications and an independent life can be achieved by many of those affected. Klinefelter syndrome is caused by an extra X chromosome in boys. The specific chromosomal aberration is often not recognized because those affected can lead an almost normal life. They often experience speech and reading difficulties and have delayed and weaker testicular development with decreased testosterone production. Because affected individuals are infertile, the chromosomal disorder is often first detected due to an unfulfilled desire to have children. Pätau syndrome, also known as trisomy 13, is due to a replication error on chromosome 13. Either an additional chromosome 13 is present (free trisomy 13) or a translocation trisomy 13 is present, in which only a certain section of the chromosome is supernumerary. Affected babies suffer from severe malformations affecting the brain, digestive tract, heart, kidneys and eyes. A cleft lip and palate is also characteristic.
