Meiosis: Meaning, Division and Needs

In this article we will discuss about:- 1. Meaning of Meiosis 2. Divisions of Meiosis 3. Significance 4. Need 5. Types.

Meaning of Meiosis:

Meiosis (Gk. meioum or meio— to lessen) is a double division which occurs in a diploid cell (or nucleus) and gives rise to four haploid cells (or nuclei), each having half the number of chromosomes as compared to the parent cell. The term meiosis was coined by Farmer and Moore in 1905. The division was first of all studied by Van Benedin (1887), Strasburger (1888), Sutton (1900) and Winiwater (1900).

Interphase occurs prior to meiosis. It is similar to interphase of mitosis except that S- phase is prolonged. DNA replication occurs during S-phase. A distinct G₂ phase is either short or absent.

At this time each chromosome comes to have two chromatids. Chromosome replication occurs once but meiosis has two M-phases each with its own Karyokinesis and Cytokinesis. As a result chromosome number is halved. The transition period between M- phase I (meiosis I) and M-phase II (meiosis II) is short and without DNA replication. It is called interkinesis.

Divisions of Meiosis:

Meiosis consists of two divisions, meiosis I and meiosis II. The first division of meiosis is called heterotypic or reduction division. During this division the number of chromosomes is reduced to half. Segregation of chromosomes is, however, random. The two chromatids of a chromosome become genetically different due to crossing over. These chromatids are separated in the second division of meiosis.

The second meiotic division is known as homotypic (= homoeotypic) or equational division, because the chromosome number remains the same as produced after the end of the first division. Like mitosis, meiosis also involves indirect nuclear division.

The essential features of meiosis are:

(i) Two successive divisions but no DNA replication prior to second division,

(ii) Cross­ing over and formation of chiasmata between homologous chromosomes,

(iii) Separation of sister chromatids which have undergone change due to crossing over.

I. Karyokinesis:

Meiosis I:

The heterotypic or reduction division is the first division of meiosis. Like mitosis, it is studied under four stages— prophase, metaphase, anaphase and telophase.

Prophase I:

It is more complicated and prolonged as compared to the similar stage of mitosis. For the sake of convenience, prophase I is divided into five sub-phases— leptotene, zygotene, pachytene, diplotene and diakinesis. Another sub-phase called preleptonema is sometimes recognised prior to leptonema.

In this phase chromosomes are not distinguishable because of their thinness but sex chromosomes (if present) are often seen as heterochromatic (heteropycnotic) bodies.

1. Lepotene or Leptonema (Gk. leptos- slender, tainia- band, nemа- thread):

Nucleus enlarges. Their ends, however, remain attached to nuclear envelope through a special struc­ture called attachment plate. The chromatin fibres of interphase nucleus shorten and elongated chromosomes become clear.

They possess a string of swollen areas called chromomeres. Chromomeres are often believed to represent genes. The chromosomes are replicated but the chromatids are not distinguishable due to the presence of nucleoprotein core between them.

In many animal cells the chromosomes show a peculiar arrangement called bouquet stage. Here the ends of chromosomes converge towards the side having replicated centrosomes or centriole pairs. One of the two centiole pairs begins to move to the opposite side. Both the centriole pairs or centrosomes develop astral rays from the pericentriolar satellites. Each centriole pair and its astral rays together constitute aster.

In cells undergoing meiosis, there are two sets of chromosomes, that is, the chromo­some number is diploid. There are two similar chromosomes of each type. Such chromo­somes are called homologous chromosomes. The two homologous chromosomes are con­tributed by different parents.

One of them belongs to the father parent and is called paternal chromosome. The other chromosome of homologous pair belongs to the mother parent and is called maternal chromosome. The homologous chromosomes resemble each other in the position of their centromeres, position of chromomeres, shape and size.

2. Zygotene or Zygonema (Gk. zygon- yoke or tied, tainia- band):

The two homolo­gous chromosomes get attached to each other laterally due to development of nucleoprotein between them. It is similar to nucleoprotein core present between two chromatids of a chromosome. Pairing is such that the genes of the same character present on the two chromosomes come to lie exactly opposite.

The process of attachment of the homologous chromosomes is known as synapsis or syndesis. Depending upon the place of origin of pairing, synapsis is procentric (starting from centromeres and proceeding to­wards ends), pro-terminal (starting from ends and proceeding towards centromeres) and interme­diate (at various places in between centromeres and ends).

It produces a complex known as synaptonemal complex. In a synaptonemal or synaptinemal complex, ribo- nucleoprotein core has a tripartite structure, one central and two lateral longitudinal elements which are connected by lateral elements.

Each lateral element occurs in between two chromatids of a chromosome. Central element lies between the two homologous chromosomes (Fig. 10.11). On account of synapsis, chromosomes form pairs or bivalents. The number of bivalents is half the number of the total chromosomes.

3. Pachytene or Pachynema (Gk. pachys- thick, tainia- band):

Soon after completion of synapsis, the cell enters pachytene stage. In this stage it can remain for days. Chromosomes are paired and occur in synaptonemal (= synaptine­mal) complexes. The paired chromosomes or bivalents shorten.

Each bivalent or chromosome pair is made up of actually four chromatids, two of each chromosome. The two chromatids be­longing to the same chromosome are called sister chromatids. Chromatids belonging to the two different chromosomes of a homologous pair are termed as non-sister chromatids.

Dense areas appear here and there over the bivalents (Fig. 10.12). They are called recombi­nation nodules. Nodules contain multi-enzyme complexes called recombinase. Recombinase is made of endonu­clease, exonuclease, unwindase, R-protein, etc.

In the presence of enzyme endonuclease breaks develop in the in­dividual chromatids. The process is called nick­ing. In most of the cases, the nicks get healed but in one out of 1000, gaps develop in the re­gion of nicks by the activity of another enzyme called exonuclease.

Separation of chromatid segments occurs in between two gaps by U-protein or enzyme unwindase. The separated segments of non-sister chromatid segments may exchange position if they happen to show the same degree of nicking. The phenomenon is called crossing over (Fig. 10.13).

Crossing over is a process of exchange of genetic material or chromatid segments between two homologous chromosomes. It is an enzyme mediated process. The separated chromatid segments soon get re-united with the help of an enzyme known as R-protein. The process is called re-annealing.

4. Diplotene or Diplonema (Gk. diplos- double, tainia- band):

The nucleoprotein fusion complex of the synapsed chromosomes dissolves partially. Therefore, the homolo­gous chromosomes separate except in the region of crossing over. The chromatids also become distinguishable (tetrad stage).

The points of attachment between the homologous chromosomes after the partial dissolution of nucleoprotein complex are called chiasmata. Chiasmata may be terminal or interstitial. Depending upon their position the homologous chromosomes appear cross-like, ring-like or chain like. However, they are not permanent structures. They shift side-ways or even disappear at places.

Diplotene is extended and metabolically active in animal cells, especially oocytes because bulk of gametic growth occurs during this phase. In some oocytes, diplotene stage can last for months and years. In such cases, chromosomes de-condense and get engaged in RNA synthesis. Lampbrush chromosomes are actually de-condensed diplotene chromosomes.

5. Diakinesis (Gk. dia- through, kinesis- movement):

Chiasmata shift towards the ends of the chromosomes. The phenomenon is called terminalisation. The bivalent of satellite chromosomes remains united with the nucleolus for some time. The nucleolus ultimately degenerates. Simultaneously, nuclear envelope disintegrates.

Metaphase I:

A colourless bipolar spindle apparatus appears in the region of degenerated nucleus. It consists of fine fibres. The fibres converge towards the two ends called poles. In animal cells the poles are formed by asters. The spindle fibres are formed of microtubules.

The spindle apparatus has the maximum diameter in the middle region which is called equator. The bivalents arrange themselves on the equator of the bipolar spindle. The limbs of the chromosomes are usually short and lie horizontally on the equa­tor. The centromeres slightly project towards the periphery.

Since, there are two centromeres in each bivalent the centromeres of all the bivalents produce a double metaphasic plate. The distribution of bivalents is at random so that the individual paternal and maternal chromo­somes can face either of two poles of the spindle.

Each chromosomes gets attached to the spindle pole of its side by means of a chro­mosome fibre or tractile fibril which arises in the region of the centromere. (This is in contrast to the development of two tractile fibrils from the same centromere in mitosis). The fibres of the homologous chromosomes are always in the opposite direc­tions.

Anaphase I:

The homologous chromosomes break their connections and separate out. The process of separation is named as disjunction (dis- separate, junction- union).

The separated chromosomes or univalents are also called dyads (Gk. dyas— two) because each of them consists of two chromatids which lie at an angle to each other. The double stranded chromosomes of anaphase I are in sharp contrast to their single-stranded nature in anaphase of mitosis.

The chromosomes move towards the spindle poles along the path of their tractile fibrils. At the end of anaphase I, two groups of chromosomes are produced, with each group having half the number of chromosomes present in the parent nucleus.

Telophase I:

The polar groups of chromosomes arrange themselves into haploid or dyad nuclei. The chromosomes elongate. A nucleolus is formed by the satellite chromosome. It is followed by the appearance of nucleoplasm and nuclear envelope.

The elongated chromo­somes usually remain straight and do not enter the interphase. In some cases telophase is completely omitted when the anaphase chromosomes directly enter the metaphase of homotypic division (e.g., Trillium). Similarly, cytokinesis may or may not follow division I of meiosis.

Significance of Meiosis I:

1. It separates the homologous chromosomes and reduces the chromosome number to one half. This is essential for sexual reproduction.

2. Crossing over occurs during this division. It introduces new combinations of genes or recombination’s. Recombination’s result in variations.

3. There is random distribution of paternal and maternal chromosomes into daughter cells. It is a sort of independent assortment and produces variations.

4. Due to disturbance in disjunction, chromosomal and genomatic mutations take place.

5. Meiosis I induces the cells to form spores or gametes.

Interkinesis or Intra meiotic Interphase:

It is metabolic stage between telophase of meiosis I and prophase of meiosis II. Chro­mosomes are elongated but chromatin reticulum is not formed. Protein and RNA synthesis may occur. Centrosomes or centriole pairs undergo replication in animal cells. However, there is no DNA synthesis. It is important for bringing true haploidy (haploidy of DNA) in daughter cells.

Meiosis II:

It is shorter than the typical mitotic division because of the shortening of prophase of this division. The division maintains the number of chromosomes produced at the end of reduction division. It is hence called homotypic or equational division.

Though it is similar to mitosis, meiosis II is not mitosis because:

(i) It always occurs in haploid cells,

(ii) It is not preceded by DNA replication,

(iii) The two chromatids of a chromosome are often dissimilar,

(iv) The daughter cells formed after meiosis II are neither similar to each other nor similar to the parent cell.

The main function of homotypic division or meiosis II is to separate the chromatids of univalent chromosomes which differ from each other in their linkage groups due to crossing over. Meiosis II is divisible into prophase, metaphase, anaphase and telophase.

Prophase II:

This stage of nuclear division is very brief. It takes place simultaneously in the two nuclei. In animal cells, the centriole pairs develop asters and move to the regions of future spindle poles. The dyad chromosomes shorten a little.

Nucleolus and nuclear envelope degenerate. The chromatids of individual chromosomes are usually divergent and are hence much looser than the ones in somatic division. In case where telophase I is omitted, the prophase II is completely absent.

Metaphase II:

Achromatic bipolar fibrous spindles are formed in the areas of dividing nuclei. They are amphiastral in case of animal cells and anastral in case of plant cells. The spindles are arranged in isobilateral or tetrahedral fashion.

The chromosomes reach the respective spindle and arrange themselves in such a fashion that their centromeres come to lie at the equator. Each chromosome gets connected with both the spindle poles by means of chromosome fibres or tractile fibrils that develop from both the surfaces of its cen­tromere. The centromeres give out chromosome fibres or tractile fibrils from both their surfaces towards the spindle poles.

Anaphase II:

The centromere of each chromosome divides into two so that there is one centromere for each chromatid. The two chromatids of a chromosome separate completely and are called daughter or new chromosomes. The daughter chromosomes move towards the spindle poles along the path of their fibres or tractile fibrils. At the end of anaphase II, four groups of chromosomes are produced, each group having haploid number.

Telophase II:

The four groups of chromosomes arrange themselves into haploid nuclei. For this, chromosomes elongate very much to form chromatin. A nucleolus is also produced. This is followed by the formation of nucleoplasm and a nuclear envelope. The spindle fibres usually degenerate during telophase II.

II. Cytokinesis:

Cytokinesis can be of two types, successive and simultaneous. In successive type, cytokinesis occurs after every nuclear division. It produces two cells after the heterotypic division. Both the cells undergo homotypic divisions and then give rise to four cells. The four cells formed by successive cytokinesis can be arranged either in a linear or isobilateral tertrad.

In the simultaneous type, cytokinesis takes place only at the end of both the divisions. The nuclei are generally arranged in the form of a tetrahedron. Cytoplasm cleaves or con­stricts in between the nuclei. Four furrows are formed.

They deepen and meet in the centre of the parent cell. In plants, wall material is deposited in the furrows. It gives rise to four haploid cells. These haploid cells are arranged tetrahedrally and are collectively called tetra­hedral tetrad.

Significance of Meiosis:

1. Formation of Gametes:

Meiosis forms gametes that are essential for sexual repro­duction.

2. Genetic Information:

It switches on the genetic information for the development of gametes or gametophytes and switches off the sporophytic information.

3. Maintenance of Chromosome Number:

Meiosis maintains the fixed number of chromosomes in sexually reproducing organisms by halving the same. It is essential since the chromosome number becomes double after fertilization.

4. Assortment of Chromosomes:

In meiosis paternal and maternal chromosomes assort independently. It causes reshuffling of chromosomes and the traits controlled by them. The variations help the breeders in improving the races of useful plants and animals.

5. Crossing over:

It introduces new combination of traits or variations.

6. Mutations:

Chromosomal and geomantic mutations can take place by irregularities of meiotic divisions. Some of these mutations are useful to the organism and are perpetuated by natural selection.

7. Evidence of Basic Relationship of Organisms:

Details of meiosis are essentially similar in the majority of organisms showing their basic similarity and relationship.

Need for Meiosis:

Meiosis is essential for all sexually reproducing organisms. It occurs in reproductive cells so that the gametes formed are haploid or have half the number of chromosomes of those cells, which are directly derived from zygote. Two types of gametes fuse during zygote formation.

As a result, zygote comes to have double the number of chromosomes contained in gametes. Meiosis by halving the number of chromosomes maintains a fixed number of chromosomes of a species. In the absence of meiosis, the number of chromosomes will double with every generation resulting in excessive enlargement of nucleus, genetic degen­eration and death of living beings.

Types of Meiosis:

The cells in which meiosis takes place are called meiocytes. In animals, meiocytes are of two types, spermatocytes and oocytes. In higher plants, meiocytes are differentiated into microsporocytes and macrosporocytes. Depending upon the stage when meiosis occurs, the latter is of three types— gametic, zygotic and sporic.

1. Gametic Meiosis:

Meiosis in most of animals takes place during the formulation of gametes (gametogenesis). It is termed as gametic meiosis. When two gametes fuse in fertilization, a diploid zygote is formed. Gametic meiosis results in diplontic life cycle.

2. Zygotic Meiosis:

In some lower plants meiosis takes place in the zygote and the resulting organisms are haploid. It is called zygotic meiosis. Organisms having zygotic meiosis have haplontic life cycle.

3. Sporic Meiosis:

In plants meiosis generally occurs at the time of sporogenesis (formation of spores or microspores and megaspores). It is called sporic meiosis or inter­mediate meiosis. Spores produce a new gametophytic phase in the life cycle. Gametes are formed by gametophytes.

Because of the presence of two distinct multicellular phases, diploid and haploid, life cycle of plants is diplohaplontic. Diploid cells have two genomes while haploid cells have a single genome. A genome is a complete complement or set of chromosomes where each kind is represented by a single chromosome.

Colchicine:

It is an alkaloid widely used in plant breeding for doubling the chromosome number. Colchicine is extracted from the corms of Autumn Crocus (Colchicum autumnale). The alkaloid does not allow the formation of spindle because it prevents assembly of microtu­bules.

It is, therefore, called “mitotic poison”. The chemical does not inhibit chromosome replication. As a result the colchicine treated meristematic cells show doubling of chromo­somes. This property of increasing the number of chromosome sets or genomes is called polyploidy.

Polyploidy provides:

(i) New varieties and species

(ii) Vigorous offspring.

Colchi­cine induced polyploidy has been used in raising several varieties of horticultural and agri­cultural plants, e.g., Potato.