Biological Life Cycle

Biological Life Cycle

A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction (a period from one generation of organisms to the same identical). For example, a complex life cycle of Fasciola hepatica, a parasitic flatworm, includes three different multicellular generations: 1) "adult" hermaphroditic; 2) sporocyst; 3) redia.

In regard to changes of 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 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.