In the biology of eukaryotic organisms, a life cycle includes the major sexual stages of a species, especially in regard to its ploidy (basic number of chromosomes). There are three types of cycles: zygotic meiosis, gametic meiosis, and sporic meiosis. All sexual cycles feature alternating haploid and diploid phases. The haploid phase becomes diploid through a process of combining (fertilization) of gametes, resulting in a zygote (see also Reproduction). To return to the haploid stage, meiosis occurs.

The cycles differ not only in the products of meiosis, but when mitosis (growth; see also Cell division) occurs. Zygotic and gametic meioses have one mitotic phase and macrothallic form, so are termed haplobiontic. Sporic meiosis, on the other hand, has two growth forms and is thus called diplobiontic.

Table of contents
1 Zygotic meiosis
2 Gametic meiosis
3 Sporic meiosis
4 External links

Zygotic meiosis


Zygotic meiosis

After karyogamy in zygotic meiosis, the zygote immediately undergoes meiosis, ending its diploid phase and producing several haploid cells. These cells then 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 resulting from mitosis are haplonts, hence this life cycle is also called haplontic life cycle. Haplonts include:

Gametic meiosis


Gametic meiosis

In gametic meiosis, instead of immediately dividing meiotically to produce haploid cells, the zygote divide mitotically to produce a multicellular diploid individual. 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 are the only haploid cells; mitosis occurs only in the diploid phase. The diploid multicellular individual is a diplont, hence a gametic meiosis is also called a diplontic life cycle. Diplonts include:

Sporic meiosis


Sporic meiosis

In sporic meiosis, also known as intermediary meiosis, mitotic divisions occur in both the diploid and haploid phases. These organisms exhibit an alternation of generations that features spore-producing multicellular sporophytes and gamete-producing multicellular gametophytes. Diagramatically, sporic meiosis looks like the complex halves of Gametic meiosis and Zygotic meiosis merged into one cycle.

This type of cycle is diplobiontic (also known as diplohaplontic, haplodiplontic, or dibiontic). Sporic meiosis occurs in:

External links