Biological life cycle

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Biological life cycle

A life cycle is a period involving one generation

Generation

Generation , also known as reproduction, is the act of producing offspring. In a more generic sense, it can also refer to the act of creating something inanimate such as electricity generation or cryptography code generation....
of an organism

Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
through means of reproduction, whether through asexual reproduction

Asexual reproduction

Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction....
or sexual reproduction

Sexual reproduction

Sexual reproduction is characterized by processes that pass a Genetic recombination of Genetics material to offspring, resulting in Genetic diversity....
. In regard to its ploidy

Ploidy

Ploidy is the number of complete sets of non-homologous chromosomes in a biological cell. In humans, the somatic cells that comprise the body are diploid , but sex cells are haploid....
, there are 3 types of cycles:

These three types of cycles feature alternating haploid and all germinates

Germination

Germination is the process whereby growth emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an flowering plant or gymnosperm....
. To return to a haploid stage, meiosis

Meiosis

In biology or life science, meiosis is a process of reductional division in which the number of chromosomes per cell is halved. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores....
must occur (see Cell division

Cell division

Cell division is a process by which a cell , called the parent cell, divides into two or more cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle....
). The cycles differ in the product of meiosis, and whether mitosis

Mitosis

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Haplontic life cycle

A zygotic meiosis is a meiosis

Meiosis

Zygote

A zygote is a cell that is the result of fertilization. That is, two ploidy cells—usually an ovum from a female and a sperm cell from a male—merge into a single ploidy cell called the zygote ....
immediately after karyogamy

Karyogamy

Karyogamy is the fusion of pronuclei of two cell s, as part of syngamy. It is one of the two major modes of reproduction in fungi. It is also the fusion of the pronuclei of two cells, as occurs in fertilization or true bacterial conjugation......
, which is the fusion of two cell nuclei

Cell nucleus

In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in all eukaryote cell ....
. 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 (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
Green algae

The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic ....
Many protozoa
Protozoa

Protozoan are microorganisms classified as unicellular eukaryotes. While there is no exact definition of the term "protozoan", most scientists use the word to refer to a unicellular heterotrophic protist, such as an amoeba or a ciliate....

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 gamete

Gamete

A gamete is a Cell that fuses with another gamete during fertilization in organisms that sexual reproduction. In species which produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual which produces the larger type of gamete?called an ovum ?and a male produces th...
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
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s
Some brown algae
Brown algae

The Phaeophyceae or brown algae, is a large group of mostly Ocean multicellular algae, including many seaweeds of colder Northern Hemisphere waters....
Some fungi, e.g. brewer's yeast

Haplodiplontic life cycle

In sporic meiosis, the zygote divides mitotically to produce a multicellular diploid "sporophyte

Sporophyte

All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or alga that has a double set of chromosomes....
". The sporophyte creates spores via meiosis which also then divide mitotically producing haploid indivicuals called "gametophyte

Gametophyte

In plants and algae that undergo alternation of generations, a gametophyte is the multicellular structure, or phase, that is haploid, containing a single set of chromosomes:...
s". Gametophytes now produce gametes via mitosis. In many plants the gametophyte is not only small-sized but also short-lived.

In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase.

Haplodiplonts are:

Most plant
Plant

Plants are Life organisms belonging to the Kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae....
s
Some fungi

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.

Life history theory

In animal and human biology life history theory

Life history theory

Life history theory is an analytical framework widely used in animal and human biology, psychology, and evolutionary anthropology which postulates that many of the physiology traits and behaviors of individuals may be best understood in terms of the key maturational and reproductive characteristics that define the life course....
is a method of understanding evolved behaviors and strategies to optimize reproductive success.