As meiosis (from Greek μείωσις meiosis 'reduction', 'reduction') or meiosis is a special type of nuclear division of eukaryotic cells referred to in which, in two steps - meiosis I and meiosis II the number of - chromosomes is halved and genetically distinct nuclei arise. In this way, meiosis differs fundamentally from the normal nucleus division, mitosis , which leaves the chromosome stock unchanged and produces genetically identical cell nuclei. The term reduction division is used differently: in a broad sense synonymous with meiosis , in the narrow sense for the first of its two sub-steps, i.e. synonymous with meiosis I.
Meiosis is one of the most important events in sexual reproduction . The halving of the number of chromosomes in meiosis compensates for the doubling that occurs when a paternal and maternal cell nucleus merges ( karyogamy ) during fertilization . Without this compensation, the number of chromosomes would double with each generation. The sequence of these two processes is known as the nuclear phase change , the presence of only a simple set of chromosomes as haploidy and the condition after fertilization as diploidy . (However, there are also polyploid organisms with higher degrees of ploidy .)
In multicellular animals and 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, in find plants between meiosis and the formation of gametes mitosis instead; The haploid phase is not limited to the gametes, but forms a separate haploid generation . However, this is very small in seed plants 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 (just like before mitosis), the chromosomes are internally doubled so that they each consist of two identical chromatids . At the beginning of meiosis I, the homologous chromosomes of maternal and paternal origin are paired together. In this state, there is very often a mutual exchange of sections ( crossing-over ), which results in newly assembled chromosomes with genetically different combinations. Then the chromosomes of a pair are separated and randomly assigned to one of the two daughter nuclei. 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 usually genetically different as a result of the crossing-over. Therefore, meiosis II now follows as an obligatory second step, in which the sister chromatids are separated as in an ordinary mitosis. In total, four genetically different haploid nuclei result from a diploid cell nucleus.
The recomposition ( recombination ) of the maternal and paternal part of the genetic material that takes place in this way is the second essential function of meiosis, in addition to reducing the degree of ploidy. It means that offspring can arise with a combination of traits that did not exist before.
Discovery and designations
After Édouard van Beneden had described in 1883 that the number of chromosomes doubled during fertilization of the egg cell of the roundworm ( Ascaris ), Eduard Strasburger and August Weismann postulated that a reduction division had to take place to compensate for the formation of the gametes . This was first fully described by Oscar Hertwig in 1890 and also in the roundworm in a way that is still valid today. At that time, the chromosomes were known as structures that occur during nuclear division, but nothing was known about their function. Only after the previously neglected rules of inheritance , which the Augustinian monk Gregor Mendel had clarified and described as early as 1866, were rediscovered and confirmed by several scientists in 1900 , Walter Sutton noticed in 1902 that the behavior of the chromosomes corresponded to Mendel's rules and therefore suspected one Context. In 1904 Theodor Boveri then postulated that the chromosomes are the material carriers of the genetic make-up ( chromosome theory of inheritance ).
Farmer and Moore coined the term meiosis in 1905.
The two stages of meiosis have been called differently by different authors:
- Chapter One: the first meiotic division , the first meiotic division , meiosis I and meiosis
- Section Two: second meiotic division , the second meiotic division , meiosis II or equational division .
The term “reduction division” is also used for meiosis as a whole.
Point in time in the life cycle
The change between a haploid and a diploid phase in the course of sexual reproduction is known as the nuclear phase change . This can occur in several variants. In humans, as in all multicellular animals , the diploid phase is in the foreground; only the gametes are haploid. Such organisms are called diplonts . The opposite case is represented by many fungi , many algae and some unicellular organisms ( flagellates ), which are normally haploid and whose diploid phase is restricted to the zygote ( haplonts ). Thirdly, there are diplohaplons in which haploid and diploid generations alternate, as is the case with all plants and most algae. In organisms with higher degrees of ploidy there is also a halving during meiosis, for example from tetraploid (four sets of chromosomes) to diploid.
In asexual reproduction, there is no change in the core phase and therefore no meiosis either. It occurs in numerous forms in plants, algae, fungi and invertebrates. This is to be distinguished from unisexual reproduction, in which female individuals produce offspring without fertilization. In animals, this is known as parthenogenesis or virgin generation. Meiosis can be omitted entirely or can be reversed by a subsequent karyogamy . Parthenogenesis is widespread in the animal kingdom (with the exception of mammals). Mostly it takes place alternately with sexual reproduction; but the latter can also be omitted entirely. One group of animals in which this has apparently been the case for millions of years are the Bdelloida , which are classed as rotifers . Many flowering plants can produce seeds without fertilization ( agamospermia ). This can take place both unisexually, in that meiosis does not occur (as is the case with various composites such as the dandelion ), and asexually, in that the embryo emerges from vegetative tissue (such as in the citrus family ).
Course of normal meiosis
Chiasmatic meiosis is present in the vast majority of organisms, including humans. It is described in this section. (For the other variants, see below.)
As in normal nuclear division, mitosis, meiosis is also preceded by DNA replication . The diploid ( 2 n ) cell nucleus thus has two chromatid chromosomes and thus four copies of the DNA double strand ( 4 c ) in four chromatids of one type of chromosome (for example, chromosome 1) . Meiosis creates four cell nuclei with a haploid ( 1 n ), not doubled ( 1 c ) set of chromosomes from this one nucleus , whereby all four nuclei differ genetically.
Both the first and the second meiotic division, like mitosis, are divided into prophase, metaphase, anaphase and telophase. The processes that take place in the pro, meta and anaphase of meiosis I differ significantly from those in mitosis as well as from meiosis II. The prophase I, i.e. the prophase of the first meiotic division, is due the many successive processes are still divided into sub-phases.
The result of spermatogenesis is four equally sized gametes . During oogenesis (in humans and animals) daughter cells of different sizes arise, of which only one with a large cell volume becomes an egg cell ; the small ones become polar bodies .
Meiosis I (reduction division)
In the prophase of meiosis I, more precisely in pachytan, recombination takes place between homologous chromosomes. In contrast to mitosis and meiosis II, in the anaphase of meiosis I the sister chromatids are not separated, but remain bound to one another via their centromere . Instead, the homologous chromosomes are split up.
The first meiotic division begins with the prophase I . This is divided into five stages:
- In Leptotene (of gr. Leptos , thin 'and lat. Taenia , band') begin to condense the chromosomes. By the end of prophase I, the ends of the chromosomes, the telomeres , are attached to the inner cell nuclear membrane . Each chromosome consists of two identical chromatids.
- The zygotene (Gr. Zygón , yoke ') is characterized by the pairing of homologous chromosomes, i.e. the juxtaposition of the copies of a chromosome type obtained from the two parents. This exact pairing of chromosomes, also called synapsis , runs like a zipper from the ends, in that the synaptonemal complex is formed between the two chromosome strands, which holds both strands together.
- In Pachytän (Gr. Pachýs , thick ') there is further condensation and the paired chromosomes each form a bivalent tetrad consisting of four chromatids . In this phase the crossing-over occurs , which initiates the exchange of homologous chromatid segments between non-sister chromatids of paired chromosomes. The synaptonemal complex then disintegrates again.
- In the diplotene (Greek diplóos , double), the pairs of two chromosomes are clearly shown with double chromatids separated from each other. Now, on the tetrads, what are known as chiasmata are those places where two of the four chromatids are cross-connected, if a crossover has taken place.
- With the diakinesis (gr. Diakinein , set in motion '), the prophase I ends by the bivalents shortened, the nucleolus dissolves, the shell of the nucleus is decomposed and the spindle apparatus is formed.
The pairing of sex chromosomes
As sex chromosomes or Gonosomen such chromosomes are referred to, which differ in the two sexes. For example, in mammals there are two X chromosomes in the female sex, but one X and one Y chromosome in the male sex . In prophase I, the two X chromosomes in the female sex can therefore pair as well as all other chromosomes, the autosomes . This is not possible in the male sex, as the X and Y chromosomes differ significantly in sequence and length.
However, there is a pseudoautosomal region at each end of the X and Y chromosomes , in which the sequence on the X and Y chromosomes (as with two homologous autosomes) is the same. A pairing and also a crossing-over is possible in these sections. In the male sex too, the two sex chromosomes are recognized as a chromosome pair during the subsequent meta- and anaphase.
Metaphase I, Anaphase I and Telophase I
In metaphase I , the paired chromosomes gather in the equatorial plane of the spindle apparatus. In this phase, too, the chiasmata can be seen in the light microscope. In the subsequent anaphase I, in contrast to the mitotic anaphase, not individual chromatids, but rather pairs of chromatids are moved to the two spindle poles. Due to the previous crossover, the two related chromatids are no longer identical.
In telophase I there is only one chromosome (with two chromatids) of each type at each pole. So there has been a reduction in the number of chromosomes. As with mitotic telophase, the chromosomes now decondense and the nuclear envelope is formed again. The completed nucleus division is followed by cell division. The subsequent period until the beginning of the second mitotic division is known as interkinesis .
In a human euploid set of chromosomes, the nucleus of the diploid cell contains 23 pairs of duplicated chromosomes, i.e. 46 chromosomes or 92 chromatids, before meiotic division. After meiosis I, each of the two daughter nuclei received 23 chromosomes, each consisting of a pair of chromatids, i.e. 46 chromatids. In terms of quantity, this corresponds to a haploid chromosome set ( 1 n ) that has been doubled ( C value 2 c ). However, if the former sister chromatids sometimes contain different DNA sequences as a result of a crossing-over, the set of chromosomes can, strictly speaking, no longer be called "haploid". Although the number of chromosomes resembles a haploid one, some genes are now present with more than one allele each.
Meiosis II (equation division)
The interkinesis is followed by meiosis II. In terms of its sequence, it corresponds to mitosis with the only difference that, due to the crossing-over, the chromatids are not identical in the chromosomes concerned.
After condensation of the chromosomes in prophase II , the chromosomes still consisting of two chromatids are arranged in the equatorial plane in metaphase II , separated at the centromere and individually assigned to the daughter nuclei in anaphase II .
The cell nuclei resulting from telophase II each contain a haploid, unduplicated set of chromosomes ( 1 n ; 1 c ). Thus the four chromatids of each tetrad of prophase I have been randomly distributed to four different haploid cell nuclei.
Variants of the process
In addition to chiasmatic meiosis, which is present in the vast majority of organisms and takes place as described above, there are two other variants:
- achiasmatic meiosis, in which there is no genetic exchange between homologous chromosomes and therefore no chiasmata arise,
- the reverse meiosis, in which first an equation division and then the reduction division takes place.
Achiasmatic meioses have been described sporadically in many taxonomic groups of invertebrates , especially in insects , as well as in a chess flower ( Fritillaria japonica , a lily plant ). As a rule it occurs in butterflies and caddis flies . It is also very common in enchytrae , a family of annelids , and in two-winged birds . Only one sex can be affected, so in the case of butterflies the female and in the case of two-winged birds the male, or both as in some enchytrae. In evolution, achiasmatic meiosis has probably emerged independently from chiasmatic meiosis many times.
Reverse meioses are also known mainly in insects, especially in scale insects , aphids and bed bugs , but also in other animals and plants. The organisms in question have holocentric chromosomes which are not connected to the dividing spindle via a point-localized kinetochore, but rather over their entire length. This is accompanied by a behavior of the chromosomes that differs significantly from the normal.
Non-random segregation and transmission
Normally, homologous chromosomes are randomly assigned to one or the other daughter nucleus during meiotic divisions (random segregation), and it is therefore random which homologous chromosomes, i.e. also which homologous genes, in which combinations are passed on to the offspring (random transmission) . However, many cases are known in very different organisms in which these processes do not occur by chance. Most of them are grouped under the keyword Meiotic Drive . Hybridogenesis in the pond frog and some fish species is a special case .
Since only one of the four daughter nuclei survives in female meiosis, a non-random segregation also results in a non-random transmission. The behavior of the chromosomes during meiosis therefore has an impact on which genes are passed on to potential offspring. Examples of this have been discovered and described in many living things, especially in plants and insects, but also in mammals (including humans) and birds. It is difficult to estimate how frequent such non-random occurrences are, because they can only be found by carefully examining individual cases. Due to the wide taxonomic distribution of the known examples, it can be assumed that the prerequisites for this are generally given.
In male meiosis, all four daughter nuclei are generally passed on. Well-known counterexamples are the gall gnats and the sciarid gnats , whose spermatogenesis results in only two sperm or one sperm and these sperm only contain chromosomes of maternal origin, while the paternal chromosomes are completely eliminated (see gull gnats # Genetics and Sciarid gnats # Genetics ) . A non-random transmission of homologous chromosomes is very common in the male sex, which is not based on non-random segregation during meiosis, but only comes into effect after meiosis, in that those gametes that do not contain the chromosome in question develop are disturbed (see Meiotic Drive , keyword "genetic drive").
A standstill in female meiosis
Prophase I, during which homologous chromosomes are paired and crossed over, takes an unusually long time compared to mitotic prophase. In most animals and also in humans, it is also much more delayed, especially in the female sex, because it is stopped at a certain stage - sometimes several times.
In mammals (and thus also in humans), female meiosis usually begins at an early stage in the development of the ovaries (in humans, shortly after birth). However, it is then already stopped in prophase I, namely in diploma. (This resting stage is also known as dictyotene .) Only after sexual maturity is meiosis continued in the oocyte (egg mother cell) that then enters the fallopian tube as an egg cell during ovulation and can be fertilized there. In metaphase II, however, there is another standstill, and only fertilization by a sperm triggers the continuation and completion of the meiosis.
In amphibians , too, female meiosis is interrupted in the diploma. In the process, the chromosomes condense and take on a characteristic “ lamp brush ” shape, with numerous loops forming. These loops are characterized by intense gene activity ( transcription ) (which is otherwise not the case in prophase). In this phase, the oocytes accumulate large amounts of substances, which then enable the embryo to develop very quickly after fertilization: A series of rapidly occurring and rapidly successive nucleus and cell divisions can produce around 4,000 cells within just eight hours.
The fact that meiosis - as in mammals - persists in the metaphase and is only reactivated through fertilization is also the norm in the rest of the animal kingdom. In vertebrates, the resting stage is metaphase II, whereas in invertebrates it is metaphase I. Only in molluscs and echinoderms (to which the popular example of the sea urchin belongs) meiosis is completed before fertilization. In all other invertebrates, segregation of the homologous chromosomes (of maternal and paternal origin) does not take place until the sperm has penetrated, and even in vertebrates, homologous chromosome segments are in part not yet separated as a result of the crossing over .
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