Meiosis is also called reduction division occurring in the reproductive germ cells at the time of gamete formation. In plants it takes place during spore formation. The somatic or vegetative cells having complete number of chromosomes are called diploid (2n). White gametes containing half of original number of chromosomes are called haploid (n) or monoploid. Meiosis has two successive divisions of a mother cell. First division is reduction division during which the chromosome number (2n) in both the daughter cells is reduced to half (n), the second division is simple a mitotic division resulting in four cells, each having same reduced number (n) of chromosomes. It is divided into first and second meiotic division. First meiotic division:
(1) PROPHASE I
It is longer in duration and has following sub stages:
(i) Leptotene: Leptotene initiates meiosis. The cell becomes larger in size and possesses a large nucleus. It has diploid chromosome number chromosomes are thin long threads and single. Each chromosome presents beaded appearance due to dense granules of chromosomes at irregular intervals along its entire length.
(ii) Zygotene: It occurs with the movement of similar chromosomes brought together by attraction between them. Thus the chromosomes of each homologous pair approach each other and become associated to form a bivalent. The pairing of homologous chromosomes is known as synapsis.
It starts at one or more points along the length of chromosomes and the chromosome of one homologue synapse exactly with the corresponding one of the other. The nucleus now appears to have half the number of chromosomes.
(iii) Pachytene: In this stage the paired chromosomes of each bivalent get shortened and thickened and are distinguishable. The homologous chromosomes twine around each other and each starts splitting into two sister chromatids by longitudinal splitting each pair forming four chromatids called tetrads.
(iv) Diplotene: In this stage the synaptic forces of attraction between each bivalent consisting ones uncoil and separate. Separation is incomplete and paired chromosomes are in contact with each other at one or more points. These points of contact are known as chiosmata. 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 due to which whole blocks of genes are transferred between non-sister chromatids of a tetrad.
Diakinesis: It is characterized by disappearance of nuclear membrane, nucleolus and completion of spindle apparatus. The separation of bivalents is completed by the process of terminallization in which the movement of the chiasmata from centrosome towards the ends of chromosome arms take place like a zipper and at the end of diakinesis two chromotids are held together only at their ends by the centrosomes. Now the bivalents become more thickened, contracted and visible. Prophase I ends here.
Metaphase I: The bivalents now line up at the equatorial plane. The tetrads then attach themselves to half spindle fibres at the centrosomes. Each chromosome of the bivalent becomes connect to the half spindle fibres of one pale and the other half with the half spindle fibres of opposite pole.
Anaphase I: In this stage each chromosome of bivalent of homologues is pulled towards the opposite pole by the contraction of half spindle fibres. The anaphase is completed when two sets of chromosomes reach the opposite poles of the cell.
Telophase I: When the chromosomes reach opposite poles, two new nuclei begin to form. New nuclear membranes appear and the chromosomes uncoil. A brief pause in miotic process may follow. 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 a short interphase which corresponds with mitotic interphase. At the interphase between two meiotic divisions there is no replication of chromosomes. These are now haploid in number although each one consist of two chromatids.
Prophase II: The centrioles divide and the spindles are formed which are at right angles to the spindle of first meiotic division. The nuclear membrane disappears and the splitted chromosomes (diads) arrange themselves at the equatorial plane.
Metaphase II: The half or discontinuous spindle fibres attach at the chromosomes of the diads and the two chromatids get separated at the centrosome from each other.
Anaphase II: Movement of two sets of chromatids at each spindle starts towards the opposite poles. Each chromatid is now called monad. The anaphase finishes when the monads reach the pole.
Telophase II: The chromosomes uncoil and form separate groups and around each group a nuclear membrane is formed.
Cytoplasmic division or cytokinesis is followed resulting is four daughter cells, each with haploid number of chromosomes.
Significance of Meiosis:
(1) Meiosis is a logical and necessary part in the life cycle of sexually reproducing animals and plants 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 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 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 the process of Meiosis crossing over takes place between two homologous chromosomes. The crossing over and chismata formation results in exchange of chromosome pieces between two homologues. Thus new combinations of genetic material are facilitated which lead to the evolution of new forms.
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