Meiosis is a process of cell division of the reproductive cells of both plants and animals in which the diploid number of chromosomes is reduced to haploid.
Meiosis is also known as reduction division(RD) since the number of chromosomes is reduced to half. It takes place only in the reproductive cells during the formation of gametes. Meiosis consists of two complete divisions. As a result of this a diploid cell produces four haploid cells. The two divisions of meiosis are meiosis I or heterotypic division and meiosis II or homotypic division. The first division is meiotic or reductional in which the number of chromosomes is reduced to half and the second division is mitoticor equational.
In all the sexually reproducing organism the chromosome number remains constant generation after generation. During sexual reproduction the two gametes male and female, each having single set oof chromosomes (n) fuse to form a zygote.The zygote thus contains twice as many chromosome as a gamete (n+n=2n). In these two sets of chromosomes one set is derived from the male parent and the other set from the female parent. This is how diploids come to possess tow identical sets of chromosomes called homologous chromosomes Meiosis may take place in the life cycle of a plant during any one of the following events.
At the time of spore formation ie. During the formation of pollen grains in anther and megaspores in ovules.
At the time of gamete formation
At the time of zygote germination.
Each meiotic division cycle is divided into same four stages as in mitosis.
Prophase, Metaphase, Anaphase and Telophase. The name of each stage is followed by I or II depending on which division of cycle is involved.
It consists of four stages namely.
A. Prophase I
B. Metaphase I
C. Anaphase I
D. Telophase I
A. Prophase I
It is the first stage of first meiosis. This is the longest phase of the meiotic division. It includes 5 sub stages namely
1.Leptotene 2.Zygotene 3.Pachytene 4.Diplotene 5.Diakinesis
The word leptotene means 'thin thread' . The chromosomes uncoil and become large and thinner. Each chromosome consists of two chromatids.
Homologous chromosomes come to-gether and lie side by side throughout their length. This is called pairing or synapsis. The paired chromosomes are now called bivalents. The adjacent non-sister chromatids are joined together at certain posints calledchiasmata.
The chromosomes condense further and become very shorter and thicker. They are very distinct now. The two sister chromatids of each homologous chromosome become clearly visible. The bivalent thus becomes a tetrad with four chromatids. In the region of chiasmata, segments of non-sister chromatids of the homologous chromosomes are exchanged and this process is called crossing over
The homologous chromosomes condense further. They begin to separate from each other except at the chiasmata. Due to this separation the dual nature of a bivalent becomes apparent and hence the name diplotene.
The Chromosomes continue to contract. The separation of chromosome becomes complete due to terminalisation. The separation starts from the centromeres and goes towards the end and hence the name terminalisation:
The nucleolus and nuclear membrane disappear and spindle formation starts.
B. Metaphase I
The spindle fibres become prominent. The bivalents align on the equatorial plane. Spindle fibres from opposite poles get attached to the centromeres of homologous chromosomes.
C. Anaphase I
The two chromosomes of each bivalent (with chromatids still attached to the centromere) separate from each other and move to the opposite poles of the cell. Thus, only one chromosome of each homologous pair reachers each pole.
Consequently at each pole only half the number of chromosomes (haploid) is received. These chromosomes are, however not the same as existed at the beginning of prophase. Each chromosome consists of one of its original chromatids and the other has a mixture of segments of its own and a segment of chromatid from its homologue (due to crossing over).
D. Telephase I
This is the last stage of meiosis I. Reorganization of the chromosomes at poles occurs to form two haploid nuclei. Nuclear membrane and nucleolus re-appear. The spindle disappears. There is no cytokinesis after meiosis I. The second meiotic division may follow immediately or after a short inter phase. The DNA of the two haploid nuclei does not replicate.
The second meiotic division is very much similar to mitosis.
The events of prophase II are similar to mitotic prophase. Nucleolus and nuclear membrane disappear. Spindle fibres are formed at each pole.
Chromosomes move to the centre of the equatorial plane. They get attached to spindle fibres centromere.
The sister chromatids separate from one another and are pulled to opposite poles of the spindle due to contraction of the spindle fibres.
The chromosomes begin to uncoil and become thin. They reorganize into nucleus with the reappearance of nucleolus and nuclear membrane in each pole. Cytokinesis follows and four haploid daughter cells are formed and thus the meiotic division is completed.
Meiosis helps to maintain the chromosome number constant in each plant and animal species. In meiosis four haploid daughter cells are formed from a single diploid cell. This is very important in sexual reproduction during the formation of gametes.
The occurrence of crossing over results in the recombination of genes.
The recombination of genes results in genetic variation.
The genetic variations form raw materials for evolution