a branch of general genetics that studies bacteria, microscopic fungi, actinophages, animal and plant viruses, bacteriophages, and so on.
Until the 1940’s it was thought that microorganisms did not have a nuclear apparatus or undergo meiosis and that therefore Mendel’s laws and the chromosomal theory of heredity did not apply to them. In the 1940’s, however, microorganisms became the object of intensive genetic research. They were useful in solving many major problems in modern genetics. For example, the first indication that deoxyribonucleic acid (DNA) is the material carrier of heredity was obtained in experiments on pneumococci (the American geneticists O. T. Avery, C. MacLeod, and M. McCarty). Intensive research was begun at about the same time on the bread mold Neurospora. Study of numerous biochemical mutants of Neurospora (by G. W. Beadle and E. L. Tatem, USA) led to the formulation of a very important principle: “one gene, one enzyme.” (This principle is more precisely formulated today as “one gene, one polypeptide chain.”)
Genetic research on microorganisms accelerated after the American geneticists S. Luria and M. Delbrück demonstrated in the colon bacillus (Escherichia coli) that bacteria obey the laws of mutation. The earlier concept of adequate, adaptive variation in bacteria arose from the methodological error of studying a culture as a unit of variation. A new principle, clonal analysis, was proposed—the study of the descendants of a single cell, the progenitor of the clone. An important benchmark in the history of microorganism genetics was the replication or imprint method devised by the American geneticists J. and E. Lederberg. With this method it was possible to show that mutations arise in bacteria regardless of the conditions of cultivation. It also greatly simplified the task of breeding variants of microorganisms with the desired properties. It was found that mutations arise spontaneously in large populations of bacterial cells.
The discovery of a sexual process in bacteria (conjugation) in 1946 made it possible to use genetic analysis to investigate them. It resulted in the detection of recombination and genetic linkage groups in bacteria and the construction of genetic maps of their chromosomes. A parasexual process discovered in fungi (G. Pontecorvo, Great Britain) at almost the same time broadened the possibilities of genetic analysis of fungi lacking in a sexual cycle of reproduction. Soon bacteriophages and other viruses (notably, tobacco mosaic virus, TMV) were used in genetic research. It “Was discovered that genetic information can be transmitted from one bacterial cell to another by means of bacteriophagic genetic transduction. This laid the foundation for the study of genetic interrelations in the so-called phage-bacteria system (J. Lederberg and N. Zinder, USA). Recombination was subsequently found in bacteriophages (A. Hershey and M. Delbrück, USA). While the use of bacteria as an object of genetic research greatly increased the resolving power of genetic analysis, bacteriophages made it possible to study the phenomena of heredity at the molecular level. Of great value was the research on TMV (the German geneticists H. Schuster and A. Gierer), which made it possible to induce a genetic effect in experiments with pure ribonucleic acid (RNA). The latter remained infectious and when applied to tobacco leaves produced regular TMV particles in the cells.
Special research methods were devised for each group of microorganisms, with due regard for their characteristics, in accordance with the general principles of investigating genetic processes in microorganisms.
The genetic mechanisms in fungi and algae that preserved the sexual process have some unusual features. The main one is that the products of meiosis (spores) remain joined together in a definite order, and after the spores are cultured separately, the genotype of each product of meiosis can be studied directly. This method, called tetrad analysis, supplements the statistical methods of studying the segregation process. The application of genetic analysis to organisms lacking in the sexual process became possible after they were found to have a great variety of parasexual processes. For example, when the hyphae belonging to two genetically different strains of imperfect fungi grow together, the two haploid nuclei unite and then coalesce into ā single diploid nucleus. Genetic material can occasionally be exchanged in this system.
A peculiarity of the sexual process in bacteria is the fact that generally only part of the genetic material is transmitted from the donor cell to the recipient cell, giving rise to a partly diploid zygote (the so-called merozygote). Bacteria are known to have several mechanisms for transmitting genetic material. The most complete form of the sexual process in bacteria, conjugation, was studied in detail in E. coli. Conjugation takes place during direct contact between two cells if one of them contains the specific sex, or crossability (fertility), factor. The sex factor contains DNA and can exist either autonomously or integrated into the cell genome. In the former case, only the sex factor passes into the recipient cell during conjugation; in the latter, the sex factor promotes the directed transfer of genetic material from the donor cell to the recipient cell. As a rule, only part of the donor’s genome is transmitted; the entire chromosome of the donor together with its sex factor is transmitted extremely rarely. Homologous genetic segments may be exchanged between the fragment of donor DNA and recipient DNA. Such a “crossing-over” results in the appearance of recombinants, that is, of cells with the altered combination of characters. Genetic analysis of E. Coli recombinants revealed the presence of a linkage group and a linear arrangement of a large number of genes in its chromosome. It also permitted the construction of a ring-shaped genetic map.
The transfer of genetic material during conjugation is a strictly determined process whereby the sequence in which genes are transmitted (as well as the probability of their participating in a crossing-over) is dependent on the location of the genes in the chromosome and the site of integration (inclusion) of the sex factor. When the sex factor becomes autonomous, the genes located in the chromosome next to the integration site may combine with the sex factor and subsequently be transmitted with it as a single entity, converting the recipient cells into diploids according to the given genetic segment. This process of gene transfer with the sex factor, called sexduction, may also give rise to recombinants.
Transduction, another mechanism that gives rise to recombinants in bacteria, is stimulated by so-called moderate bacteriophages that have the capacity for a specific type of symbiosis with bacteria, lysogenesis. In lysogenic bacteria, the DNA of moderate bacteriophages is integrated with the DNA of the bacterial cell and is replicated simultaneously with it. This latent form of bacteriophages (prophage) may survive through many cellular generations, but from time to time it changes into the vegetative state (that is, it begins to reproduce) and destroys the bacteria. In doing so, it may capture a small fragment of the host cell DNA and subsequently be transferred to another cell in which the transferred portion of the genome may enter into a genetic exchange with the homologous region of the recipient cell. The genes situated next to the site of prophage in the chromosome of the bacteria are usually transmitted during transduction. However, some bacteriophages effect a transduction during which any portion of the bacterial genome can be transferred to another cell with equal probability. Sometimes the lysogenization process itself—that is, the incorporation of moderate bacteriophages into the bacterial genome —may result in the cell’s acquiring new properties, such as virulence.
Still another type of sexual process in bacteria, called transformation, involves the transfer of genetic material not mediated by the sex factor or moderate bacteriophages and the subsequent appearance of recombinants (because of genetic exchange between the DNA fragment that penetrated into the cell and the recipient cell DNA).
The characteristics of the genetic mechanisms of bacterial viruses (bacteriophages) make them a very convenient model for studying the reproduction and functioning of genetic material. They are very simply constructed, reproduce quickly, and have a very short life cycle. Hence the genetics of bacteriophages, especially T2, T4, and λ, has been studied in great detail. Bacteriophages are crossed and bacteria are infected with a mixture of two or more variants, in which event not only the original phage particles but also recombinants with altered combinations of characters appear. The use of recombination analysis made it possible to construct genetic maps for several bacteriophages. The molecule of bacteriophage DNA was found to be its chromosome. Study of gene ultrastructure in T4 phage (S. Benzer, USA) showed that there are many portions within a gene capable of changing (mutating) with different frequency when exposed to a variety of mutagens.
The genetics of animal and plant viruses is largely based on progress made in bacteriophagic genetics, but owing to technical difficulties it has not yet developed adequately. The possibility of obtaining recombinants was demonstrated in DNA viruses of the smallpox-vaccinia group (with mixed infection of cells by different members of the group), in herpes virus (between different variants of this virus), and between the tumorigenic simian SV40 virus and different representatives of the adenoviruses. The possibility of obtaining recombinants between mutants of foot-and-mouth disease and poliomyelitis viruses as well as between variants of influenza viruses was demonstrated in RNA viruses. The last discovery is particularly important in that it shows the possible ways in which this virus varies under natural conditions.
Among the plant viruses, tobacco mosaic virus (TMV) has been the most studied. Specifically, the sequence of amino acids in TMV protein was completely decoded. The nature of the amino acid substitutions occurring in membrane proteins was established in various TMV mutants. The work done on TMV was an important contribution to the elucidation of both the mechanism of mutagenesis and the nature of the genetic code.
The emergence of a new branch of the economy, the microbiological industry, gave rise to applied microorganism genetics, which is also called selection of microorganisms. New forms of microorganisms were investigated: penicillia (Penicillium chrysogenum), actinomycetes (Actinomyces streptomycini and A. rimosus, for example), and actinophages. A parasexual process was discovered in penicillia and aspergilli; the mechanism of recombination was studied in actinomycetes; and genetic recombination in actinophages and genetic transduction in actinomycetes were discovered. Extensive research was conducted on induced variation of the quantitative characteristics in actinomycetes.
Breeders of microorganisms in the Soviet Union also use such genetic methods as induction of mutations, hybridization, and infection of actinomycetes with actinophages. The highly active strains obtained have led to a manifold increase in the production of antibiotics, amino acids, vitamins, and other biological active substances.
The growing importance of microorganism genetics and the need to expand the microbiological industry resulted in the organization in Moscow in 1968 of the All-Union Institute of Genetics and Breeding of Industrial Microorganisms of the Main Administration for the Microbiological Industry. It is now the leading research center in this field.
The genetics and breeding of microorganisms are also studied in other research organizations in such cities as Moscow, Leningrad, Kiev, and Yerevan. Located in Moscow are the N. F. Gamaleia Institute of Epidemiology and Microbiology of the Academy of Medical Sciences of the USSR, the Institute of General Genetics of the Academy of Sciences of the USSR (AN SSSR), and the I. V. Kurchatov Institute of Atomic Energy of the AN SSSR. Leningrad is the site of the A. F. Ioffe Physics and Engineering Institute of the AN SSSR and the Genetics Department of Leningrad State University.
Microorganism genetics played a major role in the history of modern genetics by supplementing several concepts of the genetics of higher organisms. Microorganism genetics, in turn, became the basis for progress in molecular genetics.
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