Biological life cycle
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biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cells that proce ...
, a biological life cycle (or just life cycle or lifecycle 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 Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the ovum, egg to adult. The term can also be used to refer to t ...
, but differs from them in stressing renewal." Transitions of form may involve growth,
asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the ...
, or
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...
. In some organisms, different "generations" of the species succeed each other during the life cycle. For plants and many
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
, 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.Dixon, P.S. 1973. ''Biology of the Rhodophyta.'' Oliver & Boyd. Life cycles that include sexual reproduction involve alternating haploid (''n'') and diploid (2''n'') stages, i.e., a change of
ploidy Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
is involved. To return from a diploid stage to a haploid stage,
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
must occur. In regard to changes of
ploidy Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
, 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
gamete A gamete (; , ultimately ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also r ...
s are formed, meiosis is "gametic". *''haplodiplontic'' life cycle (also referred to as ''diplohaplontic'', ''diplobiontic'', or ''dibiontic'' life cycle) — multicellular diploid and haploid stages occur, meiosis is "sporic". The cycles differ in when
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
(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 "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).


Discovery

The study of
reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual org ...
and development in organisms was carried out by many botanists and zoologists. Wilhelm Hofmeister demonstrated that alternation of generations is a feature that unites plants, and published this result in 1851 (see plant sexuality). Some terms (haplobiont and diplobiont) used for the description of life cycles were proposed initially for algae by Nils Svedelius, and then became used for other organisms. Other terms (autogamy and gamontogamy) used in protist life cycles were introduced by Karl Gottlieb Grell. The description of the complex life cycles of various organisms contributed to the disproof of the ideas of spontaneous generation in the 1840s and 1850s.


Haplontic life cycle

A zygotic meiosis is a
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
of a
zygote A zygote (, ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual ...
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: * In archaeplastidans: some
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
(e.g., '' Chlamydomonas'', '' Zygnema'', '' Chara'')Díaz González, T.E., C. Fernandez-Carvajal Alvarez & J.A. Fernández Prieto. (2004). ''Curso de Botánica''. Gijón: Trea. Online material:
Botánica: Ciclos biológicos de vegetales
'' (Vegetal life cycles, in Spanish). Departamento de Biología de Organismos y Sistemas, Universidad de Oviedo.
* In
stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...
: some golden algae * In
alveolate The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and Biological classification, superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with ...
s: many
dinoflagellate The dinoflagellates (Greek language, Greek δῖνος ''dinos'' "whirling" and Latin language, Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually consid ...
s, e.g., Ceratium, Gymnodinium, some apicomplexans (e.g., '' Plasmodium'') * In
rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foramini ...
ns: some euglyphids, ascetosporeans * In excavates: some parabasalids * In amoebozoans: '' Dictyostelium'' * In opisthokonts: most
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
(some chytrids, zygomycetes, some
ascomycetes Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...
, basidiomycetes)


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 A gamete (; , ultimately ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also r ...
s. Haploid cells may divide again (by mitosis) 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: * In archaeplastidans: some
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
(e.g., '' Cladophora glomerata'', '' Acetabularia'') * In
stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...
: some brown algae (the Fucales, however, their life cycle can also be interpreted as strongly heteromorphic-diplohaplontic, with a highly reduced gametophyte phase, as in the
flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek language, Greek words ('container, vessel') and ('seed'), and refers to ...
s), some xanthophytes (e.g., '' Vaucheria''), most
diatom A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...
s, some oomycetes (e.g., '' Saprolegnia'', '' Plasmopara viticola''), opalines, some " heliozoans" (e.g., '' Actinophrys'', '' Actinosphaerium'') * In
alveolate The alveolates (meaning "pitted like a honeycomb") are a group of protists, considered a major clade and Biological classification, superphylum within Eukarya. They are currently grouped with the stramenopiles and Rhizaria among the protists with ...
s:
ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
s * In excavates: some parabasalids * In opisthokonts:
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage i ...
s, some
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
(e.g., some
ascomycetes Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...
)


Haplodiplontic life cycle

In sporic meiosis (also commonly known as intermediary meiosis), the zygote divides mitotically to produce a multicellular diploid sporophyte. The sporophyte creates spores via meiosis which ''also'' then divide mitotically producing haploid individuals called gametophytes. The gametophytes produce gametes via mitosis. In some plants the gametophyte is not only small-sized but also short-lived; in other plants and many algae, the gametophyte is the "dominant" stage of the life cycle. Haplodiplonts are: * In archaeplastidans: red algae (which have two sporophyte generations), some
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
(e.g., ''
Ulva Ulva (; gd, Ulbha) is a small island in the Inner Hebrides of Scotland, off the west coast of Isle of Mull, Mull. It is separated from Mull by a narrow strait, and connected to the neighbouring island of Gometra by a bridge. Much of the island ...
''), land plants * In
stramenopiles Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...
: most brown algae * In
rhizaria The Rhizaria are an ill-defined but species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foramini ...
ns: many foraminiferans, plasmodiophoromycetes * In amoebozoa: myxogastrids * In opisthokonts: some
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
(some chytrids, some
ascomycetes Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...
like the brewer's yeast) * Other eukaryotes: haptophytes Some animals have a sex-determination system called haplodiploid, but this is not related to the haplodiplontic life cycle.


Vegetative meiosis

Some red algae (such as ''Bonnemaisonia'' and '' Lemanea'') and green algae (such as '' Prasiola'') have vegetative meiosis, also called somatic meiosis, which is a rare phenomenon. Vegetative meiosis can occur in haplodiplontic and also in diplontic life cycles. The gametophytes remain attached to and part of the sporophyte. Vegetative (non-reproductive) diploid cells undergo meiosis, generating vegetative haploid cells. These undergo many mitosis, and produces gametes. A different phenomenon, called vegetative diploidization, a type of apomixis, occurs in some brown algae (e.g., ''Elachista stellaris''). Cells in a haploid part of the plant spontaneously duplicate their chromosomes to produce diploid tissue.


Parasitic life cycle

Parasites depend on the exploitation of one or more hosts. Those that must infect more than one host
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
to complete their life cycles are said to have complex or indirect life cycles. '' Dirofilaria immitis,'' or the heartworm, has an indirect life cycle, for example. The microfilariae must first be ingested by a female
mosquito Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small Diptera, flies within the family Culicidae (from the Latin ''culex'' meaning "gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish ...
, where it develops into the infective larval stage. The mosquito then bites an animal and transmits the infective larvae into the animal, where they migrate to the pulmonary artery and mature into adults. Those parasites that infect a single species have direct life cycles. An example of a parasite with a direct life cycle is ''Ancylostoma'' ''caninum'', or the canine hookworm. They develop to the infective larval stage in the environment, then penetrate the skin of the dog directly and mature to adults in the
small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...
. If a parasite has to infect a given host in order to complete its life cycle, then it is said to be an obligate parasite of that host; sometimes, infection is facultative—the parasite can survive and complete its life cycle without infecting that particular host species. Parasites sometimes infect hosts in which they cannot complete their life cycles; these are accidental hosts. A host in which parasites reproduce sexually is known as the definitive, final or primary host. In intermediate hosts, parasites either do not reproduce or do so asexually, but the parasite always develops to a new stage in this type of host. In some cases a parasite will infect a host, but not undergo any development, these hosts are known as paratenic or transport hosts. The paratenic host can be useful in raising the chance that the parasite will be transmitted to the definitive host. For example, the cat lungworm ('' Aelurostrongylus abstrusus'') uses a slug or snail as an intermediate host; the first stage larva enters the mollusk and develops to the third stage larva, which is infectious to the definitive host—the cat. If a mouse eats the slug, the third stage larva will enter the mouse's tissues, but will not undergo any development.


Evolution

The primitive type of life cycle probably had haploid individuals with asexual reproduction.
Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
and
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
exhibit a life cycle like this, and some eukaryotes apparently do too (e.g., Cryptophyta, Choanoflagellata, many Euglenozoa, many Amoebozoa, some red algae, some
green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophyte, Embryophytes) have emerged deep in the Charop ...
, the imperfect fungi, some rotifers and many other groups, not necessarily haploid). However, these eukaryotes probably are not primitively asexual, but have lost their sexual reproduction, or it just was not observed yet. Many eukaryotes (including animals and plants) exhibit
asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the ...
, which may be facultative or obligate in the life cycle, with sexual reproduction occurring more or less frequently. Individual organisms participating in a biological life cycle ordinarily age and die, while cells from these organisms that connect successive life cycle generations (germ line cells and their descendants) are potentially immortal. The basis for this difference is a fundamental problem in biology. The Russian biologist and historian Zhores A. Medvedev considered that the accuracy of
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gene ...
replicative and other synthetic systems alone cannot explain the immortality of germ lines. Rather Medvedev thought that known features of the biochemistry and genetics of
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...
indicate the presence of unique information maintenance and restoration processes at the gametogenesis stage of the biological life cycle. In particular, Medvedev considered that the most important opportunities for information maintenance of
germ cell Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during emb ...
s are created by recombination during meiosis and
DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolism, metabolic activities and environmental factors such as r ...
; he saw these as processes within the germ line cells that were capable of restoring the integrity of DNA and
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s from the types of damage that cause irreversible ageing in non-germ line cells, e.g. somatic cells. The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life. It is not actually cells that are immortal but multi-generational cell lineages. The immortality of a cell lineage depends on the maintenance of
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death (
apoptosis Apoptosis (from grc, wikt:ἀπόπτωσις, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemistry, Biochemical events lead to characteristic cell changes (Morp ...
) during development. Maintenance of cell division potential of the biological life cycle over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly
DNA damage DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolism, metabolic activities and environmental factors such as r ...
. In sexual organisms, continuity of the germline over successive cell cycle generations depends on the effectiveness of processes for avoiding DNA damage and repairing those DNA damages that do occur. Sexual processes in
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
s provide an opportunity for effective repair of DNA damages in the germ line by
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in Cell (biology), cellular organi ...
.


See also

* * * * * *
Metamorphosis Metamorphosis is a biological process by which an animal physically developmental biology, develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell cell growth#Cell ...
- Profound change in body structure during the postembryonic development of an organism * *


References


Sources

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Further reading

* * * {{Authority control Biology Reproduction