Reproduction is inherent in all living organisms property of reproduction of their own kind, ensuring the continuity and continuity of life . Different breeding methods are divided into two main types: asexual and sexual . For organisms with a cellular structure, cell division is the basis of all forms of reproduction [1] .
Content
Asexual reproduction
Asexual reproduction is a type of reproduction in which the next generation develops from somatic cells without the participation of reproductive cells - gametes .
Asexual reproduction is the oldest and easiest way of reproduction and is widely distributed in unicellular organisms ( bacteria , blue-green algae , chlorella , amoebas , ciliates ). This method has several advantages over the sexual method: it requires less energy and time, and there is no need to search for a sexual partner, which allows for high rates of reproduction. At the same time, the harmful changes resulting from mutations are also fixed for a long period of time. In addition, in unfavorable, changing environmental conditions, almost all individuals will die, since on average they are almost identical to one parent. It should be noted that the ability of the species to asexual reproduction does not exclude the ability to sexual process , but then these events are separated in time.
The most common method of reproduction of unicellular organisms is division into two parts, with the formation of two separate individuals.
Among multicellular organisms, almost all plants and fungi have the ability to asexually reproduce - the exception is, for example, velvichia . Asexual reproduction of these organisms occurs vegetatively or by spores .
Among animals, the ability for asexual reproduction is more common in lower forms, but is absent in more developed ones. The only way of asexual reproduction in animals is vegetative.
There is a widespread erroneous opinion that individuals formed as a result of asexual reproduction are always genetically identical to the parent organism (if one does not take mutations into account). The most striking counterexample is plant propagation by spores, since during sporulation, reduction cell division occurs, resulting in disputes containing only half of the genetic information available in sporophyte cells (see Plant Life Cycle ).
Reproduction by division
The division is primarily characteristic of single-celled organisms . As a rule, it is carried out by simple division of a cell into two. Some protozoa (for example, foraminifera ) divide by a larger number of cells. In all cases, the resulting cells are completely identical to the original. The extreme simplicity of this method of reproduction, associated with the relative simplicity of the organization of single-celled organisms, allows you to multiply very quickly. Thus, in favorable conditions, the number of bacteria can double every 30-60 minutes. Asexually reproducing organism is capable of infinitely reproducing itself, until a spontaneous change of genetic material occurs - mutation . If this mutation is favorable, it will remain in the progeny of the mutated cell, which will be a new cell clone. One parental organism participates in same-sex breeding, which is capable of forming many organisms identical to it.
Division of prokaryotic cells
The division of prokaryotic cells is the process of formation of procaryotic daughter cells from the maternal one. The key events of the cell cycle in both prokaryotes and eukaryotes are DNA replication and cell division . A distinctive feature of the division of prokaryotic cells is the direct participation of replicated DNA in the division process [2] . In the overwhelming majority of cases, prokaryotic cells divide into two daughter cells of the same size, therefore this process is sometimes also called binary division . Since prokaryotic cells most often have a cell wall , binary division is accompanied by the formation of a septum - the septum between the daughter cells, which then stratifies in the middle. The process of dividing a prokaryotic cell is studied in detail by the example of Escherichia coli [3] .
Amitoz
Amitoz , or the direct division of a cell (from the ancient Greek. Ἀ- is a prefix with the absence value and μίτος - “thread”) - cell division by simple division of the nucleus into two.
It was first described by the German biologist Robert Remak in 1841 , a term suggested by the histologist Walter Flemming in 1882 . Amitosis is a rare but sometimes necessary phenomenon [4] . In most cases, amitosis occurs in cells with reduced mitotic activity: these are senescent or pathologically altered cells, often doomed to death (mammalian germ- cells, tumor cells, etc.).
In amitosis, the interphase state of the nucleus is morphologically preserved, the nucleolus and the nuclear membrane are clearly visible. DNA replication is absent. Spiraling of chromatin does not occur, chromosomes are not detected. The cell retains its inherent functional activity, which almost completely disappears during mitosis. When amitosis, only the core is divided, and without the formation of a spindle of division , so the hereditary material is distributed randomly. The absence of cytokinesis leads to the formation of binuclear cells, which are no longer able to enter into a normal mitotic cycle. With repeated amitoses, multinucleated cells can be formed.
This concept still appeared in some textbooks before the 1980s. At present, it is believed that all phenomena attributable to amitosis are the result of incorrect interpretation of insufficiently prepared microscopic preparations , or interpretation as cell division of phenomena accompanying cell destruction or other pathological processes. At the same time, some variants of eukaryotic nuclear division cannot be called mitosis or meiosis. Such, for example, is the division of macronuclei of many ciliates , where segregation of short fragments of chromosomes occurs without the formation of a spindle.
Mitosis
Mitosis ( μίτος - “warp thread”) is an indirect cell division , the most common way of reproducing eukaryotic cells . The biological significance of mitosis consists in the strictly identical distribution of chromosomes between the daughter nuclei , which ensures the formation of genetically identical daughter cells and maintains continuity in a number of cell generations [5] .
Mitosis is one of the fundamental processes of ontogenesis . Mitotic division ensures the growth of multicellular eukaryotes by increasing the tissue cell populations. As a result of mitotic cell division of meristem cells, the number of plant tissue cells increases. The crushing of the fertilized egg and the growth of most tissues in animals also occurs through mitotic divisions [6] .
Based on morphological features, mitosis is conditionally divided into stages: prophase, prometaphase, metaphase, anaphase, telophase. The first descriptions of the phases of mitosis and the establishment of their sequence were undertaken in the 70-80s of the XIX century . In the late 1870s - early 1880s, the German histologist Walter Fleming introduced the term “mitosis” to refer to the process of indirect cell division [7] .
The duration of mitosis on average is 1-2 hours [8] . Mitosis of animal cells, as a rule, lasts 30-60 minutes, and plants - 2-3 hours. For 70 years, about 10 14 cell divisions in total have been carried out in the human body [9] .
Meiosis
Meiosis (from the ancient Greek μείωσις - “reduction”) or reduction cell division - division of the nucleus of a eukaryotic cell with a decrease in the number of chromosomes by half. It takes place in two stages (reduction and equational stages of meiosis). Meiosis should not be confused with gametogenesis — the formation of specialized germ cells , or gametes , from undifferentiated stem cells .
With a decrease in the number of chromosomes as a result of meiosis in the life cycle , a transition from the diploid phase to the haploid phase occurs. The restoration of ploidy (the transition from the haploid phase to the diploid phase) occurs as a result of the sexual process .
Due to the fact that in the prophase of the first, reduction stage, pairwise fusion (conjugation) of homologous chromosomes occurs, the correct flow of meiosis is possible only in diploid cells or in even polyploids (tetra, hexaploid, etc., cells). Meiosis can also occur in odd polyploids (tri-, pentaploid, etc.) cells, but because of the inability to provide a pairwise chromosome fusion in prophase I, chromosome discrepancies occur with disorders that threaten the viability of the cell or the developing from it a multicellular haploid organism.
The same mechanism underlies the sterility of interspecific hybrids . Since interspecific hybrids in the nucleus of cells combine the chromosomes of parents belonging to different species, chromosomes usually cannot enter into conjugation. This leads to disturbances in chromosome discrepancies in meiosis and, ultimately, to the non-viability of germ cells, or gametes . Chromosome rearrangements (large-scale deletions , duplications , inversions or translocations ) also impose certain restrictions on the conjugation of chromosomes.
Dispute propagation
Often, asexual reproduction of bacteria is preceded by the formation of spores. Bacterial spores are resting cells with reduced metabolism, surrounded by a multilayer membrane, resistant to drying and other unfavorable conditions that cause the death of ordinary cells. Sporulation serves both for experiencing such conditions and for settling bacteria: once in a suitable environment, the spore germinates, turning into a vegetative (dividing) cell.
Asexual reproduction with unicellular spores is also characteristic of various fungi and algae . Spores in many cases are formed by mitosis (mitospores), and sometimes (especially in fungi) in large quantities; during germination, they reproduce the maternal organism. Some fungi, such as the malicious pest of phytophthora plants, form motile spores with flagella, called zoospores or strollers. Having melted in droplets of moisture for some time, such a vagabond “calms down”, loses its flagella, becomes covered with a dense shell and then, under favorable conditions, germinates.
Vegetative reproduction
Vegetative reproduction is the formation of a new individual from the multicellular part of the body of the parent individual, one of the methods of asexual reproduction characteristic of multicellular organisms. In algae and fungi, it occurs through the separation of unspecialized sites of thallus or through the formation of specialized sites (brood buds of algal flora and others).
In higher plants, occurs as the disintegration of the maternal individual into two or more daughter individuals (for example, when creeping shoots or rhizomes die off, separating root shoots ), or as separating the seedlings of the daughter offspring from the maternal individual (for example, tubers , bulbs , brood buds ).
In some plants, shoots ( willow leaves ) or leaves that have separated from the mother plant can take root [10] . In animals, vegetative propagation (which zoologists often call asexual) is carried out either by division or by budding.
Vegetative propagation is based on processes similar to regeneration ; As a rule, in the absence of the ability to regenerate this group of organisms (for example, rotifers , nematodes , leeches ) is absent and vegetative reproduction, and in the presence of developed regenerative capacity ( annelids , hydroids , flatworms , echinoderms ), vegetative reproduction is also found.
Budding
Budding is a type of asexual or vegetative reproduction of animals and plants, in which daughter individuals are formed from the outgrowths of the body of the maternal organism (kidneys). Budding is characteristic of many fungi , liver mosses and animals ( protozoa , sponges , intestinal cavities , some worms , tunicates , some flagellates , hydras , sporoceans ). In a number of animals, budding does not reach the end, the young individuals remain connected to the maternal organism. In some cases, this leads to the formation of colonies .
Some species of unicellular characteristic of such asexual reproduction, as budding . In this case, the mitotic division of the nucleus occurs . One of the nuclei formed is transferred to the emerging local protrusion of the mother cell , and then this fragment buds. The daughter cell is significantly smaller than the maternal cell, and it takes some time for the growth and completion of the missing structures, after which it takes on the appearance of a mature organism. Budding is a type of vegetative reproduction. Many lower fungi , such as yeast and even multicellular animals, such as freshwater hydra, multiply by budding. When yeast buds, a thickening is formed on the cell, gradually turning into a full-fledged yeast daughter cell. On the body of the hydra, several cells begin to divide, and gradually a small hydra grows on the mother, in which a mouth with tentacles and an intestinal cavity are formed , which is connected with the intestinal cavity of the "mother".
Fragmentation (division of the body)
Some organisms can multiply by dividing the body into several parts, and from each part grows a full-fledged organism, in everything similar to the parent individual ( flat and ringed worms , echinoderms ).
Sexual reproduction
Sexual reproduction is associated with the sexual process (cell fusion), and, in the canonical case, with the fact of the existence of two complementary sex categories ( male organisms and female organisms).
During sexual reproduction, the formation of gametes , or germ cells, occurs. These cells have a haploid (single) set of chromosomes. Animals possess a double set of chromosomes in normal (somatic) cells, therefore gamete formation in animals occurs during the process of meiosis . In many algae and all higher plants, gametes develop in the gametophyte, which already possess a single set of chromosomes, and are obtained by simple mitotic division.
According to the similarity-difference between the emerging gametes, several types of gamete formation are distinguished between themselves:
- isogamy - gametes of the same size and structure, with flagella
- anisogamy - gametes of various sizes, but of similar structure, with flagella
- oogamy - gametes of various sizes and structures. Small ones that have male flagellar gametes are called spermatozoa , while large ones that do not have flagellated female gametes are called ova .
When two gametes merge (in the case of oogamy, the gamete of different types of gametes must be merged) a zygote is formed, which now has a diploid (double) set of chromosomes. From the zygote, a daughter organism develops, whose cells contain genetic information from both parents.
Hermaphroditism
An animal that has both male and female gonads is called a hermaphrodite (on behalf of Hermaphrodite , a mythical bisexual creature). Hermaphroditism is widespread among lower animals and to a lesser extent among higher animals. A similar feature in plants is called monoeciousness (as opposed to dioeciousness) and is associated with the overall evolutionary advancement of the species to a lesser extent than in animals.
Parthenogenesis and apomixis
Parthenogenesis is a special type of sexual reproduction, in which a new organism develops from an unfertilized egg, thus the exchange of genetic information does not occur, as it does in asexual reproduction. A similar process in plants is called apomixis .
Parthenogenesis, which is a sexual, but same-sex type of reproduction, arose in the process of evolution in dioecious organisms. In those cases when some species are represented only by females (always or periodically), one of the main biological advantages of parthenogenesis is to accelerate the rate of reproduction of the species. Same-sex taxa breeding parthenogenetically often occupy the periphery of species ranges , where hybridization and competition with bisexual populations do not prevent the establishment and distribution of same-sex (female) populations [11] . Parthenogenesis has been described for aphids , daphnids , lizards , some fish, and other animals [12] . Parthenogenesis does not occur in mammals in which parthenogenetic embryos die in the early stages of embryogenesis [13] .
During parthenogenesis, the egg can be haploid and diploid. When developing from a haploid egg cell, developing individuals can only be males, females only, or both, depending on the sex determination mechanism. For example, in bees , parasitic wasps , worms , ticks, males emerge from an unfertilized haploid egg cell. Parthenogenesis can be permanent or cyclic. In daphnia, aphids, rotifers, parthenogenetic generations alternate with sexual ones. In Daphnia in particular, females are diploid, and males are haploid. Under favorable conditions, daphnia does not have meiosis , and the eggs remain diploid. They develop without fertilization and give rise only to females. In adverse conditions, females start laying haploid eggs from which males hatch. As a result of the sexual process, diploid zygotes are formed, again giving rise to females [14] .
Progenesis
Progenesis is gametogenesis in the larval stage. It is subdivided into:
- Neoteny - a temporary delay in the development of the organism at the larval stage, with the acquisition of the ability to sexual reproduction. For example, in some species of ambystomas, amphibians from the Ambystomaceae family ( Ambystomidae ) of the caudate order ( Caudata ), the neothenic axolotl larva is formed due to a hereditary deficiency of the hormone thyroidin.
- pedomorphosis - neoteny with a complete loss of ability to metamorphosis. Occurs in caudate amphibians of the Protea family.
- pedogenesis - parthenogenetic reproduction in the larval stage, often it occurs when the embryo is still in the mother's body. This type of reproduction is characteristic of some arthropods and parasitic flatworms - trematodes , which use it to significantly increase the number of descendants in a short time.
Alternation of generations
In many algae , in all higher plants , in some of the simplest and intestinal cavities in the life cycle alternation of generations reproduces, respectively, sexually and asexually , is metagenesis . Heterogony is observed in some worms and insects - alternation of different sexual generations, for example, alternation of dioecious generations with hermaphroditic , or with breeding parthenogenetic .
Generational alternation of plants
The gametophyte develops from a spore , has a single set of chromosomes and has organs of sexual reproduction - gametangia . In various organisms, male gametangia, that is, producing male gametes, are called antheridia , and female organisms are called arhegonia. Since the gametophyte, like the gametes it produces, has a single set of chromosomes, the gametes are formed by simple mitotic division.
At gamete fusion, a zygote is formed, from which the sporophyte develops. The sporophyte has a double set of chromosomes and carries organs of asexual reproduction - sporangia . In multi-spore organisms, male gametophytes develop from microspores , carrying only antheridia, and from megaspores , female ones. Microspores develop in microsporangia , megaspores in megasporangia . When sporogenesis occurs, meiotic reduction of the genome occurs, and a single set of chromosomes characteristic of the gametophyte is restored in disputes.
The evolution of breeding
The evolution of reproduction proceeded, as a rule, in the direction from asexual to sexual forms, from isogamy to anisogamy, from the participation of all cells in reproduction to the division of somatic and sexual cells, from external fertilization to internal with pre-natal development and care for offspring.
The rate of reproduction, the number of offspring, the frequency of change of generations, along with other factors determine the rate of adaptation of the species to environmental conditions. For example, high rates of reproduction and frequent change of generations allow insects to develop resistance to pesticides in a short time. In the evolution of vertebrates - from fish to warm-blooded - there is a tendency to reduce the number of offspring and increase its survival.
See also
- Parthenogenesis
- Cloning
Notes
- ↑ Reproduction - an article from the Great Soviet Encyclopedia .
- ↑ Benjamin Lewin. Chapter 13: The replicon // Genes VIII. - Upper Saddle River, NJ: Pearson Prentice Hall, 2004. - ISBN 0131439812 .
- ↑ de Boer PA. Advances in understanding E. coli cell fission (Neopr.) // Curr Opin Microbiol .. - 2010. - V. 13 . - p . 730-737 . - DOI : 10.1016 / j.mib.2010.09.015 . - PMID 20943430 .
- ↑ mitosis: Meiosis and Amitosis . The Columbia Electronic Encyclopedia , 6th edition (2007). The appeal date is September 9, 2011. Archived February 26, 2013.
- ↑ Biological encyclopedic dictionary / Ch. Editor Gilyarov M. S .. - M .: Owls. encyclopedia, 1986. - 831 p. - 100 000 copies
- ↑ Gilbert S. Developmental Biology: in 3 volumes. - M .: "The World ", 1995. - T. 3. - 352 p. - 5000 copies - ISBN 5-03-001833-6 .
- ↑ History of biology from ancient times to the beginning of the XX century / Edited by S. R. Mikulinsky . - M .: " Science ", 1972. - 564 p. - 9600 copies
- ↑ Alberts, B. , et al. Molecular biology of the cell: In 3 tons. - 2nd, processed. - M .: “ Peace ”, 1993. - T. 2. - 539 p. - ISBN 5-03-001987-1 .
- ↑ Buldakov L. A., Kalistratova V. S. Radioactive radiation and health . - M .: Inform Atom, 2003. - 165 p. Archive dated December 2, 2003 on Wayback Machine
- ↑ Stalk // Encyclopedic Dictionary of Brockhaus and Efron : 86 t. (82 t. And 4 extra.). - SPb. , 1890-1907.
- ↑ Viktorov A. G. Geographical parthenogenesis in island insects (Rus.) // Nature . - Science , 2006. - № 9 . - pp . 82-83 .
- ↑ Aslanyan, Soldatova, 2010 , p. 17-19.
- ↑ Biological encyclopedic dictionary / GL.red. M.S.Gilyarov. - M .: Owls. encyclopedia, 1986. - 831 p.
- ↑ Inge-Vechtomov, 2010 , p. 221-223.
Literature
- Aslanyan M.M., Soldatova O.P. Genetics and Genesis. A manual for students studying in the direction of "biology". - M .: Author's Academy; Fellowship of scientific publications KMK, 2010. - 114 p. - ISBN 978-5-91902-001-1 .
- Inge-Vechtomov S. G. Genetics with the basics of selection. - SPb. : Publisher NL, 2010. - 718 p. - ISBN 987-5-94869-105-3.
Links
- Reproduction : Wikimedia Commons
- Reproduction - an article from the Great Soviet Encyclopedia .