Unit Three. The Continuity of Life
The life cycles of all sexually reproducing organisms follow the same basic pattern of alternation between diploid chromosome numbers (the blue areas of the life cycles illustrated in figure 9.3) and haploid ones (the yellow areas). In most animals, fertilization results in the formation of a diploid zygote, shown in figure 9.3b, that begins to divide by mitosis. This single diploid cell eventually gives rise to all of the cells in the adult frog shown in the figure. These cells are called somatic cells, from the Latin word for “body.” Each is genetically identical to the zygote.
In unicellular eukaryotic organisms like the protists shown in figure 9.3a, individual haploid cells function as gametes, fusing with other gamete cells. In plants like the fern shown in figure 9.3c, the haploid cells that meiosis produces divide by mitosis, forming a multicellular haploid phase, the heart-shaped structure in the figure. Some cells of this haploid phase eventually differentiate into eggs or sperm, which fuse to form a diploid zygote.
Figure 9.3. Three types of sexual life cycles.
In sexual reproduction, haploid cells or organisms alternate with diploid cells or organisms.
In animals, the cells that will eventually undergo meiosis to produce gametes are set aside from somatic cells early in the course of development. These cells are often referred to as germ-line cells. Both the somatic cells and the gamete-producing germ-line cells are diploid, as indicated by blue arrows in figure 9.4. Somatic cells undergo mitosis to form genetically identical, diploid daughter cells. The germ-line cells undergo meiosis, indicated by the yellow arrows, producing haploid gametes.
Figure 9.4. The sexual life cycle in animals.
In animals, the completion of meiosis is followed soon by fertilization. Thus, the vast majority of the life cycle is spent in the diploid stage. In this text, n stands for haploid and 2n stands for diploid. Germ-line cells are set aside early in development and undergo meiosis to form haploid gametes (eggs or sperm). The rest of the body cells are called somatic cells.
Key Learning Outcome 9.2. In the sexual life cycle, there is an alternation of diploid and haploid phases.