The process of Meiosis and its significance or various stages of reduction division of cell

Meiosis or reduction division is complicated type of cell division occurring in the reproduction of germ cells at the time of gametic formation. In animals it occurs during gametes formation. Every species has definite number of chromosomes and that this number remains constant in its somatic cells, generation after generation. To keep the number constant the chromosome number in gametes is reduced to half. When male and female gametes and their nuclei fuse, normal number of chromosomes characteristic of each species is restored. The somatic or vegetative cells having the complete number of chromosomes are called diploid (2n). While gametes containing half of the original number of chromosomes are called haploid (n) or monoploid.

The events of Meiosis:

Meiosis consists of two successive divisions of mother cells. First division is reduction division, during which the chromosome number (2n) in both daughter cells is reduced to half (n), the second division is simple mitotic division resulting in four cells, each having same reduced number (n) of chromosomes.

Both, first and second divisions are divisible into four stages known as prophase I, metaphase I, Anaphase I and Telophase I in first meiotic division and prophase II, Metaphase II, Anaphase II and Telophase II in second meiotic division.

First Meiotic division:

Prophase I:

It is division of longer duration and has five sub stages (i) Leptotene (ii) Zygotene (iii) Pachytene (iv) Diplotene and (v) Diakinesis.

(i) Leptotene: Leptotene initiates meiosis. Cells become large in size and has large nucleus. It has diploid chromosome number. Chromosomes in this stage are observed to be thin, long threads and longitudinally single rather than double as in mitosis. Each chromosome presents beaded appearance due to the presence of dense granules of chromosomes at irregular intervals along its entire length.

(ii) Zygotene: Zygotene commences with the movement of similar chromosomes brought together by attraction between them. Thus the chromosomes of each homologous pair approach each other and become intimately associated to form bivalent. Pairing of homologous chromosomes is known as synapsis. It starts at one or more paints along the length of chromosomes and the chromomere of one homologue synapse exactly with corresponding one of the other. This nucleus now appears to have half the number of chromosomes.

(iii) Pachytene: During this stage paired chromosomes of each bivalent get shortened and thickened and are more readily distinguishable. The homologous chromosomes twice around each other and each starts splitting into two sister chromatids longitudinally splitting each pair into four chromatids called tetrads.

(iv) Diplotene: in these stage synaptic forces of attraction between each bivalent consisting of four chromatids laps and the chromosomes uncoil and separate. The separation is incomplete and paired chromosomes are in contact with each other at one or ore points. These points of contact are known as chiasmata. Exchange of chromatid parts takes place between paired chromosomes due to the breakage and rejoining of segments of chromatids at chiasmata. This is known as crossing over. The important thing to keep in mind is that whole blocks of genes are transferred between non sister chromatids of a tetrad during crossing over.

(v) Diakinesis: It takes place by the disappearance of nuclear membrane, nucleolus and completion of spindle apparatus. The separation of bivants is completed by the process terminalization in which the movement of chiasmata from the centromere towards the ends of the chromosome arms take like a zipper, and at the end of diakinesis two chromatids are held together only at their ends by the centromeres. Now the bivalents become more thickened, contracted and prominently visible. Prophase I end here.

Metaphase I:

The bivalents now line up at the equatorial plane. The tetrads then attach themselves to half spindle fibres at the centromeres. Each chromosome of the bivalent becomes connected to the half spindle fibres of one pole and the other half with half spindle fibres of the opposite pole.

Anaphase I:

In this stage each chromosome of homologoues is pulled towards the opposite pole by the contraction of half spindle fibres. Anaphase is completed when two sets of chromosomes reach the opposite poles of the cell.

Telophase I:

When chromosomes reach opposite poles, two new nuclei begin to form. New nuclear membranes appear and the chromosomes uncoil. A brief pause in the meiotic process may follow. However the chromosomes are still double stranded (consist of two chromatids) thus they are ready to divide again and second meiotic division begins.

Second Meiotic Division:

Telophase I is followed by short interphase which corresponds with mitotic interphase. Sometimes the interphase may persist for considerable length of time. At the interphase between two meiotic divisions there is no replication of chromosomes. These are now haploid in number although each one consists of two chromatids.

Prophase II:

The centrioles divide and the spindles are formed which are at right angles to the first meiotic division. The nuclear membrane disappears and the splitted chromosomes (diads) arrange themselves at the equatorial plane.

Metaphase II:

Half or discontinuous spindle fibres attach at the centromeres of the diads and two chromatids get separated at the centromere from each other.

Anaphase II:

Movement of two sets of chromatids of each spindle starts towards the opposite poles. Each chromatid is now called monad. Anaphase finishes when the monads reach the poles.

Telophase II:

Chromosomes uncoil and form separate groups and around each group a nuclear membrane is formed. Cytoplasmic division or cytokinesis is followed resulting in four daughter cells, each with haploid number of chromosomes.

Significance of Meiosis:

(1) It is logical and necessary part in the life cycle of sexually reproducing animals as it helps in restoring the definite number of chromosomes. As a result of meiosis the gametes are formed, each gamete possesses haploid (n) number of chromosomes and the fusion of gametes at the time of fertilization results in the diploid  (2n) number. Thus meiosis helps in the maintenance of chromosomal number in species generation after generation. In the absence of meiosis, the number of chromosomes would have been doubled giving rise to abnormal growth, changes in species characteristics and at the same time may prove fatal.

(2) During process of meiosis crossing over takes place between two homologous chromosomes. Crossing over and chiasmata formation result in exchange of chromosome species between two homologues. Thus new combination of genetic material are facilitated which lead to the evolution of new forms.