Everything about Biological Life Cycle totally explained
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
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
A
zygotic meiosis is a
meiosis of a
zygote immediately after
karyogamy, which is the fusion of two
cell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide
mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (for example, 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
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 gametes. Haploid cells may divide to form more haploid cells, as in many yeasts, but the haploid phase isn't the predominant life cycle phase. In most diplonts, mitosis occurs only in the diploid phase, for example 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:
Animals
Some brown algae
Some fungi, for example 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" isn't suitable.
Life history theory
In animal and human biology life history theory is a method of understanding evolved behaviors and strategies to optimize reproductive success.
Further Information
Get more info on 'Biological Life Cycle'.
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