Lesson 3.6 Viruses

 3.6.3 Replication of viruses

Replication of a virus requires a living host cell, since a virus doesn’t possess all of the required components to create more of itself. As a result, viruses are often referred to as obligate intracellular parasites. They cannot replicate by themselves!

The virus has to get its nucleic acid inside the host cell in order to start the process of replication. In the case of enveloped animal viruses, the envelope of the virus contacts the host cell membrane and is drawn into the cell. In the case of bacteriophages, which replicate inside of bacteria, this is accomplished by injection of the nucleic acid into the host.

Once inside the host cell, the process of viral replication depends upon the type of nucleic acid that is present in the capsid. DNA containing viruses may immediately take over the host cell’s protein synthesis processes forcing it to create new viruses. In some cases, the viral DNA may embed itself into the host’s DNA and remain inactive for a period of time. Certain RNA containing viruses may also immediately take over the host cell. Other RNA containing viruses need to undergo additional steps in order to force the host cell to create virus. The end result, however, is the same: the host cell is taken over by the virus, becomes a factory for the creation of more viruses, and is destroyed as these newly assembled viruses are released.

Using a DNA containing bacteriophage as an example,let’s take a look at Figure 3.6.5 to

further explore viral replication.

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Figure 3.6.5

Figure 3.6.5 is a diagram illustrating two cycles associated with viral infection of a bacterial cell. The red asterisk (*) shows the viral DNA embedded into the host cell DNA, which only occurs during the lysogenic cycle.

 You will notice that there are two cycles listed in Figure 3.5.5: the lytic cycle and the lysogenic cycle. In the lytic cycle, the viral nucleic acid immediately begins to take over the host cell’s protein synthesis system, directing it to create virus parts. These parts are then assembled and the host cell bursts, destroying the host cell and releasing many new virus particles These new particles then find host cells to continue the replication of the virus.

In the lysogenic cycle, the viral nucleic acid embeds itself into the host nucleic acid. It does not immediately begin to take over the host. Every time a cell infected in this manner divides, it provides a copy of the viral nucleic acid to the new cells produced. This can result in a large number of host cells carrying viral nucleic acid. At some point, due to a trigger event (such as exposure to UV light or certain chemicals), the viral nucleic acid activates and begins to take over the host cell’s protein synthesis system. As with the lytic cycle, the host cell will produce, assemble, and eventually release new viruses.

Review Activities:

1. a. View the following video link which reviews the structure and replication of viruses. Be sure to write down any questions you may have.

Viruses: Viral Replication and The Mysterious Common Cold

 b. Follow your instructor’s directions for remaining questions.

2. a. Create a concept map that illustrates the similarities and differences between viruses, prokaryotic cells, and eukaryotic cells.

 b. Share your concept map as directed by your instructor.