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In biology, a biological life cycle (or just life 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. In , the concept of a single generation is a cohort of people who, on average, are born around the same period of time, it is related though distinct from the biological concept of generations.
"The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, or sexual reproduction.
In some organisms, different "generations" of the species succeed each other during the life cycle. For Embryophyte and many algae, 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 is involved. To return from a diploid stage to a haploid stage, meiosis must occur. In regard to changes of ploidy, there are three types of cycles:
The cycles differ in when mitosis (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).
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.Svedelius, N. 1931. Nuclear Phases and Alternation in the Rhodophyceae. In: Beihefte zum Botanischen Centralblatt. Band 48/1: 38–59. 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.Moselio Schaechter (2009). Encyclopedia of Microbiology. Academic Press. Volume 4, p. 85.
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 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:
Haplodiplonts are:
Some animals have a sex-determination system called haplodiploid, but this is not related to the haplodiplontic life cycle.
A different phenomenon, called vegetative diploidization, a type of apomixis, occurs in some (e.g., Elachista stellaris). Cells in a haploid part of the plant spontaneously duplicate their chromosomes to produce diploid tissue.
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.
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 paratenicSchmidt and Roberts. 1985. Foundations of Parasitology 3rd Ed. Times Mirror/Mosby College Publishing 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.
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 replicative and other synthetic systems alone cannot explain the immortality of . Rather Medvedev thought that known features of the biochemistry and genetics of sexual reproduction 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 are created by recombination during meiosis and DNA repair; he saw these as processes within the germ line cells that were capable of restoring the integrity of DNA and from the types of damage that cause irreversible ageing in non-germ line cells, e.g. .
The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the abiogenesis. It is not actually cells that are immortality but multi-generational cell lineages.
Discovery
The study of reproduction and Embryogenesis in organisms was carried out by many botanists and zoologists.
Haplontic life cycle
A zygotic meiosis is a meiosis of a zygote immediately after karyogamy, which is the fusion of two cell nucleus. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide 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.
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 . 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.
(2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 some "" (e.g., Actinophrys, Actinosphaerium)
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 . 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.
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.
Parasitic life cycle
Parasites depend on the exploitation of one or more hosts. Those that must infect more than one host species 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 Microfilaria must first be ingested by a female mosquito, where it develops into the infective Larva 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.
(2025). 9781416048893 ISBN 9781416048893
Evolution
The primitive type of life cycle probably had haploid individuals with asexual reproduction. Bacteria and archaea 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 imperfect fungi, some 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, which may be facultative or obligate in the life cycle, with sexual reproduction occurring more or less frequently. (2025). 9789048127702, Springer Science & Business Media. ISBN 9789048127702
(1999). 9783642084911 ISBN 9783642084911 The immortality of a cell lineage depends on the maintenance of cell 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, particularly DNA damage. In sexual organisms, continuity of the germline over successive cell cycle generations depends on the effectiveness of processes for avoiding DNA damage and DNA repair that do occur. Sexual processes in provide an opportunity for effective repair of DNA damages in the germ line by homologous recombination.
See also
Sources
(1995). 9780521316873, Cambridge University Press. ISBN 9780521316873
Further reading
(1995). 9780691001517, Princeton University Press. ISBN 9780691001517
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