Life cycle of the single-celled parasite |
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." Transitions 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 (n) and diploid (2n) stages, i.e., a
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 3 types of
cycles
haplontic life cycle — the haploid stage is multicellular and the diploid stage is a single
cell, meiosis is "zygotic".
Zygotic meiosis |
diplontic life cycle — the diploid stage is multicellular and haploid gametes are formed,
meiosis is "gametic".
Gametic meiosis |
haplodiplontic life cycle (also referred to as diplohaplontic, diplobiontic, or
dibiontic life cycle) — multicellular diploid and haploid stages occur, meiosis
is "sporic".
Sporic meiosis |
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 2n phase in
gametic meiosis. Therefore, zygotic and gametic meiosis are collectively termed
haplobiontic (single mitotic phase, not to be confused with haplontic). Sporic
meiosis, on the other hand, has mitosis in two stages, both the diploid and
haploid stages, termed diplobiontic (not to be confused with diplontic)
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