1. VIRUSES

2. Viruses
Viruses are infectious agents with both living and nonliving characteristics

3. Viruses 1. Living characteristics of viruses
a. They reproduce but only in living host cells
b. They can mutate
2. Nonliving characteristics of viruses
a. They contain no cytoplasm or cellular organelles
b. They must replicate using the host cell's energy and materials (Viruses don't grow and divide on their own)
c. The vast majority of viruses possess either DNA or RNA but not both.

4. Why are viruses not considered living?
Viruses must invade healthy cells in order to reproduce. They are known as parasites.(They need a host cell function)
Viruses that infect bacteria are known as bacteriophages 

5. The Structure Of a Virus
Viruses are composed of two parts:
A Protein Coat
Inner core of genetic material
The Inner Core of nucleic acid is surrounded by a protein coat called a capsid
The Nucleic core is either made up of DNA or RNA but never both

6. How Viruses Multiply VIRUSES CAN EXHIBIT TWO TYPES OF LIFE CYCLES.
1. Active Viruses (Lytic Cycle)
2. Hidden Viruses (Lysogenic Cycle)
SOME VIRUSES ARE ACTIVE ALL THE TIME.
SOME VIRUSES CAN BE HIDDEN AND BECOME ACTIVE LATER.
(See pages 212 and 213)

7. Active vs. Hidden Viruses3
Prophage in chromosome
Attachment 1
Lambda phage 2
Entry
Active Cycle
Hidden Virus
Cycle 6
Synthesis 8
Release 4
Replication of chromosome (and progeny) cell division 7
Assembly
Further cell division replications 5
Induction

8. Cool Video Clips
feature=related

9. Viral Replication The Lytic (Active) and Lysogenic (Hidden) Cycles:
How HIV (a retro-virus) replicates and causes AIDS

10. How small are viruses????
Viruses are so small that most can only be seen by the electron microscope ( 1935-tobacco mosaic virus)
nanometers long (one millionth of an inch) and about one thousand times smaller than bacteria
can only grow in other cells (host cells)

11. Tobacco Mosaic Virus

12. Adenovirus

13. Smallpox

14. Ebola virus

15. Why do we get infected with some viruses but not others?
Each virus is specific for the type of cell (respiratory, intestinal, bacterial) that they invade because the receptor sites on the virus must match up with the receptor site on the host cell (embedded in the plasma membrane)
Example: Rhinovirus (cold virus) will only
infect respiratory tissue

16. How did viruses evolve?
Some people think that viruses were some of the first life forms
Other people think that they are descendents of complex living organisms but they dropped or lost their cell structure
Others think they arose from DNA that escaped from cellular organisms
Some even think they came from aliens (Mr. Kay)

17. What works against viruses?
Vaccines or immunizations
By pre-infecting the body with small
amounts of a particular virus, our body can build up antibodies that will fight it for our lifetime
* Antibiotics DO NOT work against viruses because they attack cell walls (which viruses do not have)

18. Vaccine
Antigens (Dead/Weakened Viruses) are deliberately introduced into the immune system to produce immunity
Because the virus has been killed or weakened, minimal symptoms occur
Have eradicated or severely limited several diseases from the face of the Earth, such as polio and smallpox

19. Cell before infection…
Viruses
Viruses enter body cells, hijack their organelles, and turn the cell into a virus making-factory. The cell will eventually burst, releasing thousands of viruses to infect new cells.
…and after.
Cell before infection…

20. The Second Line of Defense ~Interferon~
Virus-infected body cells release interferon when an invasion occurs
Interferon – chemical that interferes with the ability to viruses to attack other body cells
What happens to already infected cells?

21. White Blood Cells ~T-Cells~
T-Cells, often called “natural killer” cells, recognize infected human cells and cancer cells
T-cells will attack these infected cells, quickly kill them, and then continue to search for more cells to kill

22. The Third Line of Defense ~Antibodies~
Most infections never make it past the first and second levels of defense
Those that do trigger the production and release of antibodies
Proteins that latch onto, damage, clump, and slow foreign particles
Each antibody binds only to one specific binding site, known as an antigen

23. Vaccine
Antigens (Dead/Weakened Viruses) are deliberately introduced into the immune system to produce immunity
Because the virus has been killed or weakened, minimal symptoms occur
Have eradicated or severely limited several diseases from the face of the Earth, such as polio and smallpox

24. Vaccines Viruses grown on chicken embryos are attenuated vaccines
Another type of vaccine is made by heat killing the virus