Inbiology, abiological life cycle (or justlife cycle when the biological context is clear) is a series of stages of the life of an organism, that begins as a zygote, often in an egg, and concludes as an adult that reproduces, producing an offspring in the form of a new zygote which then itself goes through the same series of stages, the process repeating in a cyclic fashion. Inhumans, the concept of a singlegeneration is a cohort of people who, on average, are born around the same period of time,[1] it is related though distinct from the biological concept of generations.
"The concept is closely related to those of the life history,development andontogeny, but differs from them in stressing renewal."[2][3] Transitions of form may involve growth,asexual reproduction, orsexual reproduction.
In some organisms, different "generations" of the species succeed each other during the life cycle. Forplants and manyalgae, there are two multicellular stages, and the life cycle is referred to asalternation of generations. The termlife history is often used, particularly for organisms such as thered algae which have three multicellular stages (or more), rather than two.[4]
Life cycles that include sexual reproduction involve alternatinghaploid (n) anddiploid (2n) stages, i.e., a change ofploidy is involved. To return from a diploid stage to a haploid stage,meiosis must occur. In regard to changes ofploidy, there are three types of cycles:
The cycles differ in whenmitosis (growth) occurs. Zygotic meiosis and gametic meiosis have one mitotic stage: mitosis occurs during then 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).[citation needed]
The study ofreproduction anddevelopment in organisms was carried out by many botanists and zoologists.
Wilhelm Hofmeister demonstrated thatalternation of generations is a feature that unites plants, and published this result in 1851 (seeplant 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.[5][6] Other terms (autogamy and gamontogamy) used inprotist life cycles were introduced by Karl Gottlieb Grell.[7] The description of the complex life cycles of various organisms contributed to the disproof of the ideas ofspontaneous generation in the 1840s and 1850s.[8]
A zygotic meiosis is ameiosis of azygote immediately afterkaryogamy, which is the fusion of twocell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells dividemitotically 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 include:
In gametic meiosis, instead of immediately dividingmeiotically to produce haploid cells, the zygote dividesmitotically 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 orgametes. 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.[15]
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 sporic meiosis (also commonly known as intermediary meiosis), the zygote divides mitotically to produce a multicellular diploidsporophyte. The sporophyte creates spores via meiosis whichalso then divide mitotically producing haploid individuals calledgametophytes. 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.[20]
Haplodiplonts are:
Some animals have asex-determination system calledhaplodiploid, but this is not related to the haplodiplontic life cycle.
Somered algae (such asBonnemaisonia[21] andLemanea) and green algae (such asPrasiola) have vegetative meiosis, also called somatic meiosis, which is a rare phenomenon.[22] 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 ofapomixis, occurs in somebrown algae (e.g.,Elachista stellaris).[23] Cells in a haploid part of the plant spontaneously duplicate their chromosomes to produce diploid tissue.
Parasites depend on the exploitation of one or morehosts. Those that must infect more than one hostspecies 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. Themicrofilariae must first be ingested by a femalemosquito, where it develops into the infectivelarval stage. The mosquito then bites an animal and transmits the infective larvae into the animal, where they migrate to thepulmonary artery and mature into adults.[24]
Those parasites that infect a single species have direct life cycles. An example of a parasite with a direct life cycle isAncylostoma 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 thesmall intestine.[25][verification needed]
If a parasite has to infect a given host in order to complete its life cycle, then it is said to be anobligate parasite of that host; sometimes, infection isfacultative—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 asparatenic[26] 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.[citation needed]
The primitive type of life cycle probably had haploid individuals with asexual reproduction.[13] Bacteria andarchaea exhibit a life cycle like this, and some eukaryotes apparently do too (e.g.,Cryptophyta,Choanoflagellata, manyEuglenozoa, manyAmoebozoa, some red algae, somegreen algae, theimperfect fungi, somerotifers and many other groups, not necessarily haploid).[27] However, these eukaryotes probably are not primitively asexual, but have lost their sexual reproduction, or it just was not observed yet.[28][29] Many eukaryotes (including animals and plants) exhibitasexual reproduction, which may be facultative or obligate in the life cycle, with sexual reproduction occurring more or less frequently.[30]
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 historianZhores A. Medvedev[31] considered that the accuracy ofgenome replicative and other synthetic systems alone cannot explain the immortality ofgermlines. Rather Medvedev thought that known features of the biochemistry and genetics ofsexual reproduction indicate the presence of unique information maintenance and restoration processes at thegametogenesis stage of the biological life cycle. In particular, Medvedev considered that the most important opportunities for information maintenance ofgerm cells are created byrecombination during meiosis andDNA repair; he saw these as processes within the germ line cells that were capable of restoring the integrity ofDNA andchromosomes from the types of damage that cause irreversible ageing in non-germ line cells, e.g.somatic cells.[31]
The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to theorigin of life. It is not actually cells that areimmortal but multi-generational cell lineages.[32] The immortality of a cell lineage depends on the maintenance ofcell division 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) 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, particularlyDNA damage. In sexual organisms, continuity of thegermline over successive cell cycle generations depends on the effectiveness of processes for avoiding DNA damage andrepairing those DNA damages that do occur. Sexual processes ineukaryotes provide an opportunity for effective repair of DNA damages in the germ line byhomologous recombination.[32][33]
