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What is the Lytic Cycle?

written by: dgemmellaro•edited by: Paul Arnold•updated: 10/14/2009

Since viruses do not have a cellular structure, they cannot replicate through the process of cell division. But we know that populations of viruses grow. How do they do this? Through either the lytic cycle or the lysogenic cycle, or sometimes both. This article will explore the lytic cycle.

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    The lytic and lysogenic cycle differ mainly in the fact that while the latter does not cause damage to the host cell, the former causes the host cell to burst (though there are a few viruses which go through processes that are exceptions to this rule). During the lysogenic cycle, viral DNA is integrated into host cell DNA.

    Viruses often referred to as “virulent" viruses, however, replicate mainly through the lytic cycle and are often considered very serious viruses that can evade normal host defense mechanisms.

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    Viral Entry into a Host Cell

    The lytic cycle starts when a virus infects a host cell by penetrating the cell wall, where this is present, and the plasma membrane. Viruses can pass through the membrane either by attaching themselves to one of the receptors that are normally present on the external surface of the plasma membrane or by releasing enzymes that digest their way through the membrane. Once the plasma membrane is breached, the virus injects into the cell its genetic material, which can be single-stranded or double-stranded DNA or, in the case of retroviruses, RNA. At this point the host cell is “officially" infected, and has become a target for the host’s immune system.

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    The Synthesis of Viral Components

    Once the viral genetic material is inside the cell, viral enzymes break down host DNA, so that the host cell can no longer carry on its normal cellular processes. The host cell now has no “choice" but to follow the directions encoded in the viral genetic material, and these directions say: "make more viruses."

    The DNA filaments of DNA viruses transcribe themselves into mRNA, and, in their turn, these molecules head straight for the ribosomes of the host cells. Ribosomes are the “protein factories", and the mRNA molecules represent the “instructions" to make proteins. The ribosomes of the host cell start churning out viral proteins. Some of these viral proteins are actually the enzymes mentioned above, which destroy host cell DNA. Indeed, in some cases, destruction of host cell DNA occurs only after some of the viral genetic material is translated and transcribed into proteins. In other cases, enzymes get injected into the host cell along with the genetic material and start degrading host DNA immediately.

    HIV Virion-en-2 - graphic released into the public domain by US Federal Govt. Retroviruses, like HIV, are slightly different in that their genetic material is not DNA, but RNA. The RNA gets injected into the host cell and this must be transcribed into DNA with the help of an enzyme called reverse transcriptase (which is encoded for by the viral genome or injected along with the RNA filaments). The viral DNA is then re-transcribed into RNA in order to be “read" by the ribosomes to make proteins.

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    Host Cell Fate

    As the viral proteins and other components are being synthesized inside the host cell, they are assembled into new viruses; sometimes even parts of the host cell are included in the assembly. While the mechanism remains unclear, once a certain number of viruses has been reached (usually between 100 and 200), an enzyme called lysozyme is produced. This enzyme creates a hole in the host’s cell membrane, allowing extra-cellular fluid to enter. This influx of liquid, along with the presence of the newly created viruses, causes an increase in the volume of the cell; the volume continues to increase until the cell bursts, or lyses. This releases the newly synthesized viruses, which can now move around freely to infect other cells.

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    Generalizations and Variations

    Often, the generalized view of the lytic cycle described above is divided into four stages:

    1. Entry

    2. Synthesis

    3. Assembly

    4. Release

    There are certain viruses that, even though they replicate according to this general scheme, represent a peculiarity with regards to their way of leaving the host cell. These viruses, of which the HIV virus is a classic example, enter the host cell through the membrane and start replicating their genetic material following the normal steps of the lytic cycle. However, once new viruses have been created and assembled, the cell membrane is not broken down and extracellular fluids do not enter. Instead, the new viruses leave the cell by budding off from it, “wrapped around" a portion of the host cell’s membrane. This is a particularly efficient way for viruses to evade being detected by the defense mechanisms of the host that they have infected.