Introductory Microbiology

Lytic and lysogenic cycle

 
• In a typical lytic virus it is not the presence of viral DNA that leads to the production of new virions and host cell death. Rather it is expression of the viral genome that is deleterious. Host cells can harbor viral genomes without harm if the expression of the viral genes can be controlled. This is the situation found in lysogens. However if this control is lost, the virus enters the lytic pathway and produces new virions, eventually lysing the host cell lysogeny can thus be considered a genetic trait of a bacterial strain.
• The temperate virus does not exist in its extracellular form inside the cell. Instead, the prophage is integrated into the bacterial chromosome and replicates along with the host cell as long as the genes controlling its lytic pathway are not expressed. Typically this control is maintained by a phage encoded repressor protein. The virus repressor protein not only control the lytic genes on the prophage but also prevents the expression of any incoming genes of the same virus. This results in the lysogens having immunity to infection by the same type of virus.
• If the phage repressor is inactivated or if its synthsis is prevented, the prophage is induced. Induction results in the production of new virions and the lysis of the host cell. In some cases, induction can be brought about by environmental conditions. If the virus loses the ability to leave the host genome, becomes a cryptic virus. Genomic studies have shown that many bacterial chromosomes contain DNA sequences that were clearly one part of a viral genome. Thus the establishment and breakdown of the lysogenic state is likely a dynamic process in prokaryotes.
• Replication of bacteriophage involves four main phases:

  1) the adsorption of the phage to the bacterium
  2) injection of nucleic acid into the host cytoplasm
  3) the replication of the phage nucleic acid and synthesis of phage specific proteins
  4) assembly, maturation and release of the bacteriophage.

  Generally, viruses which kill the host cell during release are referred as lytic viruses and the cycle is referred as lytic cycle. But some bacteriophages will multiply inside the host cell and release to the environment after assembly without killing the host cell are referred as temperate phages and the cycle is referred as lysogenic cycle.

Animal viruses:
* Animal viruses are both DNA and RNA viruses. Retrovirus is a RNA virus but can have DNA as an intermediate.
* The animal viruses enter into host cell as whose viron through phagocytosis.
* The viral nucleic acid will be produced at the nucleus of host cell and the protein coat will be produced at cytoplasm and assembly takes place.
* Most of the animal viruses are enveloped.

Types of infections:
* lytic infection- destroys host cells.
* Persistent infection- host cell continues to shed viruses over long time.
* oncogens – infection by certain viruses causes cells to change, become cancerous.
* latent infection – virus genes may not be expressed for long time.
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