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VIRUSES Genetic Information and Viruses • Among the most important basic properties of life is the ability to replicate and evolve. • Viruses are tiny particles that have no cells, yet they replicate and evolve. • Because they cannot do these things without help, viruses depend on the living host cells they infect. Is a virus alive? • Viruses are not cells; they are not made up of cells. • Viruses ARE small segments of nucleic acid that are surrounded by a protective protein coat. • Viruses ARE 10 to 100 times smaller than bacteria. • Viruses ARE pathogens—they cause disease in living organisms. Viral Structure • The capsid is made of protein and holds either DNA or RNA • The tail fibers attach to the cell • The plug makes an opening in the membrane of the living cell • The sheath injects the nucleic acid into the cell Viruses come in all shapes and sizes, making them very unique… Viral Replication Viruses must rely on living cells (host cells) for replication. An animal virus enters the cell by endocystosis. A bacterial virus punches a hole in the cell wall. Comparison of the two cycles: • • • • Lytic Cycle Viral genes enter the cell and duplicate using proteins inside of the host cell New viruses are made inside of the host cell The host cell is flooded with new viruses and bursts open The host cell is killed • • • • Lysogenic Cycle Sometimes viruses inject their genes but do not duplicate inside of the host cell Instead, the virus’ DNA (or RNA) combines with the host cell’s DNA The host cell divides normally through mitosis The viral genes are replicated without killing the host cell. Impact of Viruses • Viruses live at the expense of the host organism. • Antibiotics are effective against bacterial infections because they interfere with aspects of bacterial metabolism that are different from their hosts’ metabolism • Antibiotics are useless against viruses because viruses do not have their own metabolism. What are vaccines? • Vaccines are killed or weakened viruses that are introduced into the body to build immunity. • Immunity is your body’s defense mechanism that recognizes harmful viruses when they try to attack and prevents them from making you sick. Why give vaccines? • Immunizations help a child’s immune system do its work. The child develops protection against future infections, the same as if he or she had been exposed to the natural disease. What else do vaccines do? 1. Protect a child’s friends, schoolmates, and others from those same diseases. Some children can’t get certain vaccines for medical reasons, or some children are not able to respond to certain vaccines. For these children, the immunity of people around them is their only protection. 2. Protect your grandchildren, their grandchildren, and future generations from diseases. If we stopped vaccinating, diseases that are under control would eventually come back to cause epidemics. This has happened in several countries. 3. Help rid the world of diseases that have been crippling and killing children for centuries. Immunization allowed us to eradicate smallpox. Today polio is nearly gone, and in the future measles and other diseases will follow.