Biological life cycle
In biology, a life cycle includes the major sexual stages of a species, especially in regard to its ploidy. There are three types of cycles: zygotic meiosis, gametic meiosis, and sporic meiosis. All sexual cycles feature the alternating haploid and diploid phases. The haploid organism becomes diploid through fertilizations of gametes, resulting in a zygote. (See Reproduction) To return to the haploid stage, meiosis occurs. (See Cell division)
In the whole cycle, zygotes are the only diploid cell; mitosis occurs only the haploid phase.
The cycles differ in the product of meiosis, and if mitosis (growth) occurs. Zygotic and gametic meiosis have one mitotic stage and form, so collectively they are haplobiontic. Sporic meiosis, on the other hand, has two (diplobiontic).
After karyogamy in zygotic meiosis, the zygote immediately undergoes meiosis, ending its diploid phase and producing several haploid cells. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes from these individuals or cells fuse to become a zygote.
The individuals or cells as a result of mitosis are haplonts, hence this life cycle is also called haplontic life cycle. Haplonts are:
In gametic meiosis, instead of immediately dividing meiotically to produce haploid cells, the zygote divide mitotically to produce a multicellular diploid individual or more diploid cells. Cells from the individual then undergo meiosis to produce gametes. The gametes (haploid) do not divide mitotically, however. Two opposite types of gametes fuse and produce the diploid zygote.
In the whole cycle, gametes only diploid cell; mitosis occurs only the diploid phase.
The diploid multicellular individual is a diplont, hence a gametic meiosis is also called a diplontic life cycle. Diplonts are:
In sporic meiosis, also known as intermediary meiosis, mitoses occur in both the diploid and haploid phases. It exhibits alternation of generations, which features of spore-producing multicellular sporophytes and gamete-producing multicellular gametophytes. Diagramatically, sporic meiosis looks like the complex halves of #Gametic meiosis and #Sporic meiosis are merged into one.
This type of cycle is diplobiontic (also known as diplohaplontic, haplodiplontic, or dibiontic).
See also: Death