Viral Reproduction

Viruses can replicate (reproduce themselves) only at the expense of their host cells. In doing so, they first become attached to a susceptible cell. Then they penetrate to the interior, some types leaving their coats on the outside. Inside the cell, their DNA or RNA directs the synthesis of new virus molecules, which are then assembled into complete viruses. These are released from the host cell, usually as it dies (Fig. 17.18).

Some viruses (e.g., those causing influenza) can mutate (see Chapter 15) very rapidly, allowing them to attack organisms that previously had been immune to them. As a result, new vaccines constantly have to be developed to combat new strains of viruses. Some viruses greatly affect the metabolism of their host cells. For example, in botulism bacteria, the botulism toxins are produced only if specific phages are present and active. Evidence is mounting that many forms of cancer, which usually involves abnormal cell growth, are caused by viruses. Scientists also suspect that all living organisms carry viruses in an inactive form in their cells, and they are trying to discover what causes the inactive viruses suddenly to become active.

Reproduction Within Viruses

tail fibers—

Botulism Virus Cell
Figure 17.17 Phage viruses. Insert drawing shows some of the detail, x20,000. (Electron micrograph courtesy D. Kay)

Chapter 17

Chapter 17

Viral Reproduction

Figure 17.18 Stages in the development of a phage virus within a bacillus bacterium. A. The virus becomes attached to the bacterium. B. The DNA of the virus enters the cell. C. Various components of the virus are synthesized from the DNA of the bacterium. D. The viral components are assembled into units. E. The assembled viruses are released as the bacterial wall breaks down. After M. J. Pelczar, Jr., and R. D. Reid. 1972. Microbiology, 3d ed. The McGraw-Hill Companies. All rights reserved.

Figure 17.18 Stages in the development of a phage virus within a bacillus bacterium. A. The virus becomes attached to the bacterium. B. The DNA of the virus enters the cell. C. Various components of the virus are synthesized from the DNA of the bacterium. D. The viral components are assembled into units. E. The assembled viruses are released as the bacterial wall breaks down. After M. J. Pelczar, Jr., and R. D. Reid. 1972. Microbiology, 3d ed. The McGraw-Hill Companies. All rights reserved.

Cells of higher animals that are invaded by viruses produce a protein called interferon, which is released into the fluid around the cells or into the bloodstream. Minute amounts of interferon in contact with cells cause the cells to produce a protective protein that prevents or inhibits the propagation of many types of viruses within the protected cells and also inhibits viruses from causing tumors that transform normal cells into tumor cells.

Because of these properties of interferon, it is now being produced in large quantities for use in controlling certain cancers and many other viral infections. The use of bacteria as hosts for donor DNA is especially promising for the future. This process, in effect, turns the bacteria into interferon synthesis centers (see Fig. 14.12). In 1981, scientists at the University of Washington and the Genentech Corporation of San Francisco announced that they had succeeded in producing a form of interferon by splicing interferon genes into yeast cells. Because yeast cells are larger than bacteria, the process can potentially produce much larger quantities of interferon at considerably lower cost than is possible using bacteria.

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