Alternation of Generations

Alternation of generations is the production of alternating asexual and sexual reproductive stages, consisting of a multicellular gametophyte generation and a multicellular sporophyte generation. This process occurs in the life cycles of all plants. Plants have different types of multicellular structures. A structure is haploid if it has a single set of chromosomes, which is half the total of the parent; the haploid condition is designated by 1n or n. A structure is diploid if it has a double set of chromosomes in homologous pairs; the diploid condition is designated by 2n. The structures are unlike those of animals in which haploid cells are sex cells that carry half of the chromosomes needed for reproduction. Each sex cell is either a sperm, the male reproductive cell or gamete, or an egg, the female reproductive cell or gamete. In alternation of generations, there are two phases (generations) within one life cycle of a plant.

Alternation of generations is a cyclical process involving both a sporophyte generation and a gametophyte generation. A sporophyte is an organism that produces spores in a multicellular diploid form. The diploid cells undergo meiosis (cell division to create gametes), producing haploid cells, or spores. In meiosis, cell division results in four daughter cells from the original parent cell, and each cell has half the number of chromosomes of the parent cell. Haploid spores divide by mitosis, a type of cell division that produces two daughter cells with the same chromosome count as the parent cell. Haploid spores produce two forms of new haploid cells, called gametophytes. A gametophyte is the multicellular haploid form taken by a plant during alternation of generations. Gametophytes give rise to gametes, also called sex cells. Gametes have similar structures, but they develop into separate types of gametophytes that are male and female. Male gametophytes release sperm cells. Female gametophytes are egg cells that release a chemical signal that attracts sperm to the egg. Fertilization, the union of sperm and egg, occurs, the egg becomes a diploid zygote, or fertilized egg, and the cycle begins again.