Studies the phenomena of transmission of hereditary traits in Bacteria and viruses, so it is more correct to speak of genetics of microorganisms. This field of research has assumed considerable importance because of the practicality with which it is possible to conduct work on living material and because of the very large number of individuals on which it is possible to experiment. These possibilities allow the discovery of genetic events that in other living things cannot be determined in vivo and that, often, are deferred in time so they become extremely rare to observe. It takes only a few agar plates (a kind of gelatin) to cultivate and assay billions of generations of Bacteria in a very short time; thus it has been established that they are capable of recombining their genetic material by three mechanisms: transformation, transduction and conjugation.
The latter has been noticed mainly among colibacilli (Escherichia coli). If two cultures labeled, for example, with P+R genes one and PR+ genes the other are mixed, recombinant strains such as PR and P+R+ can be obtained. Today the main features of this phenomenon are clear, and it is known that when two bacteria conjugate, continuity is established between them by means of cytoplasmic bridges. At this point the injection of genetic material from one bacterium to the other takes place. It is customary to denote by F+ the donor bacterium (male) and by F– the recipient bacterium (female). The bacterium that received the genetic material carries, for a time, a diploid set-up that subsequent division by cleavage changes back to haploid. Now in bacteria derived by cleavage from a bacterium that had behaved as F– there may be recombinant types present, if crossing-over has occurred.
Based on the above example, if P and R result in the inability to synthesize two substances “q” and “s,” the P+R strain of bacteria grows only if “s” is present in the laboratory culture, and the PR+ strain grows only if “q” is present. Among recombinant types PR grows only if both “q” and “s” are present, while P+R+ grows well even on normal (called minimal) culture medium in which both substance “q” and “s” are absent. It is thus possible to trace, from the frequencies of occurrence of the recombinant strains, the distance of the two P and R gene loci and locate (map) them on the chromosome of the Bacteria under investigation. Another method used to map the chromosome of Bacteria is one that relies on the time required for the transfer of gene complexes from F+ to F–. In fact, the chromosome takes a certain amount of time to accomplish this step, and it is possible, by interrupting conjugation, for example by violent agitation, to determine the time required for the transfer of the various gene complexes. It has been conceded that the chromosome of Bacteria is circular and can open up (by the intervention of enzymes capable of cutting DNA) to initiate transfer now at one point now at another.
This hypothesis was made because of the fact that it is not always the same gene complex that is transferred first. The electron microscope later confirmed this model because of the possibility it gave of photographing the various stages of the recombination mechanism under consideration. The enormous importance, from a practical point of view, of bacterial genetics lay until recently only in the possibility of obtaining strains of microorganisms capable of producing ever new types of antibiotics. Research in recent years, shedding new light on the concept of genes and the way they operate, has also opened up new possibilities for the study and treatment (gene therapy) of certain hereditary diseases and cancers.
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