Three types of cell division have been distinguished:
1. Amitosis or direct cell division
2. Mitosis or indirect cell division
3. Meiosis or reduction division
Meiosis is a specialized and rather complicated type of cell division, occurring only in the diploid or reproductive cells results in the formation of haploid sex cells or gametes.
The gametes formed as a result of meiosis possess half the number of chromosomes as found in the parent cells and their chromosome number is represented by n, whereas zygote formed by the fusion of male and female gametes and the cells derived from it are known as diploid and their chromosome number is symbolized by 2n. The two similar chromosomes of a diploid cell are known as homologous chromosomes or homologous pair.
Process
In meiosis two complete cell divisions are followed in close sequence with or without a short interphase between them.
The first meiotic division is known as Reduction division or Heterotypic division. The second division is known as Homeotypic division. Each of these divisions is further divided into Prophase, Metaphase, Anaphase and Telophase.
Heterotypic division- In it the diploid parent cells divides into two daughter cells having haploid chromosomes.
: : First Prophase- It is longer duration and distinguished into five substages:
���� Proleptotene- The preliminary steps like the movement of centrioles, the formation of astral rays and the gradual condensation of the chromatin are taken care of in this step.
���� Leptotene- This stage initiates the meiosis. Chromosome appears in diploid number as long, thin uncoiled threads, irregularly arranged. Each chromosome presents a beaded appearance due to presence of a longitudinal series of dense, bead like swellings called chromomeres. The DNA and histone synthesis and the chromosome duplication start in this stage. The nucleus is well marked and increases in size.
���� Zygotene- Commences with movement of chromosomes. Pairing of homologous chromosomes called synapsis occurs in this stage. The pairing may be completed in any method namely; proterminal synapsis, procentric synapsis, random synapsis. At zygotene the nucleolus increases in size and the centrioles move apart initiating the spindle formation.
���� Pachytene- The paired chromosomes of bivalent get shortened and thickened and homologous chromosomes twist around each other. The exchange and recombination known as crossing over takes place in this stage.
���� Diplotene- The separation of homologous chromosomes initiates this stage. Chiasma formation takes place and move along the length of chromosomes from the centromere towards the end.
���� Diakinesis- The bivalents still contract and get thickened into deeply stained bodies. These migrate to the periphery of the nucleus. The two chromatids of each chromosome become closely oppressed together losing their individual identity. At the same time the homologous move still apart due to the force of repulsion developed in between their centromeres. In doing so chiasma move towards the ends. This is known as terminalization. Nucleolus and nuclear membrane disappears and the formation of nuclear spindle starts.
: : First Metaphase This is similar to metaphase of mitosis. Formation of spindle is completed. Bivalents move to the equator and orient themselves, centromere facing the pole of spindle and arms directing towards the equator.
: : First Anaphase Bivalent move apart towards the pole of the spindle. The tetrad separates into dyad due to the complete separation of maternal and paternal chromosomes of the bivalent. Each separated half consists of two sister chromatids attached together by a common centromere. This process is known as disjunction.
: : First Telophase Commences with formation of nuclear wall around haploid group of chromosomal dyads, which have already reached the poles of the spindle. The chromosomes elongate and uncoil. Nucleolus is formed. Cell cytoplasm segments into two. Two daughter cells are formed, each of which contain haploid number of chromosomes.
: : Interphase Resting stage of dividing meiocytes and duration differs species to species.
Homeotypic Division- The second meiotic division is essentially a mitosis going on independently in both the haploid sister cells. It may follow immediately after the first meiotic division or may not occur until much later. It consists of Second Prophase, Metaphase, Anaphase and Telophase.
At the end of meiosis four haploid cells are formed.
