1. Viruses
Dead or alive?

2. Viral structure Viruses are not cells. Basic structure:
Protein capsule
Nucleic acid core (RNA or DNA)
Parasites – need a living host
Protein Capsule
Nucleic Acid
sheath
tail fibers

3. Virus Categories DNA viruses – stable, do not mutate rapidly
Smallpox, Hepatitis B, Herpes
RNA viruses – mutate rapidly, unstable
HIV, Rhinovirus, SARS, Polio, West Nile, common cold

4. Are viruses alive?
Only 3 characteristics of life: reproduction, evolution, and genetic code (DNA/RNA)
Can only reproduce inside a host cell!
Process or reproduction = lytic cycle

5. Lytic Cycle
Virus attaches to host cell’s membrane and injects its nucleic acid into the host cell.
The viral nucleic acid takes over protein synthesis, creating new viruses.
The host cell bursts, lyses, releasing the newly formed viruses.

7. HIV

8. HIV doesn’t target just any cell, it goes right for the cells that want to kill it. “Helper" T cells are HIV's primary target. These cells help direct the immune system's response to various pathogens.

9. HIV undermines the body's ability to protect against disease by depleting T cells thus destroying the immune system.
The virus can infect 10 billion cells a day, yet only 1.8 billion can be replaced daily.

10. After many years of a constant battle, the body has insufficient numbers of T-Cells to mount an immune response against infections. At the point when the body is unable to fight off infections, a person is said to have the disease AIDS.
It is not the virus or the disease that ultimately kills a person; it is the inability to fight off something as minor as the common cold.

11. Chapter 18 Questions
Use the textbook to answer the following 3 questions:
What are the 4 main viral structures?
What is the difference between the lytic and lysogenic cycles?
Critical Thinking:
How do vaccines work to prevent viruses? Why don’t antibiotics work on viruses?
How do you think viruses can be used for good?