written by: Sonal Panse•edited by: Paul Arnold•updated: 1/21/2010
Find out about the spread of viruses with this brief look at the lytic and lysogenic cycles. These occur after a virus infects a cell.
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Lytic and lysogenic cycles are two different methods of viral replication. While they are different, they can be interchangeable or the replication can involve both methods in separate phases.
To start with, the virus has to infect the cell. So the virus attaches itself to the outer cell wall and releases enzymes that weaken the cell wall. Then, depending on whether it is a DNA virus or a RNA virus, the virus injects its double stranded DNA or its single stranded RNA into the cell.
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The Lytic Cycle
In the lytic cycle, which is considered the main cycle in viral replication, once the viral DNA enters the cell it transcribes itself into the host cell's messenger RNAs and uses them to direct the ribosomes.
The host cell's DNA is destroyed and the virus takes over the cell's metabolic activities.
The virus begins using the cell energy for its own propagation. The virus produces progeny phages. These replicate fast, and soon the cell is filled with 100-200 new viruses and liquid. As the cell starts getting overcrowded, the original virus releases enzymes to break the cell wall. The cell wall bursts – this process is called lysing - and the new viruses are released.
So, in short, in the lytic cycle, the virus hijacks the infected cell and then destroys it. The lytic cycle occurs in virulent viruses. The symptoms from a viral infection occur when the virus is in a lytic state.
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The Lysogenic Cycle
In the lysogenic cycle, the viral DNA or RNA enters the cell and integrates into the host DNA as a new set of genes called prophage. That is, the viral DNA becomes part of the cell's genetic material. No progeny particles, like in the lytic phase, are produced. Each time the host cell DNA chromosome replicates during cell division, the passive and non-virulent prophage replicates too. This may alter the cell's characteristics, but it does not destroy it.
There are no viral symptoms in the lysogenic cycle; it occurs after the viral infection is over. But the viral DNA or RNA remains in the cell and it may remain there permanently. However, if the prophage undergoes any stress or mutation or is exposed to UV radiation, the viral lysogenic cycle can change into the viral lytic cycle. In which case, there will be symptoms of a new viral infection.
Some viruses first replicate by the lysogenic cycle and then switch to the lytic cycle.
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Differences Between Lytic and Lysogenic Cycles
In the Lytic Cycle:
Viral DNA destroys Cell DNA, takes over cell functions and destroys the cell.
The Virus replicates and produces progeny phages.
There are symptoms of viral infection.
Virtulant viral infection takes place.
In the Lysogenic Cycle:
Viral DNA merges with Cell DNA and does not destroy the cell.