Viruses and Non-living Parasites of Cells Help

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to Viruses and Non-living Parasites of Cells

The Five-Kingdom System is commonly used to classify all living organisms. Note that in the preceding sentence, the word, living, was emphasized. The reason for this emphasis is the puzzling existence of a real organic oddball, the viruses .

The term virus comes from the Latin and exactly means “a poison.” This name probably derives from the fact that a virus is a non-living superchemical that always invades living cells and, in a sense, “poisons” them by becoming a parasite. A virus, you see, cannot reproduce on its own. Therefore, it parasitizes human, animal, plant, or bacterial cells and uses their DNA/RNA to reproduce itself. In the process, the invaded cell is often destroyed, and the living organism becomes ill or dies.

Viral Structure And Function

A quick glance at Figure 6.4 (A) will quickly reveal why viruses are not considered living cells – they contain no plasma membrane or other organelles! A virus basically is a tiny parasitic particle whose simple structure consists of a core of nucleic acid surrounded by a coat of proteins. This extremely simple structure is enough, because viruses do not eat or drink, grow, synthesize proteins, or reproduce by themselves. Each viral particle contains either DNA or RNA as its nucleic acid, but not both of them. Recall that both DNA and several types of RNA are required for protein synthesis. Hence, viruses cannot make their own proteins.

Helical ( HEE -lih-kal) viruses contain nucleic acid wound up tightly into a coil or spiral, surrounded by a coat of small repeating proteins. Polyhedral ( PAHL -ee- he -dral) viruses have a protein coat with “many” ( poly -) triangular faces coming together. Enveloped viruses are enclosed by an outer lipid envelope. The strangest of the lot may be the bacteriophage (back- tee -ree-oh- FAYJ ), which is sometimes just called a phage ( FAYJ ). Bacteriophage literally means “bacteria-eater”! While the bacteriophage doesn’t exactly eat bacteria, it does attack and destroy many types of bacterial cells. The bacteriophage (phage) particle is topped by a multiple-faced head portion, a slender neck within a protein sheath, and several long tail fibers flairing out at the bottom. These tail fibers attach to the cell wall of the attacked bacterium, then inject viral DNA into it. (Examine Figure 6.4, B.) The injected viral DNA uses the bacterial host cell DNA and RNA to reproduce itself in huge numbers. Eventually, there may be so many new virus particles that they release enough powerful enzymes to cause the complete lysis (rupture and breakdown) of the infected bacterium.

Bacteria and the “Homeless” Viruses Viruses: Non-living Parasites of Cells Viral Structure And Function

Fig. 6.4 Virus structure and action. (A) Four major types of virus structure. (B) Bacteriophage attacking a bacterium.

The Worldwide Aids Epidemic

Dangerous viral attacks upon human cells, not just bacterial cells, are unfortunately quite common. A prominent case in point is the deadly AIDS virus . AIDS is an abbreviation for Acquired Immunodeficiency ( im -yew-no-deh- FISH -en-see) Syndrome . This disease is caused by infection of human cells with the human immunodeficiency virus , or HIV . The HIV particles are usually transmitted from an infected person to someone else during unprotected sexual intercourse. However, the virus may also be spread when the tears or saliva of an infected person go into another person’s blood, or from blood-to-blood during a transfusion.

The HIV particles are enveloped viruses that attack the T-cells in a human’s immune (im- YOON ) or “safety”-providing system . The viruses fuse with the T-cell’s plasma membrane, then release their viral RNA into the cell. Eventually, the host T-cell produces DNA, which then directs the cell organelles to create new HIV particles. The new viruses bud-off from the surface of the host cell, and eventually infect many others.

Infection with HIV particles disturbs the immune or self-protective functions of the T-cell. When thousands of T-cells become infected, then, the immunodeficiency syndrome shows up. The AIDS virus, itself, does not kill the victim. Rather, it is the prolonged immunodeficiency of the infected person that is deadly. With very little immunity (protection) from disease in general, the AIDS patient becomes an easy victim for infection by many dangerous bacteria, such as those causing pneumonia.


Practice problems for these concepts can be found at:  Bacteria and Viruses Test

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