1. Viruses
Watch: Virus BrainPop

2. What is a virus?
Viruses are microscopic particles that invade cells and destroy them.

3. What is a virus? A virus is NOT a cell. A virus is NOT living.
Made of genetic material in a protein coat (called a capsid)
Influenza Virus H5N1 (avian influenza)
Herpes Simplex

4. What is a virus? (con't)
Although they have genetic information, viruses are not living because they:
have no cells
can’t use energy
can’t make proteins
can’t reproduce on their own (require a host)
(Remember the 7 Characteristics of Life!)

5. Bacteria vs. Virus Bacteria:
prokaryotic organisms (much simpler than eukaryotes, like plants & animals)
Small in size
Virus:
- NOT living; very simple & lacking most metabolic parts found in cells.
- VERY small!

6. 1. Genetic Material (DNA/RNA)
Structure of a Virus
Two Main Parts:
1. Genetic Material (DNA/RNA)
2. Protein Coat (Capsid)

7. double-stranded DNA or single-stranded DNA
Structure of a Virus
1. Genetic Material
A viruses genetic material could be:
double-stranded DNA or single-stranded DNA OR
double-stranded RNA or single-stranded RNA

8. Structure of a Virus 2. Protein Coat -Also called a capsid
Encloses the viral genome (DNA or RNA)
Come in various shapes.

9. A unique example of a virus is a Bacteriophage
A unique example of a virus is a Bacteriophage. Bacteriophages are viruses that infect bacteria.

10. How does a virus infect a cell?
Viruses are obligate intracellular parasites.
They can not reproduce on their own
The virus makes a host cell into a “virus making factory”.
They must insert their genetic material into the host to reproduce.
Flu attack - how a virus invades your body

11. Viral infections
Viruses identify host cells by a “lock-and-key” fit between proteins on the outside of virus and specific receptor molecules on the host’s surface.
Most viruses of eukaryotes attack specific tissues.
Examples:
* Human cold viruses infect only the cells lining the upper respiratory tract.
* The AIDS virus binds only to certain white blood cells.

12. You can see the different receptors (made of proteins) on the outside of a virus that are searching for specific cells.

13. Basic Steps of a Viral Infection
1) The genome of the virus enters the host cell.
2) The viral genome takes over its host, reprogramming the cell to copy viral nucleic acids (transcription) and manufacture proteins from the viral genome (translation).
3) The nucleic acid molecules and capsomeres then self-assemble into viral particles and exit the cell.

16. Basic Steps of a Viral Infection Continued
After entering the cell and replicating genetic material, one of two pathways might be taken:
1.)Lytic Cycle –new viruses causes the cell to burst (or “lyse”), releasing newly made viruses that will infect other cells
2.)Lysogenic Cycle – Viral DNA becomes part of the cell’s DNA and then replicates along with the cell. (does NOT cause the cell to burst right away)
Bacteriophage Lytic Cycle

18. Examples of Viruses: HIV (Human Immunodeficiency Virus)
HIV is a retrovirus that attacks helper T-Cells called lymphocytes
Helper T-cells fight infection (they are part of your immune system)
AIDS occurs when the helper T cells are almost all killed.
- Can develop from HIV within a few months or take as long as 10 years.
- During this time a person might have mild flu-like symptoms or none at all. They are still contagious during this time.
- AIDS victims die of cancer or other diseases that
a normal person would defeat.
AIDS BrainPop

19. Retroviruses Retroviruses carry RNA as their genetic code
When these viruses affect a host cell, they insert their RNA and it will undergo reverse transcription to make DNA from their RNA (this is opposite of the type of transcription we learned)
The host cell will recognize the DNA as its own and begin making viral proteins, which is harmful to the organism
Transcription:
DNA  mRNA
Reverse Transcription:
RNA  DNA

21. Examples of Viruses: The Flu (Influenza)
-There are several different viruses (and bacteria) which may infect the lung, but true flu is caused by orthomyxoviruses
-Orthomyxoviruses usually infect your upper respiratory tract (throat and upper lungs) because these tissues have plenty of the receptors for the influenza virus.
-A common way to pick up the flu virus is to rub the moist corners of your eyes, nose or mouth after having shaken hands with someone who is shedding the virus.
-Two or three days after exposure you start to shiver, have a headache and aching limbs, you feel exhausted and you get a fever

22. Why do you need a flu shot every year? (not in notes)
All influenza viruses (all orthomyxoviruses) have RNA as their genetic material.
When RNA is replicated it tends to have more errors than when DNA is replicated. These extra errors provide extra mutations upon which selection may act.
That means RNA viruses have a high mutation rate and can evolve quickly - faster than a DNA virus or even a DNA human!
Flu & Flu Vaccine Brain Pop

23. Viruses vs Cells
(not in notes)