Chromosome of a Bacteriophage

The transfer of resistance can be achieved by conjugation, transduction, and transformation. There is also a phenomenon of transposition by which resistance determinants pass from one plasmid to another or to a chromosome or to a bacteriophage, thus allowing construction of new plasmids under the pressure of new antibiotic exposure. 

The principles that govern the delivery of recombinant DNA in clonable form to a host cell, and its subsequent amplification in the host, are well illustrated by considering three popular cloning vectors commonly used in experiments with E. coli—plasmids, bacteriophages, and bacterial artificial chromosomes—and a vector used to clone large DNA segments in yeast. 

Some E. coli bacteriophages, including f2, MS2, R17, and Qj8, as well as some eukaryotic viruses (including influenza and Sindbis viruses, the latter associated with a form of encephalitis) have RNA genomes. The single-stranded RNA chromosomes of these viruses, which also function as mRNAs for the synthesis of viral proteins, are replicated in the host cell by an RNA-dependent RNA polymerase (RNA replicase). All RNA viruses—with the exception of retroviruses—must encode a protein with RNA-dependent RNA polymerase activity because the host cells do not possess this enzyme. 

NUCLEOPROTEINS AND NUCLEIC ACIDS. Nucleic adds are compounds in which phosphoric acid is combined with carbohydrates and with bases derived from purine and pyrimidine. Nucleoproteins are conjugated proteins consisting of a protein moiety and a nucleic acid. Originally, nucleoproteins were thought to occur only in the nuclei of cells, but it was later established that they are far more widely distributed, being found in cells of all types, animal and plant. They are found in the chromosomes, in the genes, in viruses, and bacteriophages. 

Many plasmids and bacteriophages have been ingeniously modified to enhance the delivery of recombinant DNA molecules into bacteria and to facilitate the selection of bacteria harboring these vectors. Plasmids are circular duplex DNA molecules occurring naturally in some bacteria and ranging in size from 2 to several hundred kilobases. They carry genes for the inactivation of antibiotics, the production of toxins, and the breakdown of natural products. These accessory chromosomes can replicate independently of the host chromosome. In contrast with the host genome, they are dispensable under certain conditions. A bacterial cell may have no plasmids at all or it may house as many as 20 copies of a plasmid. 

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