Biological life cycle

A life cycle is a period involving one generation of an organism through means of reproduction, whether through asexual reproduction or sexual reproduction. In regard to its ploidy, there are three 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 diploid phases (n and 2n). The haploid organism becomes diploid through fertilization, which joins of gametes. This results in a zygote which then 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

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Zygotic meiosis


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 (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:

Diplontic life cycle

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Gametic meiosis


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 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:

Life history theory

In animal and human biology life history theory is a method of understanding evolved behaviors and strategies to optimize reproductive success.
Generation (from the Greek γενεά), also known as procreation, is the act of producing offspring. It can also refer to the act of creating something inanimate such as electrical generation or cryptographic code generation.
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Asexual reproduction is a form of reproduction which does not involve meiosis, ploidy reduction, or fertilization. Asexual reproduction only takes one parent. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes.
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Sexual reproduction is a union that results in increasing genetic diversity of the offspring. It is characterized by two processes: meiosis, involving the halving of the number of chromosomes; and fertilisation, involving the fusion of two gametes and the restoration of the
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Ploidy is the number of homologous sets of chromosomes in a biological cell. The ploidy of cells can vary within an organism. In humans, most cells are diploid (containing one set of chromosomes from each parent), but sex cells (sperm and egg) are haploid.
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Fertilization (also known as conception, fecundation and syngamy), is fusion of gametes to form a new organism of the same species. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo.
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A gamete (from Ancient Greek γαμετης; translated gamete = wife, gametes = husband) is a cell that fuses with another gamete during fertilisation (conception) in organisms that reproduce sexually.
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For other meanings see Zygote (disambiguation).
A zygote (Greek: ζυγωτόν) is a cell that is the result of fertilization.
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Germination is the process where growth emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm.
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meiosis (IPA: /maɪˈəʊsɪs/) is the process by which one diploid eukaryotic cell divides to generate four haploid cells often called gametes.
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Cell division is a process by which a cell, called the parent cell, divides into two cells, called daughter cells. Cell division is usually a small segment of a larger cell cycle. In meiosis however, a cell is permanently transformed and cannot divide again.
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meiosis (IPA: /maɪˈəʊsɪs/) is the process by which one diploid eukaryotic cell divides to generate four haploid cells often called gametes.
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For other meanings see Zygote (disambiguation).
A zygote (Greek: ζυγωτόν) is a cell that is the result of fertilization.
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Karyogamy is the fusion of nuclei of two cells, as part of syngamy. It is one of the two major modes of fungi reproduction. It is also the fusion of the nuclei of two cells, as occurs in fertilization or true conjugation. [karyo- + Greek gamos, marriage]
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nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles]]

In cell biology, the nucleus (pl.
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Eukarya
Whittaker & Margulis, 1978
(unranked) Opisthokonta

Kingdom: Fungi
(L., 1753) R.T. Moore, 1980[1]

Subkingdom/Phyla

Chytridiomycota
Blastocladiomycota

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Green algae are microscopic protists; found in all aquatic environments, including marine, freshwater and brackish water.

The green algae (singular: green alga) are the large group of algae from which the embryophytes (higher plants) emerged.
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Protozoa (in Greek proto = first and zoa = animals) are one-celled eukaryotes (that is, unicellular microbes whose cells have membrane-bound nuclei) that commonly show characteristics usually associated with animals, mobility and heterotrophy.
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A gamete (from Ancient Greek γαμετης; translated gamete = wife, gametes = husband) is a cell that fuses with another gamete during fertilisation (conception) in organisms that reproduce sexually.
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Phaeophyceae
Kjellman

The Phaeophyceae or brown algae, (singular: alga) is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters.
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Eukarya
Whittaker & Margulis, 1978
(unranked) Opisthokonta

Kingdom: Fungi
(L., 1753) R.T. Moore, 1980[1]

Subkingdom/Phyla

Chytridiomycota
Blastocladiomycota

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Ascomycota (sac fungi)
  • Saccharomycotina (true yeasts)
  • Taphrinomycotina
  • Schizosaccharomycetes (fission yeasts)
Basidiomycota (club fungi)
  • Urediniomycetes

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Life history theory is an analytical framework widely used in animal and human biology, psychology, and evolutionary anthropology which postulates that many of the physiological traits and behaviors of individuals may be best understood in terms of the key maturational and
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