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x TIOUS DISEASE BACTERIOLOGY IMMUNOLOGY MYCOLOGY PARASITOLOGY VIRO O LECTURE BACTERIOLOGY - CHAPTER EIGHT EXCHANGE OF GENETIC INFORMATION Dr. Gene Mayer SPANISH ow what you think EDBACK EARCH KMARK PRINT THIS PAGE rey Nelson, Rush University, s and The MicrobeLibrary G: Murray et al. robiology, 6th Ed., hapter 3 G OBJECTIVES he mechanisms of nsfer in bacteria be the nature of e genetic elements d plasmids the significance of fer, transposable ents and plasmids I. INTRODUCTION In bacterial populations mutations are constantly arising due to errors made during replication. If there is any selective for a particular mutation (e.g. antibiotic resistance), the mutant will quickly become the major component of the popula the rapid growth rate of bacteria. In addition, since bacteria are haploid organisms, even mutations that might normall recessive will be expressed. Thus, mutations in bacterial populations can pose a problem in the treatment of bacterial Not only are mutations a problem, bacteria have mechanisms by which genes can be transferred to other bacteria. Th mutation arising in one cell can be passed on to other cells. Gene transfer in bacteria is unidirectional from a donor cell to a recipient cell and the donor usually gives only a small DNA to the recipient. Thus, complete zygotes are not formed; rather, partial zygotes (merozygotes) are formed. Bacterial genes are usually transferred to members of the same species but occasionally transfer to other species can Figure 1 illustrates gene transfers that have been shown to occur between different species of bacteria.
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S on e recombination n transduction transduction onversion e genetic element quence recombination ion njugative plasmid ive plasmid nt II. GENE TRANSFER MECHANISMS IN BACTERIA A. Transformation Transformation is gene transfer resulting from the uptake by a recipient cell of naked DNA from a donor cell. Certain b ( e.g. Bacillus, Haemophilus, Neisseria, Pneumococcus) can take up DNA from the environment and the DNA that is ta be incorporated into the recipient's chromosome. 1. Factors affecting transformation a. DNA size state Double stranded DNA of at least 5 X 10 5 daltons works best. Thus, transformation is sensitive to nucleases in the env b. Competence of the recipient Some bacteria are able to take up DNA naturally. However, these bacteria only take up DNA a particular time in their cycle when they produce a specific protein called a competence factor. At this stage the bacteria are said to be compe bacteria are not able to take up DNA naturally. However, in these bacteria competence can be induced in vitro by trea chemicals ( e.g. CaCl 2 ). 2. Steps in transformation a. Uptake of DNA Uptake of DNA by Gram+ and Gram- bacteria differs. In Gram + bacteria the DNA is taken up as a single stranded mo the complementary strand is made in the recipient. In contrast, Gram- bacteria take up double stranded DNA. b. Legitimate/Homologous/General Recombination After the donor DNA is taken up, a reciprocal recombination event occurs between the chromosome and the donor DN recombination requires homology between the donor DNA and the chromosome and results in the substitution of DNA the recipient and the donor as illustrated in Figure 2.
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