 Biological life cycle. In biology, a biological life cycle or just life cycle when the biological context is clear is a series of changes in form that an organism undergoes, returning to the starting state. The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal. Generations of form may involve growth, asexual reproduction, or sexual reproduction. In some organisms, different generations of the species succeed each other during the life cycle. For plants and many algae, there are two multicellular stages, and the life cycle is referred to as alternation of generations. The term life history is often used, particularly for organisms such as the red algae which have three multicellular stages or more rather than two. Life cycles that include sexual reproduction involve alternating haploid then and diploid to n-stages, i.e., the change of ploidy is involved. To return from a diploid stage to a haploid stage, meiosis must occur. In regard to changes of ploidy, there are three types of cycles. Haplontic life cycle, the haploid stage is multicellular and the diploid stage is a single cell. Meiosis is zygotic. Diplontic life cycle, the diploid stage is multicellular and haploid gametes are formed. Meiosis is dramatic. Haploid-diplontic life cycle also referred to as diplo-haplontic, diplopidontic, or dybiotic life cycle. Multicellular diploid and haploid stages occur. Meiosis is sporic. The cycles differ in when mitosis growth occurs. Zygotic meiosis and gametic meiosis have one mitotic stage. Mitosis occurs during the n-phase in zygotic meiosis and during the 2-n-phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively termed haplobiantic single mitotic phase. Not to be confused with haplontic sporic meiosis. On the other hand, has mitosis in two stages, both mod diploid and haploid stages, termed haplobiantic not to be confused with diplo-haplontic.