Alternatively, they may enter a dormant state, known as the lysogenic cycle, within the host cell if conditions are favorable. In the lytic cycle Figure 2 , sometimes referred to as virulent infection, the infecting phage ultimately kill the host cell to produce many of their own progeny.
Immediately following injection into the host cell, the phage genome synthesizes early proteins that break down the host DNA, allowing the phage to take control of the cellular machinery.
The phage then uses the host cell to synthesize the remaining proteins required to build new phage particles. The heads and sheaths are assembled separately, the new genetic material packed into the head and new daughter phage particles constructed.
During this process, the host cells gradually become weakened by phage enzymes and eventually burst, releasing on average new phage progeny into the surrounding environment. Figure 2. Depiction of the stages of the bacteriophage lytic cycle. Watch the lytic cycle in action here.
The lysogenic cycle Figure 3 , sometimes referred to as temperate or non-virulent infection, does not kill the host cell, instead using it as a refuge where it exists in a dormant state. Following the injection of the phage DNA into the host cell, it integrates itself into the host genome, with the help of phage-encoded integrases, where it is then termed a prophage. The prophage genome is then replicated passively along with the host genome as the host cell divides for as long as it remains there and does not form the proteins required to produce progeny.
As the phage genome is generally comparatively small, the bacterial hosts are normally relatively unharmed by this process. Figure 3. Depiction of the stages of the bacteriophage lysogenic cycle. If a bacterium containing prophage is exposed to stressors, such as UV light, low nutrient conditions, or chemicals like mitomycin C, prophage may spontaneously extract themselves from the host genome and enter the lytic cycle in a process called induction. A good example of a phage with this type of life cycle is the lambda phage.
During the lysogenic cycle, instead of killing the host, the phage genome which is called a prophage integrates itself to the bacterial chromosome and becomes part of the host. A bacterial host with a prophage is called a lysogen and then the entire process in which a bacterium is infected by a temperate phage is called lysogeny.
The presence of the phage may alter the phenotype of the bacterium since the phage virus can bring in extra genes e. This process of change in the host phenotype is called lysogenic conversion or phage conversion.
There are some bacteria, Vibrio cholerae and Clostridium botulinum, which are less virulent in the absence of the prophage. During the lysogenic cycle, the prophage will persist in the host chromosome until induction, which leads to the excision of the viral genome from the host chromosome. After induction takes place the temperate phage can proceed through a lytic cycle and then again undergo lysogeny in a newly infected cell. Lytic and Lysogenic Cycle Difference.
There are various differences between the lytic and lysogenic cycles of bacteriophage. The below article discusses some of them.
Lytic Cycle. Lysogenic Cycle. It is a type of viral reproduction mechanism which results in the lysis of the infected cell. The lysis of the cells produces symptoms of the viral infection in the host. Lysogenic cycle is a viral reproduction mechanism in which the viral DNA is integrated into the host genome.
The new set of genes in the host genome is called the prophage. Thereby, viral DNA becomes a part of the host genome. Once the host genome replicates, the viral genes are also replicated simultaneously. The prophage stage is shown in figure 2.
Since no new progeny is produced by the lysogenic cycle, the host cell does not lyse. Therefore, no symptoms of the viral infection are shown in the host. Some viruses first undergo lysogenic cycle and then enters the lytic cycle. Lytic Cycle: Lytic cycle is a type of a viral reproduction mechanism, which results in the lysis of the infected cell.
Lysogenic Cycle: Lysogenic cycle is a viral reproduction mechanism where the viral DNA is integrated into the host genome. Lytic Cycle: Lytic cycle does not have a prophage stage. Lysogenic Cycle: Lysogenic cycle has a prophage stage. In contrast, the lysogenic cycle does not result in immediate lysing of the host cell. Those phages able to undergo lysogeny are known as temperate phages. Their viral genome will integrate with host DNA and replicate along with it fairly harmlessly, or may even become established as a plasmid.
The virus remains dormant until host conditions deteriorate, perhaps due to depletion of nutrients; then, the endogenous phages known as prophages become active. At this point they initiate the reproductive cycle, resulting in lysis of the host cell. An example of a bacteriophage known to follow the lysogenic cycle and the lytic cycle is the phage lambda of E.
Viruses that infect plant or animal cells may also undergo infections where they are not producing virions for long periods. An example is the animal herpes viruses, including herpes simplex viruses, which cause oral and genital herpes in humans.
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