Index >> Cell Division >> Meiosis

	Meiosis The meiosis is a specialized type of nuclear division which divides a diploid cell into four haploid daughter cell. It occurs in conjunction with the formation of reproductive cells (gametes or meiospores) in sexually reproducing species. It may be sporic (vascular plants and others) or gametic (man and many other animals), and may be performed by I. spore mother cells (sporocytes) 2. primary spermatocytes and oocytes, or even, 3. by the zygote itself in algae and fungi. Any diploid cell which is undergoing meiosis in called meiocyte. The meiosis includes two successive divisions, the meiosis I and meiosis ll. The meiosis I is a reductional division producing two haploid cells from a single diploid cell. The meiosis II is an equational division which spaerates the sister chromatids of the haploid cells, the products of meiosis I.
A. Meiosis I Before undergoing the meiosis I, each meiocyte remains in the interphase during which the genetic materials are duplicated due to active DNA replication. The meiosis I includes a long prophase, metaphase, anaphase and telophase. 1. Prophase I The prophase I differs from mitotic prophase in two aspects. Firstly, it is of longer duration than the mitotic prophase and secondly, most cytogenetical events such as synapsis, crossing over, etc., occur during prophase first. For the sake of convenience, the prophase first has been subdivided into five consecutive stages: leptonema, zygonema, pchynema, diplonema and diakinesis. The name of these substages of prophase first have been derived from the specific morphology or behaviour of the chromosomes within the nuclear membrane at each substage of prophase. Thus leptotene means thin thread, zygotene means yolked thread, pachytene means thick thread, diplotene means double thread and diakinesis is essentially the end of both diplotene and prophase I (see Garber 1972).
(i) Leptonema (Leptotene)-During the leptonema stage the chromosomes appear as long single threads, unassociated with one another. The centrioles move towards the opposite poles of the cell and a definite type of orientation and polarization of chromosomes towards the centrioles take place. (ii) Zygonema (Zygotene or Synaptenes)-During the zygonema stage the homalogous chromosomes pair with one another, gene by gene, over the entire length of the chromosomes. The pairing of the homologous chromosomes is called synapsis. Each pair of homologous chromosomes is known as bivalent.
(iii) Pachynema (Pachytene)-During the pachynema stage each paired chromosome (bivalent) becomes shorter and thicker than in earlier substages and splits into two sister chromatids except at the region of the centromere. (The doubling of the amounts of DNA occured actually much earlier in meiotic interphase). As a result of the longitudinal division of each homologous chromosome into two chromatids there are four groups of chromatids in the nucleus lying parallel to each other called tetrads. During the synapsis and tetrad formation, an exchange of chromosomal or genetic material between non-sister chromatids of each tetrad occurs in most meiocytes. Such exchange of genetic materials between non-sister chromatids is called crossing-over and it provides the raw material, the hereditary variations for the organic evolution. To perform the act of crossing over, each of the two non-sister chromatids of a tetrad breaks at an identical point.
	According to most recent cytological findings of Stern and Hotta (I 969) the breaks of chromatids are caused by endonuclease enzyme. A. Leptotene A. Centriole B. Nucleolus C. Nucleus D. Nuclear envelope E. Diakinesis A. Centriole B. Chromosome C. Centromere ↓ ↓ B. Zygotene 1. Nuclear Envelope 2. Homologous Chromosomes 3. Daughter Centrioles F. Metaphase 1. Chromosomal Fibers 2. Chiasmata 3. Continuous Fibers 4. Centrioles ↓ ↓ C. Pachytene A. Centromere B. Chromatids G. Late Anaphase ↓ D. Dipotene A. Chiasmata

After the breakage of non-sister chromatids; an interchange of broken chromatid segments takes place in between the two non-sister chromatids of the same tetrad. Each broken chromatid segment unites with the non-sister chromatid of its own tetrad by the help of an enzyme called ligase (Stern and Hotta, 1969). The point of interchange is visible as characteristic X-shaped configuration of an overlapping region called chiasma (plural, chiasmata). The longer the chromosome pair, the greater likelihood of more than one chiasma, although one chasima appears to  interfere with the formation of another in a closely adjacent regions of the chromosome on the same side of the centromere. The basis of this interferance is not clear. Chiasmata have been observed in all but a few plant and animal meiocytes.

(iv) Diplonema (Diplotene)-During diplonema stage, chiasmata appear to move towards the ends of the synapsed chromosomes in the process of terminalization and ultimately to slip off the ends.

(v) Diakinesis-During diakinesis the chromosomes begin to coil and so become shorter and thicker. Terminalization is completed. The nucleolus detaches from the nucleolar organizer and disappears completely. The nuclear envelope starts to degenerate and spindle formation is well under the way.

2. Metaphase I
During the first meiotic metaphase, the bivalents orient themselves at random on the equatorial plate. The centromere of each chromosome of a terminalized tetrad is directed towards the opposite poles. The chromosomal microtubular spindle fibres remain attached with the centromers and homologous chromosomes become ready to separate.

3. Anaphase I
In contrast to mitotic anaphase in which separation of sister chromatids occurs, the meiotic anaphase I is characterized by the separation of whole chromosomes of each homologous pair (tetrad), so that each pole of the dividing cell receives either a paternal or maternal longitudinally double chromosome of each tetrad. This ensures a change in chromosome number from diploid to monoploid or haploid in the resultant reorganized daughter nuclei.

4. Telophase I
The arrival of chromosomes at the poles of the spindle signals the end of anaphase I and the beginning of telophase I. During telophase I, the chromosomes may persist for a time in the condensed state, the nucleolus and nuclear membrane may be reconstituted and cytokinesis may also occur to produce two haploid cells. In Trillium meiocytes are reported to progress directly from anaphase I to prophase II.

B.  Meiosis II
After a short interphase during which the chromosomes do not uncoil, the nuclear membrane disappears, and the dyads (chromosomes with two chromatids) arrange themselves upon the metaphase plate.

A1 Different Stages of Meiosis A1 Prophase B1 Anaphase 1. Centromere 2. Chromatids 3. Daughter cell

A2 Prophase B2 Anaphase A. Chromosome B. Chromatids C.Daughter Cells

The chromatids of each chromosome (dyad) are equivalent, with the exception of regions distal to points of crossing over. The centromere divides and thus allow each chromosome to move poleward. The arrival of the chromosome at the poes marks the close of anaphase II. Following the arrival of the monoploid number of sister chromosomes at the poles, the chromosomes return to their long attenuate, reticulate configuration, nuclear membranes are reconstituted, nucleoli reforms in the end of telephase II. Cytokinesis separates each nucleus from the others. And thus the process of meiotic nuclear division completes.