42 Overview of Meiosis
Sexual reproduction requires fertilization, a union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. The number of sets of chromosomes in a cell is called its ploidy level (Figure 1). Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. If the reproductive cycle is to continue, the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division, known as meiosis, that reduces the number of chromosome sets.
Most animals and plants are diploid, containing two sets of chromosomes; in each somatic cell (the non-reproductive cells of a multicellular organism), the nucleus contains two copies of each chromosome that are referred to as homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. Homologous chromosomes are matched pairs containing genes for the same traits in identical locations along their length (Figure 2). Diploid organisms inherit one copy of each homologous chromosome from each parent; all together, they are considered a full set of chromosomes. In animals, haploid cells containing a single copy of each homologous chromosome are found only within gametes. One haploid gamete (containing one copy of each chromosome) fuses with another haploid gamete (also containing one copy of each chromosome) to produce a diploid cell (which has two copies of each chromosome).
Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells, are produced within the gonads, such as the testes and ovaries. Germ cells are capable of mitosis to perpetuate the cell line and meiosis to produce gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state (Figure 3).
Once the two gametes have fused during fertilization, the resulting diploid cell is called a zygote. A zygote is diploid (2 copies of each chromosome), just like the original parent organism. In order to grow from a zygote into an adult organism, many more cells must be produced. The zygote goes through a cell division process called mitosis, which produces 2 identical somatic (body) cells from one original cell. After many, many rounds of mitosis, the zygote has grown into an adult organism. Because all the body cells in the adult organism were produced during the process of mitosis, the DNA within all of those cells is identical.
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