TRANSFORMATION

From Big Medical Encyclopedia

TRANSFORMATION transformatio genetic (Latin transformation) — change of hereditary properties of a cell as a result of transfer in it genetic material (DNA, chromosomes or genes) other cell. T. leads to what the cell recipient gets and steadily transfers to the descendants a sign, at it being absent earlier, but being available for a donor cell. T. it was opened at bacteria in 1928 by the English microbiologist F. Griffith. He found out that the cells of a pneumococcus (Dip-lococcus pneumoniae) differing in existence of the dense capsule can transfer property to form the capsule to an acapsular strain of diplococcuses by means of heat-killed cells or sterile extract of them. It was the first example of the directed transfer of the inherited sign (property) not living cells, but their chemical components. In 1944 Mr. of Avery (O. T. of Avery), Mac-Laud (S. M. of McLeod) and the IAC Cards and (M. of McCarty), studying the reason of emergence of virulence in avirulent strains of Diplococcus pneumoniae at simultaneous infection of mice with the inactivated heating, virulent and live avirulent strains of Diplococcus pneumoniae, came to a conclusion that the chemical substance transferring an ancestral feature is DNA (see. Deoxyribonucleic acid). This discovery made during the studying of T. bacteria, exerted huge impact on development of genetics (see) and molecular biology (see). Further it was established that ability to T. not only Diplococcus pneumoniae, but also many other microorganisms possess and that it is an ancestral feature. However this sign is inherent in not all strains of a microorganism of this look; even in sensitive to T. a strain not all cells can be transformed under the influence of exogenous DNA. It demonstrates that T. — complex process, in implementation to-rogo a big role plays a condition of a transforming factor — DNA and a condition of a cell recipient. Efficiency of T. depends on the sizes of molecules DNA, their concentration and on so-called competence of a cell (the short-term condition of a susceptibility of population of cells recipients dated for a certain phase of a cellular cycle).

At concentration of the transforming DNA the number of transformant in direct ratio is lower than 0,1 mkg/ml of its concentration, and at higher content of DNA observe «effect of saturation». T. depends also on a nativnost of DNA (its molecule shall be two-spiral), and also from fiziol. conditions of a cell, napr, from a phase of a cellular cycle, in a cut it is at the time of contact with the transforming DNA. For you. subtilis such phase is an exponential growth phase. The culture which reached a stationary state, i.e. stopped growth practically does not contain the cells capable to T. Obnaruzheno that the nadosadochny liquid received during the centrifuging of competent culture of cells contains the agent, being protein, to-ry is capable to report property of competence to culture of incompetent cells. Believe also that competence to T. it is connected with existence of proteinaceous receptor sites on a surface of cells.

For overseeing of T. genetic markers are used. Can be such markers resistance of cells to streptomycin, erythromycin, streptocides or their ability to use as the only source of carbon a maltose, a mannitol, glycerin, etc. So, culture of the cells which underwent T., through some time term necessary for manifestation of an acquired character, sow on the Wednesday unsuitable for growth of a stock strain. All cells of this strain perish, and transformed grow and breed. This method is very sensitive.

The major stage T. implementation (integration) of a fragment of DNA of a donor cell into a genome of a cell recipient is. Integration happens owing to the known process of genetic exchange — a recombination (see). At T. the fragment of single-stranded DNA of the donor copulates with the homologous site of a chromosome of the recipient with formation of a partial heterozygote. The size of a fragment of DNA of a donor cell integrated into a chromosome of the recipient is small, usually one — two genes. Frequency of joint T. on two genes or genetic markers of subjects above, than it is less than a distance between them. This phenomenon is used for creation of accurate genetic maps of certain sites of chromosomes of bacteria (see the Gene, the Chromosome map).

Attempts of T. eukaryotic cells by means of DNA 20 century were made at the beginning of the 60th. Among the first works, in to-rykh on cells of the person existence of T was rather convincingly shown., it is necessary to call W. Szybalsky and W. Szybalsky's research. As recipients they used the line of the cells steady to 8-azagipoksanti-well. At these cells completely were absent activity of a pirofosfo-rilaza inosinic to - you (inosine-phosphorylase; KF 2.4.2.1) and ability to use hypoxanthine (see) as a source of purines (see. Purine bases). As the donor of DNA served the parent line of the cells steady against 8 azaguanine and metabolizing hypoxanthine. These experiments showed a possibility of T. cells of mammals. However further researchers met the considerable difficulties connected with allocation and purification of DNA, ways of its implementation in a cell and also with methods of registration of an expression of the transforming DNA. It turned out that the exogenous DNA entered into cells of mammals shall be protected from the destroying action of cellular nucleases (see) that was reached by use of polyanions (a polylysine, polyarginine, spermine, etc.), and also use calcium - the DNA phosphatic precipitated calcium superphosphate. Unlike the transforming DNA of prokariot in experiences with eukaryotic cells of DNA there has to be as much as possible a vysokopolimerna. Development of methods of cytology allowed to carry out transformation of cells by means of the separate allocated chromosomes. The T is especially effective. with use of vectors when the site of DNA of a donor cell is built in a plasmid of a bacterium (see Plasmids) or a genome of a virus (see the Genome). In such state the transforming DNA is protected from effect of hydrolases, and the vector provides an effective recombination with a chromosome of a cell recipient. Advantage of the vector equipment is also that the vector can be multiplied in bacterial cells. In the last quarter of 20 century the phenomenon of genetic T. consider as a part of more common problem of trans-species transgenesis (see. Genetic engineering).

Bibliography: X e y from U. Genetik to a bakta

of riya and bacteriophages, the lane with English, M., 1965; Avery O. T.,

MacLeod G. M of a. McCarty of M. of Studies on chemical nature of substance inducing transformation of pneumococcal types, J. exp. Med., v. 79, p. 137, 1944; Griffith F. Significance of pneumococcal types, J. Hyg. (Lond.), v. 27, p. 113, 1928; Szybalska E. H. a. S z y b a 1 s k i W. Genetics of human cell lines, IV. DNA-me-diated heritable transformation of a biochemical trait, Proc. nat. Acad. Sci. (Wash.), v. 48, p. 2026, 1962. To. H. Greenberg.

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