Extra chromosomal DNAs

Bacteria normally harbour a single, circular chromosome that tends to be tethered to the bacterial plasma membrane and tends to have few if any closely associated proteins. Many bacteria also contain extra-chromosomal DNA in the form of plasmids, as will be discussed later. Eukaryotes (plants, animals and yeasts) posses multiple linear chromosomes contained within a cell nucleus, and these chromosomes are normally closely associated with proteins termed histones (the pro-tein-DNA complex is termed chromatin). Eukaryotes also invariably possess DNA sequences within mitochondria and in chloroplasts in plants. The (usually circular) DNA molecules are much... 

An essential feature of the cloning vector used is that it must be capable of self-replication in the cell into which it is introduced, which is usually E. coli. Two of the most commonly used types of vector in conjunction with E. coli are plasmids and bacteriophage X. Plasmids are circular extra-chromosomal DNA molecules, generally between 5000 and 350 0000 bp in length, that are found naturally in a wide range of bacteria. They generally house several... 

When genetic information (nucleic acid) is transferred between different cells, species or genera it is often carried by a specialized DNA molecule called a vector. Viruses are natural vectors, as are some kinds of small independently replicating circular extra-chromosomal DNA molecules (plasmids). A few of the basic features of plasmids used in molecular biology are reviewed in Figure 2.6. 

Plasmids are extra-chromosomal DNA, usually circular and double-stranded they occur naturally in bacteria. 

Several of the Kinetoplastida (protozoa which possess a characteristic organelle containing extra-chromosomal DNA, the kinetoplast) can be induced to differentiate in vitro. Both Trypanosoma species and Leishmania species belong to this category, and have proved to be most suitable for studies of the effects of enzyme inhibitors. 

Plasmid A small, circular, independent replicating piece of DNA in a cell that is not part of its chromosome and can be transferred to another cell. Known also as Extra-chromosomal DNA. 

TOL pathway, discovered in P. putida (arvilla) mt-2, is coded in the extra chromosomal DNA, called tol pWWO plasmid. In the initial degradation steps a methyl group is oxidized by a monooxygenase to the benzyl alcohol (Fig. 5.5), which is converted to catechol in the next steps. TOL plasmids in P putida encode the metabolic pathways also for the degradation of xylenes, as well as alcohol and carboxylate derivatives of toluene and xylenes (Assinder Williams, 1990). 

Plasmids are extra-Chrurnosomal DNA molecules present in the cytoplasm of many bacteria, which are able to replicate independently of the chromosome. 

Naked DNA No limit Especially 10-30% of cells Extra chromosomal Lymphocytic Until death of cell Easy and cheap In vivo... 

In addition to their chromosomal DNA, bacteria often carry extra small pieces of DNA as permanent parts of their genome. These plasmids (sometimes called episomes), which are about the size of the DNA of viruses, replicate independently of the host chromosomes. Each bacterial cell usually contains more than a single copy of the plasmid. For example, the "colici-nogenic" plasmid ColEl, that infects E. coli is a circular piece of DNA of molecular mass 4.2 x 106 Da. Over 20 copies are normally found per cell but in the presence of a suitable concentration of the drug chloramphenicol the number may rise to 1000-2000. 

In vitro micronucleus (IVMN, or MN) assay detects extra-nuclear chromatin present in the cell s cytoplasm arisen after cell division as a consequence of chromosome (DNA) breakage (clastogenicity) or lagging chromosome (aneu-genicity). The lagging chromosome is caused by the compound s interference... 

Katakura, K., Peng, Y., Pithawalla, R., Detke, S. and Chang, K. P. (1991) Tunicamycin-resistant variants from five species of Leishmania contain amplified DNA in extra-chromosomal circles of different sizes with a transcriptionally active homologous region. Mol. Biochem. Parasitol. 44 233-244. 

The transport of the therapeutic transgenes toward the nuclei of the target cells can be carried out both by viral [19] and nonviral vectors such as plasmid DNA [20]. Plasmid DNA molecules are extra-chromosomal carriers of genetic information that have the ability to replicate autonomously. These vectors constitute an attractive gene transfer system because they are safer and easier to produce when compared with viral vectors [20-22]. However, plasmid DNA vectors are less effective in transfecting cells when compared with viral vectors, which have a natural ability to deliver and express their genes in a wide variety of cell types and tissues [11,23]. 

This may be seen in genes related to potassium. For plants, potassium is the element of summer and of stress. When summer heat and droughts stress the plant, it needs extra potassium to balance the shifts in osmotic pressure. Plants that make extra chromosomes have more genetic space to evolve genes that move the potassium around. Extra DNA gives the plant a better ability to control its potassium and respond to its environment. 

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