Lysogenic cycle Animation

Lysogenic cycle, or lysogeny, is one of the two alternative life cycles of a virus inside a host cell, whereby the virus that has infected a cell attaches itself to the host DNA and, acting like an inert segment of the DNA, replicates when the host cell divides. This method of replication is contrasted with the lytic cycle, whereby the virus that has entered a cell takes over the cell's replication mechanism, makes viral DNA and viral proteins, and then lyses (breaks open) the cell, allowing the newly produced viruses to leave the now disintegrated host cell to infect other cells. While the lysogenic cycle causes no harm to the host cell, an induction event, such as exposure to ultraviolet light, can cause this latent stage to enter the lytic cycle.

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The lysogenic cycle is one strategy for replication of the virus without destruction of the host. The discovery of this cycle has important medical implications. Sometimes expression of the genes of a lysogenic bacteriophage can alter the phenotype of the host bacteria. This phenomenon, called lysogenic conversion, can have medical significance for humans. For example, the bacteria that causes diptheria, Corynebacterium diphtheriae, is harmless to humans unless it is infected by the phage β. Then the genes of the incorporated DNA of the bacteriophage induce the host bacteria to produce toxins.

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