REPRODUCTION

From Big Medical Encyclopedia

REPRODUCTION — process of reproduction of new individuals; the general property of all live organisms, to-rogo is the cornerstone ability of nucleic acids (carriers of genetic information) to self-doubling. At the expense of R. maintenance and (or) increase in number of individuals of population and types among generations is carried out (see).

There are various classifications of types P. The tsitogoniya — reproduction by division of separate cells on the basis of a mitosis and meiosis belongs to the first type (see. Cell division ), in a cut distinguish: an agamogoniya, or asexual reproduction (see) — the cell fission without the subsequent merge leading to formation of affiliated individuals (i.e. reproduction without formation of gametes), and a gamogoniya, or sexual R. — cell fission with the subsequent merge of two and more cells (i.e. reproduction with formation of gametes). Treats the second type P. also R. by department of group of cells (more rare than a part of one cell), a mitosis and meiosis directly do not precede Krom; such form P. is called vegetative reproduction (see).

At an agamogoniya mitotic division of a parent organism on two affiliated can be observed. At the same time at one organisms (an amoeba, diatomic seaweed) hypodispersion of cytoplasm between daughter cells, and at others (shell amoebas, yeast) — uneven is observed, at Krom the affiliated kernel, a part of organoids and a small part of cytoplasm whereas the most part of cytoplasm remains in «mother cell» (the phenomenon of pseudo-budding) passes into a pseudo-kidney.

One of forms of an agamogoniya is the sintomiya — multiple fission of a parent cell on a large number of daughter cells. The option of a sintomiya — a palintomy — is characterized by multiple fission of a cell as a result of a series of the consecutive mitoses which are not followed by increase in cytoplasm at daughter cells. The palintomy is often observed in cysts and is externally quite often similar to crushing of an oospore. At a palintomy education from one oospore of several diploid cells with a smaller volume of cytoplasm is possible, each of which then develops in normal an uniovular germ (polyembriony). Among mammals the palintomy is brightly expressed at battleships; females of a battleship give rise to 4 — 8 same-gender enzygotic twins. Other form of a sintomiya is sporogenesis (Sporozoa, foraminifera, radiolarias, seaweed, mushrooms). At sporogenesis quite often at first a series of mitotic divisions of a kernel without division of cytoplasm follows, and then there occurs isolation of cytoplasm around each of affiliated kernels. This form of sexless R. at parasitic protozoa is quite often called a schizogony. Products of sporogenesis usually have diploid chromosome number and are called agametes. Agametes of foraminifera possess pseudopodiums, agametes of mushrooms and seaweed have flagellums. Mobile agametes of plants usually are called zoospores, spore-forming cells of plants — a sporangium or a zoosporangium. At ferns at agamic sporogenesis disputes are differentiated by the sizes: microdisputes at germination give men's plants, macrospores — female plants. At an agamogoniya existence of individuals of two floors for reproduction is not required, i.e. there is a possibility of reproduction of the only individual that it is essential to parasitic protozoa. However at the same time all descendants are genetically identical each other and to the parents since at an agamogoniya there is no exchange of hereditary information. Ability of many organisms to sexless R. is used at cloning (see. Clone ).

At sexual R. (a gamogoniya, or a gametogonium) unlike sexless R. sex cells, or gametes, with haploid number of chromosomes are always formed (see. Gametogenesis , Gametes ). The Gaploidnost of gametes is reached in process meiosis (see). The conjugation of homologous chromosomes happening during meiosis, one of which is received from a fatherly organism, and the second — from maternal, is closely connected with process of exchange of the genetic material resulting from a crossing-over (decussation of chromosomes) and independent distribution of krossoverny and nekrossoverny chromosomes on 4 gametes which are formed of primitive formative cell (see. Recombination ). The second mechanism providing combinative variability is accidental connection of gametes in the course of sexual R. with formation of a diploid zygote. Sexual R. — the main, the most eurysynusic and evolutionarily the most perfect form of R. -

At sexual R. a necessary condition almost always (except for self-pollination at plants and other forms of autogamy) is a meeting of the gametes coming from two various individuals that provides still big combinative variability. At the lowest groups of organisms such gametes are quite often made by hermaphroditic individuals; at the highest animals in different groups (roundworms, arthropod, vertebral) the razdelnopolost independently is from each other evolutionarily developed (see. Floor ). At plants (see) a razdelnopolost occurs only at gonochoristic plants (hemp, etc.).

Specialized option of sexual R. is the parthenogenesis (see), or virgial reproduction, at ‘Krom an individual develops from an unfertilized gamete.

At a number of systematic groups of live organisms alternation of different forms P. — sexless and sexual, vegetative and sexual, bisexual and parthenogenetic generations is observed (see. Metagenesis ). Genetically and (or) it is exogenous the caused disturbances in a structure and functioning of a reproductive organs (see. Generative organs ) can lead to infertility (see).


Bibliography: Willie K. and D e t e V., Biology, the lane with English, M., 1974; Gartman M. The general biology, the lane with it., M. — L., 1936; Dogel V. A., Clearings and y Yu. I. and X e y with and E. M N. General parasitology, L., 1962; Meyer K. I. Reproduction of plants, M., 1937.

N. N. Vorontsov.

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