Viral chromosomes

Pieces of DNA, for example genes or cDNA probes, are multiplied using a method based on in vivo replication in bacterial cells. The sequence of DNA is incorporated into a vehicle or vector which transports it into a cell host, usually E. coli. As the bacteria culture grows, the vector also replicates so producing more of the required sequence of DNA. Two of the naturally occurring types of DNA molecule which can be used as vectors are plasmids and viral chromosomes. 

Another viral chromosome that can be used as a vector is that of the filamentous phage Ml3. The M13 chromosome is a single-stranded DNA molecule which when inserted into the bacterial host replicates outside the bacterial chromosome in the cytoplasm. The virus is then reassembled and released from the bacterial cell without cell lysis. 

To clone the restriction fragments, they must each be inserted into a vector. A vector is a piece of DNA (plasmid, viral chromosome, yeast chromosome) capable of autonomous replication in a host cell, for instance, the plasmid pBR322 shown in Figure 1-6-3. The DNA used as a vector usually has... 

When the virus was used to infect a known, fully permissive cell line from Spodoptera frugiperdaf both genes were expressed. CAT activity was detected early in the infection at about six hours, a not surprising result since the RSV LTR contains an RNA polymerase II promoter and this enzymatic activity, pre-existing in the cell, should be amongst the first to act on the viral chromosome. Late in the infection, at the time that the polyhedrin promoter is known to become active, P-galactosidase activity was detected. 

When adsorbed by a cell, the virus insinuates its nucleic acid, whereby the viral chromosome redirects the cell s metabolism to produce new viruses, which in turn initiate further infection. The virus has no metabolism of its own, and can be described as the ultimate parasite, firuses are not considered living organisms, and in the absence of the host cell, are biologically inert, the same as any other large molecule. Viruses called bacteriophages, which infect bacteria, have been used to... 

The transcriptional repression can be broken, however, leading to excision of the viral chromosome from the host genome as a circular DNA, followed by replication of viral DNA, followed by activation of genes needed to assemble virus particles. 

Bacterial cells contain only a single circular chromosome, aggregated into a nucleoid. Viral chromosomes can consist of one or several single- or double-stranded nucleic acid molecules. See also artieicial... 

Terminal redundancy the existence of identical genetic information at each end of a viral chromosome. In X-phages this constitutes up to 20 nucleotide pairs, whereas up to 6,000 nucleotide pairs may be terminally redundant in T-even phages. 

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