The term ‘Meiosis’ was proposed by Farmer and Moore. Meiosis is important for organisms which reproduce sexually. It is a special type of cell division, in which cell undergoes two successive divisions, producing four daughter cells, each containing half the number of chromosomes as compared to the parent cell.
The reduction takes place because the gametes are diploid (2n). They will produce a zygote after fertilisation, with double the number of diploid chromosomes (4n). To avoid this, reduction takes place and cell become haploid from diploid, before fertilisation. So, after the fusion of gametes, the zygote has 2n-diploid chromosome number. Meiosis involves two separate cell divisions, called Meiosis I and Meiosis II.
(1) Meiosis I:
Reductional division: Meiosis I results in a reduction of chromosomes to half with the formation of two haploid cells. It takes place in four phases, i.e., prophase, metaphase anaphase and telophase.
(i) Prophase-I. In this phase, the nucleus increases. Centrosome separates and moves toward the poles. Chromatin network converts into chromosomes. Chromosomes are present in the form of fine threads. Attraction occurs in homologous chromosomes which arrange in pairs. This is known as synapsis or pairing. The two nonsister homologous chromatids break apart (separate) at corresponding points, due to repulsion. They remain in contact with each other at many points, which are termed as chiasmata. By the formation of chiasmata a piece of a chromatid, bearing genes get transferred from one to another chromatid and vice versa. It is known as a crossover.
(iI) Metaphase I- Centrioles form a spindle. The pairs of homologous chromosomes arrange on the equatorial plate in such a way that centromeres are towards the poles, but their arms are towards mid-line.
(iii) Anaphase I- The two partners of homologous chromosomes completely separate from each other and start migrating towards the opposite poles of the spindle. In this way, two sets are formed. Each set is on a pole. Now, the number of chromosomes becomes haploid.
(iv) Telophase I– Nuclear membrane is reappears. Thus, a nucleus is reformed at each pole. Nucleolus also again appears. The division of cytoplasm (cytokinesis) by cleavage, results in the two haploid daughter cells.
(2) Meiosis II:
Equational Division: The two daughter cells then undergo Meiosis II which is exactly like mitosis. Meiosis II, is an equational division, resulting in four daughter cells with half the number of chromosomes.
This is also completed in two stages.
(A) Kaiyokinesis.
(B) Cytokinesis.
(A) Karyokinesis-
In this, four stages take place:
(1) Prophase II
(2) Metaphase II
(3) Anaphase II
(4) Telophase II
(1) Prophase II- Two asters are formed, moving towards the opposite poles. Spindle fibres develop chromosomes, which get attached to the spindle fibre.
(2) Metaphase II- Chromosomes get arranged anged on the equatorial plane and the sister centromeres separate. Each chromosome produces two daughter chromosomes.
(3) Anaphase II- Centromere of each chromosome splits into two daughter centromeres and one chromatid of chromosome attached with one daughter centromere and other with the second centromere, now known as daughter chromosomes. Two daughter chromotids move towards, the opposite poles.
(4) Telophase II- A11 of the four nuclei has one chromatid and each nucleus has a haploid number of chromosomes, as well as DNA, as compared to the parent cell.
(B) Cytokinesis-
The karyokinesis of the second meiotic division is followed by cytokinesis. It starts at anaphase II and completed by the end of Telophase II. This process is similar to mitosis.
