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Viruses
Page 8

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A temperate phage enters a bacterium in much the same way the T4 bacteriophage does, by attachment of tail fibers and injection of the DNA into the host cell. At this point, however, the lysogenic cycle differs from the lytic cycle. Instead of immediately creating new RNA and proteins the DNA of the temperate phage attaches itself to the host DNA. It becomes, in effect, an additional set of genes. The phage, now represented only by a short DNA segment, is called a prophage. When host DNA replicates or when the host cell divides, the prophage acts just like an inert segment of the DNA of the host. It causes no harm to the cell.

However, various external stimuli-exposure to radiation or certain chemicals, for instance-can cause the prophage to become virulent. It then takes over the host cell, produces new viruses, and ultimately destroys the cell.

Temperate viruses released during cell lysis may take with them a portion of the DNA of the host cell. When the phage enters a new host, it may introduce genes from the former host into the new host. In this process, called transduction, a virus transfers DNA from cell to cell and thus causes a change in the genetic code of bacterial cells. This results in genetic recombination and hence phenotypic variation in the new host bacterium.

 

Evolution

How and when did viruses evolve? No fossil evidence of viruses has been found. However, scientists form inferences about the evolution of viruses. Because they are obligate ;'ntracellular para. sites, viruses probably did not arise until cells had evolved, since their existence requires cells. If this is so, then viruses probably either formed spontaneously from existing nonliving organic material or evolved as simplifications of previously existing cells.

Whatever their origin, existing viruses often evolve very tap. idly by natural selection. Suppose that cold viruses invade a human body. The human immune system may destroy most of these viruses. The few that remain will have been naturally selected and will be resistant to immediate attack by the immune system. These resistant viruses enter cells and produce hundreds or thousands of viruses in a few days. The immune system will respond to repel the viruses eventually but not until many new ones have been formed. The short generation time of a virus means that natural selection acts quickly to create new viral types that are capable of withstanding destruction in the next host.

Section Review

1. What is a prophage, and how does it function?

2. How does a bacteriophage differ from a polyhedral virus?

3. What are the five phases of the lytic cycle?

4. Why do scientists think that viruses evolved after cells?

5. What is the significance of transduction?

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Copyright 1995 Mr. Lewis Classroom for Physics & Biology. All rights reserved.