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The alternation of generations is a process that occurs in plants, protists, and fungi, in which the organisms cycle through generations that reproduce either sexually or asexually to perpetuate the species. A complete circuit makes up a complete lifecycle for the organism, whether it's a fern or an alga. Multiple generations occur during a single lifecycle, in contrast with most animals.
In order to understand how the alternation of generations works, it's necessary to review a few basic biology terms. Diploid cells contain a complete set of genetic information in the form of chromosomes, while haploid cells only contain half of the chromosomes. Humans and other animals are diploid, producing small numbers of haploid cells for the purpose of reproduction, but organisms that undergo the alternation of generations are capable of living in both diploid and haploid forms. Meiosis is a type of cell division that results in four haploid cells, while mitosis produces two daughter cells that are identical to the parent cell, containing all of the genetic information found in the parent.
In their haploid phase, also known as the gametophyte generation, organisms produce haploid spores through the process of mitosis. These spores can fuse with each other to create a zygote that contains genetic information from a single parent, or from two different parents of the same species. The gametophyte generation is the phase in which the organism reproduces sexually, providing an opportunity to exchange genetic information with other organisms.
The fertilized zygote from the gametophyte generation develops into a sporophyte, which has diploid cells. The cells of the sporophyte undergo meiosis to create haploid cells that develop into gametophytes, thereby starting a new phase and bringing the organism completely through the cycle, from gametophyte to sporophyte and back again. Many people have noted that some organisms go through these phases at different times of the year, such as during spring pollen season.
These alternating phases can look markedly different in some species. In some organisms, the sporophyte is the dominant form; trees, for example, are sporophytes. In other organisms, the gametophyte is the dominant organism, as seen in liverworts. In flowering plants, the gametophytes develop on a microscopic level in the pollen and flowers, with people primarily seeing the sporophyte form. The alternation of generations and the various ways in which it manifests are an interesting illustration of the diversity of life.