Lytic cycle

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Lytic cycle, compared to lysogenic cycle

Lytic cycle is one one of the two alternative life cycles of a virus inside a host cell, 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. This method of replication is contrasted with the lysogenic cycle, 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. The lysogenic cycle causes no harm to the host cell, but the lytic cycle results in the destruction of the infected cell.

The lytic cycle is typically considered the main method of viral replication. Even the lysogenic cycle can lead to the lytic cycle when there is an induction event, such as exposure to ultraviolet light, that causes this latent stage to enter the lytic cycle.

Overview

Viral reproduction is most fully understood through studying viruses that infect bacteria, known as bacteriophages (or, commonly, phages). The lysogenic cycle and the lytic cycle are two fundamental reproductive processes that have been identified. Lysogenic cycles can also occur in eukaryotes, although the method of incorporation of DNA is not fully understood. The following is a description of these cycles based on bacteriophages.

The lytic cycle involves the virus entering a host cell, taking control of the host DNA to produce viral DNA and the viral proteins that provide the structural component of the bacteriophage, and then, when the cell has many new virus particles assembled, digesting the host cell wall from within and releasing the new viruses. For example, the bacteriophage lambda, after infecting an E. coli host cell, normally multiplies until several hundred progeny are produced, at which time the bacterial cell is lysed and the progeny released (Alberts et al. 1989).


Viruses of the lytic cycle are called virulent viruses. The lytic cycle is a four-stage cycle.

Penetration

To infect a cell, a virus must first enter the cell through the plasma membrane and (if present) the cell wall. Viruses do so by either attaching to a receptor on the cell's surface or by simple mechanical force. The virus then releases its genetic material (either single- or double-stranded RNA or DNA) into the cell. In doing, the cell is infected and can also be targeted by the immune system.

Biosynthesis

The virus' nucleic acid uses the host cell’s machinery to make large amounts of viral components. In the case of DNA viruses, the DNA transcribes itself into messenger RNA (mRNA) molecules that are then used to direct the cell's ribosomes. One of the first polypeptides to be translated is one that destroys the hosts' DNA. In retroviruses (which inject an RNA strand), a unique enzyme called reverse transcriptase transcribes the viral RNA into DNA, which is then transcribed again into RNA.

The biosynthesis is often (e.g. T4) regulated in three phases of mRNA production followed by a phase of protein production. [1]

Early phase
enzymes involved to modify the hosts DNA replication by RNA polymerase. Amongst other modifications, virus T4 changes the sigma factor of the host by producing an anti-sigma factor so that the host promotors are not recognized any more but now recognize T4 middle proteins.
Middle phase
virus nucleic acid (DNA or RNA depending on virus type)
Late phase
structural proteins including those for the head and the tail.

Maturation and lysis

After many copies of viral components are made, they are assembled into complete viruses. The phage then directs production of an enzyme that breaks down the bacteria cell wall and allows fluid to enter. The cell eventually becomes filled with viruses (typically 100-200) and liquid, and bursts, or lyses; thus giving the lytic cycle its name. The new viruses are then free to infect other cells.

Lytic cycle without lysis

Some viruses escape the host cell without bursting the cell membrane, but rather bud off from it by taking a portion of the membrane with them. Because it otherwise is characteristic of the lytic cycle in other steps, it still belongs to this category. HIV, influenza and other viruses that infect eukaryotic organisms generally use this method.

References
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  • Alberts, B., D. Bray, J. Lewis, M. Raff, K. Roberts, and J. D. Watson. Molecular Biology of the Cell, 2nd edition. New York: Garland Publishing, 1989. ISBN 0824036956.
  • Canchaya, C., C. Proux, G. Fournous, A. Bruttin, and H. Brüssow. Prophage genomics. Microbiol. Mol. Biol. Rev. 67(2): 238–76. PMID 12794192. Retrieved November 8, 2008.
  • Towle, A. 1989. Modern Biology. Austin, TX: Holt, Rinehart and Winston. ISBN 0030139198.

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  1. Madigan M, Martinko J (editors) (2006). Brock Biology of Microorganisms, 11th ed., Prentice Hall. ISBN 0-13-144329-1.