Meiosis: Stages and Genetic Consequences of Meiosis

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As a result of sexual reproduction a zygote is formed by the union of sexual cells or gametes. It is characterized by having double number of chromosomes (diploid) than the gametes.

In this process there is biparental inheritance. This double number is reduced to half (haploid) by another process called haplosis.

The complete process by means of which double number of chromosomes (as a result of diplosis) is reduced to its half number (by haplosis) is called meiosis or reduction division (Gr., meioum, to diminish). This term was coined by J.B. Farmer (1905) with J.E. Moore.

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Meiosis only occurs in gonads during formation of sperm and ova. The type of meiosis found in humans is characteristic of all animals and a few lower plants, and it is called terminal or gametic because the meiotic division occurs just before the formation of gametes. In most plants meiosis occurs at some time between fertilization and formation of gametes, and called intermediary or sporic. The cells in which meiosis occurs are called meiocytes.

In animals, these meiocytes are the primary spermatocytes in testes and primary oocytes in ovaries. In plants, these constitute sporocytes, each of which produces four spores, like animals. The nature of chemical substances active in initiation or induction of meiosis is not completely known. One states that adjacent somatic tissue may be responsible for the production of meiosis initiating substances.

Such somatic tissue includes nonsporogenous tissue surrounding megaspore mother cells, which divide by meiosis to produce megaspores in plants and non-germinal tissue in association with seminiferous tubles, in which sperm are produced in animals. In some cases, e.g., parasitic flagellate protozoans, meiosis may be initiated by an insect moulting hormone, ecdysone. Another possibility for starting meiosis is the relative amount of RNA and DNA.

It is stated that, if the ratio of RNA to DNA is high, cells undergo mitosis, and if this ratio is low, meiosis will take place. The later relationship has been found in potential meiotic cells of several plants. It is believed that meiosis, as well as mitosis, is under genetic control. How it is controlled is to be discovered.

Stages:

Meiosis occurs in two stages:

1. Heterotypic division:

First meiotic division is heterotypic in which diploid parent cell divides into two daughter cells, having Monoploid chromosome number.

2. Homeotypic division:

This is the second meiotic division and is equational (mitotic) in character. The two haploid cells formed as a result of heterotypic division, divide mitotically into two cells each. Thus, from a single parent cell containing 2X chromosomes (diploid) are formed four daughter cells having X number of chromosomes (haploid) each.

Successive meiotic stages are as follows:

A. Heterotypic Division:

Like mitosis, it consists of following four stages:

(1) Prophase I

(2) Anaphase I

(3) Metaphase I

(4) Telophase I

[I] First prophase:

Prior to actual prophase stage, i.e., during interphase, chromosomes are characterized by the persistent relic coils (remnant of previous division) as in premitotic interphase. As prophase starts, nucleus increases in size and this increase is much more than mitotic increase. Prophase I of meiosis is of extremely long duration as compared with mitotic prophase.

It may last 100 to 200 times longer. It is due to the reduced frequency in the number of initiation points during DNA duplication. In prezygonema about 0.3 to 0. 4 percent of the DNA remains unreplicated as observed in microsporocytes of Lillium. During Zygonema this unreplicated DNA is synthesized.

In Zygonema DNA binding protein appears which is associated with a lipoprotein fraction and which catalyzes the reassociation of single-stranded DNA. This protein has been called reassociation or r-protein. It has been suggested that it may play a role in the alignment of homologous stretches of DNA.

During pachynema, protein synthesis seems essential to maintain chromosome pairing. If protein synthesis is inhibited at late Zygonema the chromosomes pairs fall apart and are not reconstituted. DNA synthesis also occurs in a smaller magnitude than that of Zygonema.

During prophase I, homologous chromosomes pair closely and interchange hereditary material. It is subdivided into five sub-stages —

In preleptonema stage chromosomes are extremely thin and difficult to observe. Only the sex chromosomes may stand out as compact heteropycnotic bodies.

1. Leptotene or leptonema:

(Gr., leptos, thin + nemа, thread). It is the first stage of meiotic prophase, characterized by the following main features —

(1) Volume of nucleus increased and thus it assumes a larger shape.

(2) Leptoneme chromosomes look single rather than double though DNA duplication has already occurred and that they have two chromatids. Chromosomes become distinct, being quite long and uncoiled. They have a more granular appearance due to beads of unequal size (chromomeres) than the mitotic prophase chromosomes.

(3) During late Leptotene, chromosomes develop a number of small coils which vary in their degree of condensation. These meiotic coils are called major coils which grow in diameter as prophase proceeds.

The nucleolus is apparent during Leptotene, but in some cells it is relatively small at first, increasing in size during Leptotene and zygotene. Increased synthesis of RNA occurs in early prophase I, due to which nucleolus size increases.

The time of duplication of chromosomes’ (splitting into chromatids) is variable in different types of cells. In some cells this duplication occurs in Leptotene, while in others it occurs in later sub-stage of prophase. Usually this duplication of chromosomes becomes completed by the end of next sub-stage.

2. Zygotene or Zygonema:

Pairing of homologous chromosomes and synaptonemal complex formation. (Gr., Zygon, adjoining). It is the most important stage of meiotic prophase and shows the following characteristics —

(1) Two similar or homologous chromosomes begin to pair. The pairing is called synapsis and pairs so formed (consisting of one chromosome coming from paternal origin and other from maternal) are called bivalents.

Pairing is highly specific and involves the formation of synaptonemal complex. It is a linear complex of three parallel strands. Each of the two outermost elements represents an axial component of a homologue and the central element the synaptic centre. Central element is formed as a result of pairing.

It may even be absent. Axial elements sometimes twist about the central one and chromosomal fibrils may link them to one another. Both DNA and protein, including histone, are principal components of axial elements and are necessary for integrity of the complex. Each chromosome already has two chromatids; hence each bivalent consists of four chromatids called tetrad.

(2) Pairing is highly specific and involves the formation of a special structure that is called the synaptonemal complex. Pairing may occur in the following three ways — (i) Two homologous chromosomes may start to pair at the ends of chromosomes and progress towards the Centromere or kinetochore region. This type of pairing is called proterminal, i.e., beginning from terminal ends; (ii) Another type of pairing may start near the Centromere and proceed towards the ends of chromosomes. This type of pairing is termed procentric synapsis, (iii) In the third type, chromosomes undergo pairing in a scattered pattern, i.e., small pieces of chromosomes pair with similar other pieces. This is called a random synapsis. Pairing is remarkably exact and specific. It takes place point for point, and chromomere for chromomere, in each homologue. Two homologues do not fuse during pairing, but remain separated by a space about 0.15 to 0.2 µm, which is occupied by synaptonemal complex (SC).

(3) In many plants and animals, chromosomes are oriented with their one end directed towards the same side of nucleus forming bouquet. In organisms with definitely oriented or polarized chromosomes, pairing usually begins at the ends near the nuclear membrane and continues along the chromosomes, until they have completely paired. This particular state of the orientation and polarization of chromosomes is called bouquet stage.

(4) After pairing, coiling continues during zygotene, as a result of which chromosomes become shortened and thicker as the major coils increase in diameter. In addition to major coils, there is a relational coiling of chromatids, and perhaps also of two homologues.

3. Pachytene or pachynema:

(Gr., pachus, thick). It is the phase of crossing—over and recombination between homologous chromatids. After completion of synapsis nucleus is said to be in pachytene stage. Chromosomes contract longitudinally, resulting in shorter and thicker threads. By middle pachytene, nucleus contains half the number of chromosomes.

Each unit is a bivalent or tetrad which is composed of two homologous chromosomes in close longitudinal union and which contains four chromatids. Each Chromatid has its own Centromere. Thus, in a tetrad there are four centromeres, two homologous and two sister. The chromatids of each homologue are called sister chromatids.

During pachynema two of the chromatids of the homologues exchange segments, i.e., recombine.

Pachynema is a long lasting stage of prophase and may last for days, weeks or even years.

Crossing Over:

Occurs now at this stage, although exact time of this crossing over is variable. By this process bivalents exchange their segments by chromosomal breaks and union. The Chromatid which breaks at one point will unite with the Chromatid of other chromosome, which will also break at exactly the same place. The break-and-fusion usually occurs more than once in a pair of homologous chromosomes and various combinations consequently may arise.

The breaks and exchanges of partners during crossing — over produce cross-shaped figures, called chiasma (singular). Number of Chiasmata (plural) in a bivalent depends on the length of the chromosomes. They are less in short chromosomes and many in elongated chromosomes, but for average length, the number of Chiasmata per tetrad is one to three.

Ultra-structure of synapsis:

During zygotene and pachytene sub stages of meiosis, there occurs a highly organized structure of filaments between the paired chromosomes. It is called synaptonemal complex (SC). Moses in 1956 described that the complex is composed of two lateral components or arms and a central or medial element, which are interposed in between the two pairing homologues. Each lateral component is shared by the two sister chromatids of a homologue.

The morphology of SC is very similar in plant and animal meiocytes. In cross section SC appears as a flattened, ribbon-like structure. Lateral arms vary in width from 20 to 80 nm in various species. They are formed of electron dense coarse granules of fibres. These arms are joined to the adjacent chromosomes by fine fibrils.

In most plants and animals with the exception of insects, the two lateral arms are separated by an axial space of lower density. In insects the central complex may be very complex, i.e., it is ladder — like having three dense parallel lines and bridges crossing at intervals of 20 to 30 nm. These bridges are formed of fine fibrils that span the central and lateral components and are arranged perpendicularly to them.

Synaptonemal complex can be considered the structural basis for pairing and synapsis of meiotic chromosomes. At the end of leptonema the lateral arms of SC have already appeared in the space between two chromatids, while the central arm appears, with the pairing, at Zygonema.

When pairing starts, DNA is 300 times longer than the length of chromosomes. In other words, only 0.3% of DNA of homologous chromosomes is matched along the length of lateral component of SC.

As mentioned that lateral arms of SC appear in leptonema, before pairing, and that completion of SC occurs during Zygonema and becomes more conspicuous at pachynema. During diplonema, synaptonemal complex disintegrates and usually disappears.

SC stabilizes the pairing of and facilitates recombination. Because of deposition of new protein molecules in lateral arms of SC, the matching segments of DNA are placed in a way that allows interchange at the molecular level. SC can be interpreted as a protein framework that permits only proper alignment of homologues but also their recombination.

These complexes are considered as prerequisite to chiasma formation (Moses, 1955). Moreover, it may facilitate crossing over by effective synapsis due to pairing in fixed state and by providing a structural framework within which molecular recombination may occur as well as by segregating recombination DNA from most of chromosomal DNA.

King (1970) suggested that synaptonemal complex may orient nonsister chromatids of homologous chromosomes in a particular manner which facilitates enzymatically induced exchanges between their DNA molecules.

Comings and Okada (1971) reported that synapsis occurs usually at two levels, viz., at chromosomal level and second at molecular level. Synaptonemal complex pulls homologous chromosomes into close association with each other but does not play any role in molecular pairing of DNA strands.

4. Diplotene or diplonema:

It is the phase of separation of the paired chromosomes except at the Chiasmata. In this period the intimately paired chromosomes repel each other and begin to separate. However, this separation is not complete, since the homologous chromosomes remain united by their points of interchange or Chiasmata (Gr., chiasma, and crosspiece).

During diplonema, the four chromatids of the tetrad become visible. At the end of this sub stage, Chiasmata move toward the ends of chromosomes in the process of terminalization. Because of increase in coiling and resultant shortening of chromosomes, Chiasmata appear to slide off the ends of chromosomes.

5. Diakinesis:

(Gr., dia, across). It is the phase of reduction in the number of Chiasmata. In this sub stage contraction of chromosomes becomes accentuated. Number of Chiasmata diminishes. By the end of this stage homologous chromosomes are held together only at their ends. Tetrads are evenly distributed in nucleus and nucleolus disappears.

Nuclear envelope disrupts, releasing chromosomes into cytoplasm of the cell. By this time spindle fibres have organized and poles of the cell are established.

[II] First metaphase:

At the close of diakinesis, amphiaster (achromatic figure) becomes developed and bivalents move to the equatorial region of cell and finally arrange themselves in the middle of spindle. Their arrangement is peculiar, in that centromeres lie towards the poles, while the arms of chromatids directed towards the equator.

The Centromere regions of homologues repel each other to create this arrangement. Centromere of each chromosome behaves as an individual unit. In meiosis, chromosomes are not lined up along the equator.

Although the Centromere of each chromosome remains functionally single in metaphase, it is structurally double. Spindle fibres extend between the poles and are attached only to Centromere regions of chromosomes, termed chromosomal fibres.

[III] First anaphase:

During this stage, bivalents move towards the opposite poles of cell. Centromeres move first, carrying with them the arms of chromosomes and effecting complete terminalization of Chiasmata. Tetrads containing four chromatids become separated into dyads having two chromatids each.

Two chromatids of a dyad (each chromosome) diverge from each other. Anaphase chromatids are much shorter and thicker than the anaphase chromatids in a somatic mitosis. The result of this stage is a reduction of chromosome number by one half, i.e., from diploid to haploid.

[IV] First Telophase:

As the chromosomal dyads reach their respective poles, they frequently become very long due to loosening of their coils. Nucleolus reappears and nuclear membrane becomes formed around each polar group of chromosomes.

Cytokinesis may or may not occur. Sometimes, as in Trillium and certain members of Odonata, anaphase I directly passes into prophase II, omitting Telophase I and interphase.

[V] Interkinesis (Interphase):

After Telophase, two haploid daughter nuclei or cells sometimes undergo typical resting stage as in mitosis. This intervening stage between first Telophase and beginning of second prophase is termed interphase or interkinesis. This stage is of a very short duration or may be entirely absent. Most important difference between interphase at this time and other interphases is the absence of DNA synthesis.

B. Homeotypic Division:

[I] Second prophase:

This stage is like the mitotic prophase. Arms of chromatids remain separate and there is no relational coiling. Chromosomes are still coiled and each dyad consists of X-shaped figures of chromatids connected by Centromere. During the last period of second prophase nucleolus disappears, nuclear membrane disperses and achromatic figure is developed.

[II] Second metaphase:

It is of a very short duration like mitotic metaphase. Chromosomes become lined up with the centromeres on the equator. Centromeres lie along the equator while their arms are separated out. Centromere becomes functionally double. Chromosomes begin to move to opposite poles. Each Chromatid now has its own Centromere.

[III] Second anaphase:

This stage begins when chromatids along with their individual centromeres move towards opposite poles. Chromatids of second anaphase are not short, thick bodies like those of first anaphase (meiotic division), but are much more like the anaphase chromatids of mitosis.

[IV] Second Telophase:

Polar groups of daughter chromosomes (chromatids of preceding stages) uncoil, nuclear membrane surrounds each group and nucleolus reappears. Cytokinesis occurs and two cells are formed from each haploid daughter cell, as a result of homeotypic division.

Summary:

Meiotic division involves mainly two stages. In the first stage (heterotypic), chromosome number is reduced to its half resulting in the formation of two haploid daughter cells nuclei. In this heterotypic division, each chromosome pairs with its homologue and divides longitudinally into two chromatids, so that each pair contains four chromatids (tetrads).

The four chromatids of each tetrad are distributed in four nuclei formed at the close of the meiosis. Each Chromatid becomes separated or unjoined from its homologue (reduction) in one of the division (heterotypic division) and divided equationally in the other (homeotypic division).

Thus, from a single bivalent a quarter of chromatids are formed, becoming four daughter cells or nuclei.

For example, if an animal has 12 somatic chromosomes at Leptotene, there would be 12 chromosomes and 12 centromeres. After synapsis and splitting there will be 24 chromatids and only 12 effective centromeres (since centromeres do not divide but functionally they divide remaining in intimate contact to each other).

At anaphase first, 6 chromosomes with 6 centromeres would pass to one pole. In second division daughter centromeres separate, so that each chromosome now becomes two separate units. As a result of this separation, 6 chromosomes will go to each pole at the second anaphase.

Genetic consequences of meiosis:

Segregation of the homologues and recombination in meiosis have important genetic consequences. From the genetic view point, meiosis is a mechanism for distributing the genes between the gametes, allowing their recombination and random segregation.

Each one of the four gametes produced has a different genetic constitution. As a result of the crossing over, each chromosome does not consist solely of maternal or paternal material but of alternating segments of each, due to Chiasmata formation and crossing — over.

The random assortment of the genes is due not only to crossing-over but also due to the random distribution of the chromosomes in the first and second divisions. In a human being having 23 pairs of chromosomes, the possible chromosomal combinations in gametes will be a very large number, 223 or 8,388,608.

Thus, meiosis is the mechanism by which genetic variation is brought about and knowledge of its mechanism is a prerequisite for the understanding of the chromosomal basis of genetics.