Before the initiation of meiosis, there is an inter-phase stage just like mitosis in which chromosomes are duplicated. Meiotic division undergoes two successive nuclear divisions, so that four daughter cells are formed as a result of a complete division. The first division is accompanied with the reduction in the chromosome number without any splitting of the chromosomes, while the second division involves the separation of chrmatids of the chromosomes.
Consequently, the number of chromosomes which is reduced in the first division remains constant during the second division. Therefore, melosis can be studied in two parts; (i) first meiotic division and (ii) second meiotic division. The divisions of nuclei follow the same sequence as found in mitotic division. The two cell divisions follow a close sequence with only a short interphase in between.
The first meiotic division is more important than the second division because it is the reduction division. In this, four sty ages can be differentiated.
1. Prophase I
since it is of very long duration, it has been subdivided into five sub stages: leptotene, zygotene, pachytene, diplotene and diakinesis. Prophase I is complex and differs from mitotic prophase in several respects.
(a) Leptotene or Leptonema (leptos=thin)
This is the first stage of meiosis following interphase. The chromosomes at this stage appear long, thread-like structures. On the entire chromosome, characteristic bead-like structures, called chromomeres can be seen. In animal cells, the centrioles divided and move towards opposite poles.
(b) Zygotene or Zygonema (Zygone=adjoining)
This stage is characterized by the pairing of homolosgous chromosomes. This phenomenhon is known as synapsis. The pairing starts at the centromere or at any other position. The paired chromosomes are called bivalents or dyads. They gradually become thick and short.
(c) Pachytene or Pachynema (Pachus=thick)
Each chromososme of a bivalent splits longitudinally into two sister chromatids so that the bivalent becomes a tgetrad or quadrivalent. The tgwo nonsister chromatids, one from each bivalent (one paternal and the other maternal) partially coil around each other and exchange their genetic material. On each of the nonsister chromatids of the tetrad, transverse breaks occur which are followed by interchange and final fusion. This process is known as crossing over and the point where the crossing over takes place is called chiasmata (singular, chiasma). Due to coiling, the paired chromosomes become thicker and short. The nucleolus still persists.
(d) Diplotene or Diplonema (Diploos=double) At this stage, homologous chromosomes start at the centromere and moves towards the ends. The type of separation from centromere towards the end is known as terminalization. This separation makes the dual nature of a bivalent chromosome distinct and hence the name of the stage is diplotene. As the terminalization proceeds, the chiasmata (points of genetic exchange) move towards the ends of the chromosomes but the chromosomes are held together at the chiasmata. It should be remembered that crossing over always takes place between nonsister chromatids of homologous chromosomes. Chiasmata are not the cause but are only the consequence of crossing over. The number of chiasmata per bivalent varies and is dependent upon the length of the chromosomes. Nucleolus and nuclear membrane start disappearing at this stage.
(e) Diakinesis (Dia=across)
The chromosomes undergo further contraction and shortening. During this stage the nucleolus and nuclear membrane disappear. Centrioles reach the opposite poles of cell and start forming spindle apparatus.
2. Metaphase I
At this stage the bivalent chromosomes arrange themselves in the equatorial plane in such a fashion that their centromeres remain directed towards the opposite poles and the arms towards the equator. Certain spindle fibres get attached to the centromeres of chromosomes.
3. Anaphase I
The homologous chromosomes completely separate from each other and move towards opposite poles of the cells due to shortening of spindle fibres. The chromosome number of the daughter nuclel formed in thus reduced to half. However, due to crossing over, the chromatids of these chromosomes are not genetically identical.
The centromere, that holds chromatids together in each chromosome, does not divide at this stage as it does in mitosis. So, the sister chromatids do not separate but go to the same pole.
4. Telophase I
With the arrival of chromosomes at the opposite poles, telophase I starts. Only one partner of homologouschromosomes with exchanged part of chromatids goes to one pole. Now the chromosomes get uncoiled into chromatin thread. Nucleolus and nuclear membrane reappear.
Cytoplasm may or may not divide after melosis I. If telophase I is followed by cytokinesis a dyad will be formed. This is a successive division. If cytokinesis is postponed till the end of second meiotic division, the four cells will be formed by simultaneous division.
The ingterphase is a brief period here. Sometimes it may be absent. There is no duplication of chromosomes at this stage which is a different condition from that of mitosis.
The second meiotic division is essentially a mitotic division and is sometimes termed as meiotic mitosis. It can be studied under the following four stages.
1. Prophase II
In both the cells the nuclear membrane and nucleoli disappear . The centrioles duplicate and migrate towards opposite pole. Each set of centriosles is surrounded by aster rays. The formation of spindle starts. The shortening of chromosomes begins. Sometimes prophase II is absent and the telophase I is directly followed by the metaphase II.
2. Metaphase II
The chromosomes arrange themselves on the equatorial plane and centromere divides. Each chromatid gets attached to spindle fibres by its centromere.
3. Anaphase II
Spindle fibres attached to the opposite faces of cenromeres shorten in length. This causes a pull on the centromere. As a result, the centromere splits along the longitudinal axis and the chromatids are pulled to the opposite poles.
4. Telophase II
The chromatids (now the chromosomes) reach their respective poles. They uncoil and form the chromatin network. Nucleolus and nuclear membrane reappear. At the end of this phase, four haploid (n) nuclei are produced in each cell.