Genetic elements

Over 4 decades, between 1960 and 2000, the development of new antibiotics used well characterized basic structures for partial synthetic modifications, primarily to overcome resistance by increasing the pharmacodynamic properties and, secondarily, to improve the pharmacokinetic profile of older compounds. However, bacteria rapidly responded by acquiring additional genetic alterations either as mutations or by accumulating resistance genes as part of mobile genetic elements ( integrons) on transferable resistance plasmids. 

Transposons are mobile DNA elements (sizes 2.5-23 kbp) that move from one place to another in the chromosome or onto extrachromosomal genetic elements within the same cell. They are flanked by inverted repeats at then-ends and encode among other proteins a transposase that is needed for the transposition process. Resistance genes in the transposon are often parts of integrons. These are structures that cany an integrase responsible for the insertion of the resistance gene cassettes into the integron. 

Three genetic elements are responsible for acquired resistance chromosomes, plasmids and trarrsposons (Lewis 1989). Each of these will be considered in tiun. 

Bacterial resistance to biocides (Table 13.2) is usually considered as being of two types (a) intrinsic (innate, natural), a natural property of an organism, or (b) acquired, either by chromosomal mutation or by the acquisition of plasmids or transposons. Intrinsic resistance to biocides is usually demonstrated by Gram-negative bacteria, mycobacteria and bacterial spores whereas acquired resistance can result by mutation or, more frequently, by the acquisition of genetic elements, e.g. plasmid- (or transposon-) mediated resistance to mercury compounds. Intrinsic resistance may also be exemplified by physiological (phenotypic) adaptation, a classical example of which is biofilm production. 

In order for any gene to synthesize a protein, it must contain certain genetic elements such as promoter and terminator sequences. These regulatory regions signal where the DNA sequence that encodes a product (i.e., a gene) begins and ends. The recombinant... 

Nojiri H, Shintani M, Omori T (2004) Divergence of mobile genetic elements involved in the distribution of xenobiotic-catabolic capacity. Appl Microbiol Biotechnol 64 154-174... 

Top EM, Springael D (2003) The role of mobile genetic elements in baderial adaptation to xenobiotic organic compounds Curr Opin Biotechnol 14 262-269... 

Springael D, Top EM (2004) Horizontal gene transfer and microbial adaptation to xenobio-tics new types of mobile genetic elements and lessons from ecological studies. Trends Microbiol 12 53-58... 

Top EM, Springael D, Boon N (2002) Catabolic mobile genetic elements and their potential use in bioremediation of polluted soils and waters. FEMS Microbiol Ecol 42 199-208... 

Fusion of human lymphocytes with human lymphoblastoid cell lines is a very inefficient process. Fusion of human lymphocytes with murine myeloma cells lead to very unstable hybrids. Upon fusion, preferential loss of human genetic elements is often observed. Unfortunately, particularly common is the loss of chromosomes 2,14 and 22, which encode antibody light and heavy chain loci. The production yields of human monoclonals upon immortalization of the human B-lymphocyte (by whatever means) are also low. 

RNAi probably evolved initially in primitive organisms in order to protect their genomes from viruses, transposons and additional insertable genetic elements, and to regulate gene expression. The RNAi pathway was first discovered in plants, but it is now known to function in most, if not all, eukaryotes. 

Radiation causes dominant lethal mutations in the medaka (Oryzias latipes) (Shima and Shimada 1991). Mosquitofish (Gambusia spp.) from radionuclide-contaminated ponds in South Carolina differed from conspecifics in reference ponds, as judged by the frequency of DNA markers, and this is consistent with the hypothesis that these DNA markers may originate from genetic elements that provide a selective advantage in contaminated habitats (Theodorakis et al. 1998). Ionizing radiation at low-level chronic exposure reportedly has no deleterious genetic effects on aquatic populations because exposure is compensated by density-dependent responses in fecundity (IAEA 1976). However, this needs verification. 

The endoparasite C. sonorensis has evolved with the ability to generate extrachromosomal genetic elements in the form of multiple double-stranded, superhelical DNA molecules. These DNA molecules are amplified in the calyx cell nucleus, packaged into viruses, and secreted in a complex process of viral maturation, which also provides a complex double viral envelope. One viral envelope is assembled in the cell nucleus, and the other is obtained during budding from the calyx cell surface into the oviduct lumen. Viral envelopes, which are derived from cellular membranes, may mediate species-specific virus host cell and tissue interactions. This could be one important aspect of the species-specific endoparasite-host relationship fundamental to parasite survival. 

Uptain, S. M., and Lindquist, S. (2002). Prions as protein-based genetic elements. Annu. 

Plasmids and DNA Repair. Plasmids are extrachromosomal genetic elements that are composed of circular double-stranded DNA. In bacteria some can mediate their own transfer from cell to cell by conjugation they contain a set of Ira genes coding for tube-like structures, such as pili, through which a copy of plasmid DNA can pass during transfer. Plasmids range in size from 1.5 to 200 million daltons. The number... 

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