Biological life cycle
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A '''life cycle''' is a period involving 1 [[generation]] of an [[organism]] through means of [[reproduction]], whether through [[asexual reproduction]] or [[sexual reproduction]]. In regard to its [[ploidy]], there are 3 types of cycles:
*'''''haplontic''''' life cycle
*'''''diplontic''''' life cycle
*'''''diplobiontic''''' life cycle (also referred to as ''diplohaplontic'', ''haplodiplontic'', or ''dibiontic'' life cycle)
These three types of cycles feature alternating [[haploid]] and all [[germination|germinates]]. To return to a haploid stage, [[meiosis]] must occur (see [[Cell division]]). The cycles differ in the product of meiosis, and whether [[mitosis]] (growth) occurs. Zygotic and gametic meioses have one mitotic stage and form: during the ''n'' phase in zygotic meiosis and during the ''2n'' phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively term '''haplobiontic''' (single mitosis per phase). Sporic meiosis, on the other hand, has two mitosis events ('''diplobiontic'''): one in each phase.
== Haplontic life cycle ==
[[Image:Zygotic meiosis.png|thumb|250px|Zygotic meiosis]]
A '''zygotic meiosis''' is a [[meiosis]] of a [[zygote]] immediately after [[karyogamy]], which is the fusion of two [[cell nucleus|cell nuclei]]. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide [[mitotic]]ally to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (e.g., male and female) from these individuals or cells fuse to become a zygote.
In the whole cycle, zygotes are the only diploid cell; mitosis occurs only in the haploid phase.
The individuals or cells as a result of mitosis are '''haplonts''', hence this life cycle is also called '''haplontic life cycle'''. Haplonts are:
* Most [[fungi]]
* Some [[green algae]]
* Many [[protozoa]]
== Diplontic life cycle ==
[[Image:Gametic meiosis.png|thumb|250px|Gametic meiosis]]
In '''gametic meiosis''', instead of immediately dividing ''meiotically'' to produce haploid cells, the zygote divides ''mitotically'' to produce a multicellular diploid individual or a group of more unicellular diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or [[gamete]]s. Haploid cells may divide to form more haploid cells, as in many yeasts, but the haploid phase is not the predominant life cycle phase. In most diplonts, mitosis occurs only in the diploid phase, i.e. gametes usually form quickly and fuse to produce diploid zygotes.
In the whole cycle, gametes are usually the only haploid cells, and mitosis usually 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 are:
* [[Animal]]s
* Some [[brown algae]]
* Some [[fungi]], e.g. [[brewer's yeast]]
==Life history==
The term "Life history" is better than "life cycle" especially for the red algae ([[Rhodophyta]]). In the Rhodophyceae there are generally three stages and thus "alternation of generations" is not suitable.<ref name="Dixon73">'''Dixon, P.S.''' 1973. ''Biology of the Rhodophyta.'' Oliver & oyd, Edinburgh. ISBN 0 05 002485 X''</ref>
==Life history theory==
In animal and human biology [[life history theory]] is a method of understanding evolved behaviors and strategies to optimize reproductive success.
==References==
<references/>
[[Category:Biology]]
[[Category:Reproduction]]
[[ar:دورة الحياة البيولوجية]]
[[cs:Životní cyklus]]
[[de:Lebenszyklus (Biologie)]]
[[eo:Vivciklo (biologio)]]
[[fr:Cycle de vie (biologie)]]
[[he:מחזור חיים]]
[[lt:Gyvenimo ciklas]]
[[nl:Levenscyclus]]
[[ja:生活環]]
[[pl:Cykl życiowy]]
[[pt:Ciclo de vida]]
[[ru:Жизненный цикл (биология)]]
[[uk:Біологічний життєвий цикл]]