Lysogenic cycle

 Lysogeny, or the lysogenic cycle, is one of two methods of viral reproduction (the lytic cycle is the other). Lysogeny in prokaryotes is characterized by the fusion of the nucleic acid of a bacteriophage with that of a host bacterium. The newly integrated genetic material, called a prophage can be transmitted to daughter cells at each subsequent cell division, and a later event (such as UV radiation) can release it, causing proliferation of new phages via the lytic cycle. Lysogenic cycles can also occur in eukaryotes, although the method of incorporation of DNA is not fully understood.

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Mixed cycles

Following are some types of viruses that replicate by the lysogenic cycle, but also partly by the lytic cycle.

Bacteriophages

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 lysogenic state, the information contained in the viral nucleic acid is not expressed. The model organism for studying lysogeny is the lambda phage. Roughly 50-60 nucleotides are taken out of the lysogenic pathway and used.

Herpes simplex virus

The Herpes simplex virus 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 may cause disease symptoms similar to those experienced during the initial infection.

Lysogenic conversion

In some interactions between lysogenic phages and bacteria, lysogenic conversion may occur. It is when a temperate phage induces a change in the phenotype of the bacteria infected that is not part of a usual phage cycle. Changes can often involve the external membrane of the cell by making it impervious to other phages or even by increasing the pathogenic capability of the bacteria for a host.

Examples:

• Corynebacterium diphtheriae produces the toxin of diphtheria only when it is infected by the phage β. In this case, the gene that codes for the toxin is carried by the phage, not the bacteria.
• Vibrio cholerae is a non-toxic strain that can become toxic, producing cholera toxin, when it is infected with the phage CTXφ.
• Clostridium botulinum causes botulism.
• Streptococcus pyogenes causes scarlet fever.
• Shiga toxin
• Tetanus

Extra genes present in prophage genomes which do not have a phage function but (may) act as fitness factors for the lysogen are termed "morons".^[1]

References

1. ^ Canchaya C, Proux C, Fournous G, Bruttin A, Brüssow H (2003). "Prophage genomics". Microbiol. Mol. Biol. Rev. 67 (2): 238-76. PMID 12794192.