Meiosis

Meiosis (from Greek μείωσις meiosis 'reduction', 'diminution') is a special type of nuclear division of eukaryotic cells in which the number of chromosomes is halved in two steps - meiosis I and meiosis II - and genetically different cell nuclei are produced. This makes meiosis fundamentally different from the usual nuclear division, mitosis, which leaves the chromosome population unchanged and produces genetically identical cell nuclei. The term reduction division is used in different ways: in a broad sense synonymous with meiosis, in a narrow sense for the first of its two partial steps, i.e. synonymous with meiosis I.

Meiosis is one of the most important events in sexual reproduction. The halving of the chromosome number during meiosis compensates for the doubling that occurs when a paternal and a maternal nucleus fuse (karyogamy) during fertilization. Without this compensation, the number of chromosomes would double with each generation. The sequence of these two processes is called nuclear phase change, the presence of only a single set of chromosomes is called haploidy, and the state after fertilization is called diploidy. (However, there are also polyploid organisms with higher degrees of ploidy).

In multicellular animals and in humans, the two meiotic divisions are the last nuclear divisions during spermatogenesis or during and after oogenesis, i.e. during the formation of gametes (sperm and egg cells). In contrast, mitoses take place in plants between meiosis and the formation of gametes; thus, the haploid phase in them is not restricted to the gametes, but forms a separate haploid generation. In seed plants, however, this is very small and consists of only a few cells (pollen grain and embryo sac). In fungi, algae and unicellular eukaryotes, different sequences of meiosis and mitosis occur.

Before meiosis (as well as before mitosis), the chromosomes are internally duplicated, so that they then consist of two identical chromatids each. At the beginning of meiosis I, the homologous chromosomes of maternal and paternal origin are paired by juxtaposition. In this state, a mutual exchange of partial sections (crossing-over) occurs very frequently, resulting in newly assembled chromosomes with genetically different combinations. Afterwards, the chromosomes of a pair are separated and randomly assigned to one of the two daughter nuclei each. In this way, the degree of ploidy is reduced and the daughter nuclei are genetically different as a result of the random distribution. However, the chromosomes still consist of two chromatids, which are also mostly genetically different as a result of the crossing-over. Therefore, meiosis II follows as an obligatory second step, in which the sister chromatids are separated as in a normal mitosis. In this way, four genetically different haploid nuclei emerge from one diploid cell nucleus.

The recombination of the maternal and paternal parts of the genome that occurs in this way is, in addition to the reduction of the degree of ploidy, the second essential function of meiosis. It leads to the fact that descendants with a combination of characteristics can develop, which did not exist before.

Discovery and designations

After Édouard van Beneden had described in 1883 that the number of chromosomes is doubled during fertilization of the egg cell of the roundworm (Ascaris), Eduard Strasburger and August Weismann postulated that a reduction division must take place to compensate for this during the formation of the gametes. This was described for the first time in 1890 by Oscar Hertwig completely and in a way that is still valid today, also in the roundworm. At that time, chromosomes were known as structures that occur during nuclear division, but nothing was known about their function. It was not until 1900, when the hitherto unnoticed rules of heredity, which had been elucidated by the Augustinian monk Gregor Mendel and described as early as 1866, were rediscovered and confirmed by several scientists, that Walter Sutton noticed in 1902 that the behaviour of the chromosomes corresponded to Mendel's rules and therefore suspected a connection. Then, in 1904, Theodor Boveri postulated that chromosomes were the material carriers of hereditary traits (chromosome theory of heredity).

The term meiosis was coined by Farmer and Moore in 1905.

The two stages of meiosis have been called different names by different authors:

• First section: 1st meiotic division, 1st mature division, meiosis I or reduction division
• Second section: 2nd meiotic division, 2nd mature division, meiosis II or equation division.

The term "reduction division" is also used for meiosis as a whole.

Time in life cycle

The change between a haploid and a diploid phase in the course of sexual reproduction is called nuclear phase change. This can occur in several variants. In humans, as in all multicellular animals, the diploid phase is predominant; only the gametes are haploid. Such organisms are called diplonts. The reverse case is represented by many fungi, many algae, and some protozoa (flagellates), which are normally haploid and whose diploid phase is confined to the zygote (haplonts). Third, there are diplohaplounts, in which haploid and diploid generations alternate, as in all plants and most algae. In organisms with higher degrees of ploidy, halving also occurs during meiosis, for example from tetraploid (four sets of chromosomes) to diploid.

In asexual reproduction, there is no nuclear phase change and thus no meiosis. It occurs in numerous forms in plants, algae, fungi and lower animals. To be distinguished from this is unisexual reproduction, in which female individuals produce offspring without fertilization. In animals, this is known as parthenogenesis or virgin reproduction. In this process, meiosis may be omitted altogether or reversed by subsequent karyogamy. Parthenogenesis is widespread in the animal kingdom (with the exception of mammals). It usually occurs in alternation with sexual reproduction; but the latter may be omitted altogether. One group of animals in which this has apparently been the case for millions of years is the Bdelloida, which belong to the rotifers. Many flowering plants can form seeds without fertilization (agamospermia). This can occur both unisexually, in which meiosis is omitted (as in various composite plants such as the dandelion), and asexually, in which the embryo emerges from vegetative tissue (as in the citrus plants).