Biology
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The question is why meiosis occurs. In process mitosis, the number of chromosomes in the daughter cells remains the same as that of their mother cell. Mitosis is essential for the growth and asexual reproduction of organisms. In sexual reproduction, the union of male and female gametes is required. If the number of chromosomes in the reproductive cells would be the same as the vegetative cells, the zygote would contain twice number of chromosomes than that of the vegetative cell. Supposing the number of chromosomes in a vegetative and a reproductive cell of an organism is 4. In zygote the number of chromosome would be 8, and so the new organisms will have the body cells with chromosome number 8 and that is twice that of its mother organism. If every life cycle of an organism continues that way, the number of chromosome would be doubled again and again. In the second chapter, we have learnt that chromosomes bear the genes controlling the features of organisms. If the number of chromosome is increased life cycle after life cycle, the offspring will be basically different. In sexual reproduction, even through the union of male and female gametes, the number of chromosomes remains the same generation after generation as the number of chromosomes becomes half in the reproductive cells than that of the mother cell. When at the time of the development of reproductive cells and in any stage of the life cycle of plants in lower groups, the process meiosis occurs, then the feature of possessing half number of chromosome is called haploid. When the haploid cells are fused, the state of cell having two sets of chromosome is called diploid. As the meiosis cell division occurs, the features in the species of living organisms keep on the same more or less, generation after generation. Meiosis mainly occurs in the primordial germinal cells during the development of gametes. In the anthers and ovules of flowering Figure: 3.7 Meiosis plants, and in the testes and ovaries of animals meiosis occurs. During the development of pollens from diploid pollen mother cells in mosses and ferns, meiosis occurs in their zygotes. Two consecutive divisions occur with a cell in the process meiosis. The first and the second division are called meiosis-1 and meiosis-II respectively. In the first division meiosis I, the number of chromosome in a daughter cell becomes half than that of its mother cell, and Figure: 3.8 concept of meiosis cell division