Bacteriophage phage are obligate intracellular viruses that specifically infect bacteria. Phage have a very simple structure Figure 1. Their genetic material is contained in a prism shaped head, surrounded by a protein capsid. This is connected to the elongated sheath sometimes called the tail by a neck or collar region.
At the bottom of the sheath is the base plate to which the tail fibers normally six that facilitate attachment to the host cell are attached. Figure 1. Example structure of a bacteriophage. In order to reproduce, phage must first enter the host cell. They bind to specific receptors on the bacterial cell surface with their tail fibers adsorption and create a hole, a process which, along with attachment, is coordinated by the base plate 3.
A rigid tube is propelled out of the sheath, puncturing a hole in the bacterial cell membrane through which they inject their genetic material DNA or RNA, double or single stranded. 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.
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. Lysogenic Cycle: The lysogenic virus is non virulent.
Lytic Cycle: Host cell is lysed during the release of the viral particles in the lytic cycle. Lysogenic Cycle: Host cell is not lysed by the lysogenic cycle. Lytic Cycle: Viral particles are liberated in the lytic cycle. Therefore, lytic cycle produces a progeny of virus. Lysogenic Cycle: Typically, viral particles are not liberated in the lysogenic cycle.
Therefore, the lysogenic cycle does not produce a viral progeny. Lytic Cycle: Lytic cycle occurs within a short period of time. Lysogenic Cycle: Lysogenic cycle takes time. Lytic Cycle: Lytic cycle cannot follow the lysogenic cycle.
Lysogenic Cycle: Lysogenic cycle can follow the lytic cycle. Lytic Cycle: Lytic cycle shows the symptoms of viral replication. Lysogenic Cycle: Lysogenic cycle does not show symptoms of viral replication. The lysogenic cycle is much slower and may not infect a host body as fast as the lytic cycle might, but it can still be just as deadly.
Instead of replicating many copies of itself after it has inserted itself into a cell, the viral DNA or RNA incorporates itself into the genome of the cell, staying hidden.
This is usually called a lytic infection and this type of infection is seen with influenza and polio. Humans produce a new generation every 20 years or so; bacteria do it every 20 to 30 minutes, and viruses even faster. Because they reproduce so quickly, microorganisms can assemble in enormous numbers with great variety in their communities. Even though they definitely replicate and adapt to their environment, viruses are more like androids than real living organisms.
Viruses are acellular, parasitic entities that are not classified within any domain because they are not considered alive. They have no plasma membrane, internal organelles, or metabolic processes, and they do not divide.
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