Why must there be a reduction in the number of chromosomes in some stages of the life cycle?
Meiosis is a reduction division that is necessary in sexually reproducing organisms to maintain the species number of chromosomes. Gametes, or sex cells must have half the chromosomes that the parent cell has. At fertilization, two gametes fuse together to form the offspring. If these gametes had the same number of chromosomes as the parent does, each generation would have double the chromosomes as the previous generation did. To prevent that, meiosis occurs in the gonads--ovaries or testes. If the ovary or teste cell is diploid(2n), the gametes or sex cells--sperm and eggs will be haploid(n)after meiosis occurs. For example, if a human teste or ovary cell has 46 chromosomes, replication occurs. The result will be 46 pairs or 92 chromosomes. Meiosis has two rounds of cell division--meiosis I and II. After meiosis I takes place, two daughter cells, each with 46 chromosomes are the result. After meiosis II occurs, each of these daughter cells divides and four haploid cells are the result. In this example, they will each have 23 chromosomes or the haploid amount. These can be used as gametes--sperm or egg cells during sexual reproduction. In the case of males, all four haploid cells will develop into sperm. In the case of females, only one becomes the ova or egg cell, and the other three are called polar bodies.
Reduction in number of chromosomes is caused by meiosis. Meiosis reduced the chromosome number from 46 to 23. Meiosis undergoes two phases. The first phase has 46 chromosomes (2 cells created) and the second phase has 23 chromosomes (4 cells created). Meiosis only occurs in gametes (or sex cells). The reason why the number of chromosomes reduces is because the offspring must have 46 chromosomes. When egg (23 chromosomes) and sperm (23 chromosomes) combine during fertilization, the zygote (23+23=46) contain 46 chromosomes.