Life Cycle
Life cycle refers to the series of changes that the members of a species undergo as they pass from the beginning of a given developmental stage to the beginning of that same developmental stage in a subsequent generation.
In many simple organisms, including bacteria and various protists, the life cycle is completed within a single generation: an organism begins with the fission of an existing individual; the new organism grows to maturity; and it then splits into two new individuals, thus completing the cycle. In higher animals, the life cycle is also complete in a single generation. The individual animal begins with the union of male and female sex cells (gametes); it grows to reproductive maturity; and it then produces gametes, at which point the cycle begins anew.
By contrast, in most plants, the life cycle is multigenerational. An individual plant begins with the germination of a spore, which grows into a gamete-producing organism (the gametophyte). The gametophyte reaches maturity and forms gametes, which, following fertilization, grow into a spore-producing organism (the sporophyte). Upon reaching reproductive maturity, the sporophyte produces spores, and the cycle starts again. This multigenerational life cycle is called alternation of generations; it occurs in some protists and fungi as well as in plants.
The life cycle of bacteria is termed haplontic. This term refers to the fact that it encompasses a single generation of organisms whose cells are haploid (i.e., contain one set of chromosomes). The term, ploidy refers to the number of chromosomes occurring in the nucleus of a cell. In normal somatic (body) cells, the chromosomes exist in pairs. This condition is called diploidy. During meiosis the cell produces gametes, or germ cells, each containing half the normal or somatic number of chromosomes. This condition is called haploidy. When two germ cells (e.g., egg and sperm) unite, the diploid condition is restored. Thus the one-generational life cycle of the higher animals is diplontic; it involves only organisms whose body cells are diploid (i.e., contain two sets of chromosomes). Organisms with diplontic cycles produce sex cells that are haploid, and each of these gametes must combine with another gamete in order to obtain the double set of chromosomes necessary to grow into a complete organism.
The life cycle typified by plants is known as diplohaplontic, because it includes both a diploid generation (the sporophyte) and a haploid generation (the gametophyte). This phenomenon is also called alternation of generations because the life cycle includes the alternation of a sexual phase and an asexual phase. The two phases, or generations are often physiologically distinct. In algae, fungi, mosses, ferns, and seed plants, alternation of generations is common; but is not always easy to observe, however, since one or the other of the generations is often very small, even microscopic. In the sexual phase, the organism is called a gametophyte, and it produces gametes, or sex cells. In the asexual phase, the organism is a sporophyte, and produces spores asexually. In terms of chromosomes, the gametophyte has a single (haploid) set, and the sporophyte has a double (diploid) set.
Among animals, many invertebrates have an alternation of sexual and asexual generations (e.g., protozoans, jellyfish, flatworms), but the alternation of haploid and diploid generations is unknown.
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