VIRUSES

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.

• Viruses can replicate by infecting cells and using the cell to make more viruses.

• Viruses do not possess all the properties of life, therefore they are not considered living.

• But because they cause great harm to living organisms, we try to learn all we can about them.

Who Discovered the First Virus?• In the early 1900s, scientists were trying to figure

out what was killing the tobacco plants.

• They isolated a chemical in the sap of the plant and crystallized it to show some sort of poison.

• In Latin, “virus” means poison• Because of this discovery, many other viruses

could be identified and studied.

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 ReplicationViruses 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.

Prions• In 1982 an American

scientist named Prusiner discovered a new class of pathogens called prions.

• Prions are composed of proteins that have no nucleic acid.

• Mad cow disease is associated with prions.

• It is believed that the disease spreads when cows eat feed containing remnants of infected cattle.

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.