Many bacterial viruses (bacteriophages) have a lytic life cycle. In this cycle, the virus adsorbs to the surface of the host cell and injects its nucleic acid core through the outer covering.
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Once inside, the nucleic acid takes command of the host’s metabolism to synthesize more virus particles. After the synthesis is complete, the host cell is ruptured to free new virions.
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The lytic cycle takes place very quickly (about 40 minutes) and there is no delay from the time of initial penetration to lysis of the host. During the lytic cycle, the DNA of the host is broken into small segments that are about the same size as virus nucleic acid.
Occasionally, in the case of certain types of bacteriophages, during the assembly of the complete virion a small segment of host DNA is incorporated into the virus protein coat in place of the phage genome.
If the DNA found in this virus coat, for example, contains a gene for drug resistance, it may, on infection of another cell, be transferred to that cell and integrated into the endogenote, making the recipient cell drug resistant.
The integration resembles the events that take place during transformation. Because any segment of host DNA may be transferred in this way, the process is called generalized transduction.
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In the lysogenic, or latent, life cycle, lysis of the cell does not take place after the nucleic acid has penetrated the host. The virus integrates into the host chromosome, where it is called a prophage (previrus), and it replicates as part of the host chromosome.
In this condition, the host cell is called a lysogenic bacterium because it is capable of being destroyed at a later time if the virus prophage becomes active and reenters the lytic cycle.
The delay of host destruction that is characteristic of the lysogenic cycle results in the formation of many generations of lysogenic bacterial cells. During the period between virus infection and lysis, these virus-infected bacteria may function as active pathogens.
The potential for specialized transduction of bacteria begins when the prophage shifts into the lytic cycle and accidentally incorporates an adjacent section of the host DNA into the virus core.
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When the modified prophage emerges from the host chromosome, it takes command of cell metabolism and synthesizes more protein coat and nucleic acid core. Since there is a different gene on the plasmid, it too is synthesized and incorporated into all the new virions.
When the lytic cycle is completed, a hundred or so of the virions may be released from the host cell. Each of these contains the new gene. The virions may infect other bacteria of the same susceptible species and integrate this gene into the endogenote, thus transduction them to recombinant forms.
If the gene being transferred by transduction is for drug resistance, it is easy to see how portions of a bacterial population may develop this genetic trait.