Difference between revisions of "Lytic cycle" - New World Encyclopedia
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'''Lytic cycle''' is one one of the two alternative [[life cycle]]s 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 (biology)|cell]]. | '''Lytic cycle''' is one one of the two alternative [[life cycle]]s 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 (biology)|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. | + | The lytic cycle is typically considered the main method of viral replication as it is more common. 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== | ==Overview== | ||
| − | Viral reproduction is most fully understood through studying viruses that infect [[bacteria]], known as [[bacteriophage]]s (or, commonly, phages). The | + | Viral reproduction is most fully understood through studying viruses that infect [[bacteria]], known as [[bacteriophage]]s (or, commonly, phages). The lytic cycle and the lysogenic cycle are two fundamental reproductive processes that have been identified. 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). | 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). | ||
| + | The lysogenic cycle involves the virus entering the cell, and instead of creating new virus particles, is characterized by integration of the [[bacteriophage]] nucleic acid into the host bacterium's [[genome]]. The newly integrated genetic material acts as an additional set of [[gene]]s that can be replicated when the DNA of the host cell is replicated and the host cell divides. The virus is thus transmitted to daughter cells at each subsequent cell division, although the phages remain inert, causing no harm to the host cells. In the case of the bacteriophage lambda, this is a more rare occurrence than the lytic cycle (Alberts et al. 1989). However, there can be induction events, such as exposure to ultraviolet light, that leads to the lytic cycle even in these cases. | ||
| − | Viruses of the lytic cycle are called [[virulent]] viruses. The lytic cycle is a | + | ==Process== |
| + | Viruses of the lytic cycle are called [[virulent]] viruses. The lytic cycle is a five-stage cycle. | ||
| − | + | ||
| − | 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. | + | '''Attachment'''. The virus first attaches itself to a specific host cell. This is done by the tail fibers of the virus having proteins that have an affinity with the host cell wall, such as the bacterial cell wall. The virus attaches at places called ''receptor sites'' (Towle 1989). |
| + | |||
| + | '''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=== | ===Biosynthesis=== | ||
Revision as of 22:41, 29 November 2008
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 as it is more common. 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 lytic cycle and the lysogenic cycle are two fundamental reproductive processes that have been identified. 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).
The lysogenic cycle involves the virus entering the cell, and instead of creating new virus particles, is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome. The newly integrated genetic material acts as an additional set of genes that can be replicated when the DNA of the host cell is replicated and the host cell divides. The virus is thus transmitted to daughter cells at each subsequent cell division, although the phages remain inert, causing no harm to the host cells. In the case of the bacteriophage lambda, this is a more rare occurrence than the lytic cycle (Alberts et al. 1989). However, there can be induction events, such as exposure to ultraviolet light, that leads to the lytic cycle even in these cases.
Process
Viruses of the lytic cycle are called virulent viruses. The lytic cycle is a five-stage cycle.
Attachment. The virus first attaches itself to a specific host cell. This is done by the tail fibers of the virus having proteins that have an affinity with the host cell wall, such as the bacterial cell wall. The virus attaches at places called receptor sites (Towle 1989).
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.
ReferencesISBN links support NWE through referral fees
- 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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- ↑ Madigan M, Martinko J (editors) (2006). Brock Biology of Microorganisms, 11th ed., Prentice Hall. ISBN 0-13-144329-1.
