Lysogenic cycle

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Schematic sequence of the lysogenic cycle: 1) host genome 2) adsorption of the phage 3) injection of the genome 4) integration, prophage occurs 5) replication of the host genome 6) cell division

The lysogenic cycle is a form of replication of viruses and phages in which the DNA is temporarily integrated into the host's genome and the host cell is not lysed . The lysogenic cycle describes the pathogen persistence in viruses.

procedure

  • Host cell contact with a virus ( adsorption )
  • "Injection" of the phage genome or uncoating or release of the viral nucleic acid ( DNA or RNA )
  • Integration of the virus genome into the host genome or present without integration as an episome or cccDNA . With retroviruses , the RNA is first transcribed into DNA, then DNA integration.
  • dormant as prophage or provirus , episome or cccDNA
  • possibly replication together with host genome
  • when induced by certain environmental factors, the virus enters the lytic cycle one

Lysogenic cycle in phages

In addition to the virulent phages, there are also those that are referred to as temperate , ie “moderate”. This name comes from the fact that these phages infect their host cells, but do not necessarily lyse them . A lysis can but need not occur. Such a non-lytic infection cycle is called lysogenic .

In principle, the lysogenic cycle is initially analogous to the lytic infection cycle in terms of adsorption and injection. After the injection, however, neither an eclipse occurs, nor is the metabolism of the host cell switched to phage reproduction, but the phage DNA attaches itself to the circular genome of the host cell and is integrated into it.

Lysogenic cycle in phage lambda

Integration of the lambda phage genome into the host genome.

In the E. coli phage Lambda , this integration takes place at a defined point near the gal operon and is a recombination process. Here, host and phage DNA are cut using various enzymes to form cohesive ends and then re-linked. After this storage is completed, the phage genome is named prophage after André Lwoff . Such a prophage can remain integrated in the genome of the host cell for an indefinite period of time and be passed on to its daughter cells with each cell division of the host - under certain circumstances it is permanently integrated into the host's genetic material and thus represents a special form of mutation .

Transition from the lysogenic to the lytic cycle

A bacterial population that contains prophages is said to be lysogenic ; H. generating a lytic principle , denotes. Such a culture is, however, quite capable of forming phages, for example after a (slight, since UV radiation is one of the few means of lasting damage to viruses) UV radiation or the addition of mutagenic substances. This leads to what is known as induction, which means that the prophage is separated from the bacterial genome and vegetative reproduction and the formation of mature phages with subsequent lysis as in a lytic infection cycle . Some viruses, such as the HI virus, remain in the lysogenic cycle for several years or decades and then switch to a lytic cycle.

literature

  • S. Modrow, D. Falke, U. Truyen: Molecular Virology. 2nd edition, Spektrum, Heidelberg 2003, ISBN 3-8274-1086-X .
  • DM Knipe, PM Howley, DE Griffin, (Eds.): Fields Virology. 5th edition, Lippincott Williams & Wilkins, Philadelphia 2007, ISBN 978-0-7817-6060-7 .