Lysogeny
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Lysogeny, or the lysogenic cycle, is one of two methods of viral reproduction (the lytic cycle is the other). While the lytic cycle is common to both animal viruses and bacterial phages, lysogeny is more commonly found in animal viruses. Lysogeny is characterized by the fusion of the nucleic acid of a bacteriophage with that of a host bacterium so that the potential exists for the newly integrated genetic material to be transmitted to daughter cells at each subsequent cell division.
Some DNA phages, called temperate phages, only lyse a small fraction of bacterial cells; in the remaining majority of the bacteria, the phage DNA becomes integrated into the bacterial chromosome and replicates along with it. In this state, known as lysogeny, the information contained in the viral nucleic acid is not expressed. The model organism for studying lysogeny is the lambda phage.
An example of a virus that enters the lysogenic cycle is herpes, which first enters the lytic cycle after infecting a human, then the lysogenic cycle before travelling to the nervous system where it resides in the nerve fibers as an episomal element. After a long period of time (months to years) in a latent stage, the herpes virus is often reactivated to the lytic stage during which it causes severe nervous system damage.
In some interactions between lysogenic phages and bacteria, lysogenic conversion may occur.
[edit] Steps of the lysogenic cycle
- The viral genome enters the host cell from the phage through the contractile tail.
- The nucleic acid then is integrated within the genome of the regular host cell. Unlike in the lytic cycle, the assembly of viruses and the resulting of lysis does not immediately occur.
- Host cell DNA polymerase copies viral chromosomes.
- Cell divides and virus chromosomes are transmitted to cell's daughter cells
- At any moment when the virus is "triggered", the viral genome detaches from the host cell's DNA and enters stage 2 of the lytic cycle. While it remains unclear what exactly constitutes a "trigger" that activates the viral DNA from the latent stage entered in Step 4, common symptoms that appear to "trigger" the viral DNA are hormones, high stress levels (adrenaline), and free energy within the infected cell.
