Comparing the Lysogenic and Lytic Life Cycles
The viral life cycle that involves release of virions through host cell lysis is the lytic life cycle, which is common among most viruses. Many bacteriophages—viruses that infect bacteria—are also capable of following the lysogenic life cycle after their genetic material has been injected into host cells. Bacteriophages capable of the lysogenic life cycle incorporate their genetic material into the host cell's chromosome without destroying the cell. When this happens, viruses in the lysogenic cycle have entered lysogeny. Lysogeny is a state in which most viral genes in a host cell are dormant and the viral genome (provirus) and host chromosome are integrated and replicated together. Thus a provirus, or prophage, is the genome of a virus in the lysogenic life cycle that is incorporated and replicated with the host chromosome. Replication follows a different process and proceeds more slowly than in the lytic cycle, but it can continue within the same cell. The provirus also protects its host from infection by other lytic viruses. Lysogenic viruses, known as temperate viruses, can be hosted without damage until they re-enter the lytic cycle.
Sometimes lysogenic conversion occurs, the transfer of genetic information from one bacterial host cell to another by the hosted lysogenic virus. Because new genetic material is passed along, lysogenic conversion can influence the epidemiology (occurrence and control) of bacterial diseases. For example, the disease diphtheria is caused by a bacterium hosting the provirus that codes for the disease's toxin.
A well-studied temperate bacteriophage is the lambda (λ) virus. Lambda induces lysogeny in some strains of Escherichia coli (E. coli) bacteria. The incorporated lambda provirus is noninfectious and permits replication of the host cell DNA along with its own genetic material.Repression of the lytic process in a bacteriophage can be inactivated, switching the virus to a lytic life cycle. Factors regulating the choice between lytic and lysogenic life cycles can be quite complex. One example is damage to the host cell's DNA, which signals the provirus to switch from the lysogenic to the lytic life cycle.