1. Unit 5: Classification and Kingdoms
Viruses

2. Viruses
Viruses: infectious particles made up of nucleic acid, protein coat, and sometimes lipids

3. Viruses, continued Nucleic Acid can either be DNA or RNA
Nucleic Acid is protected by a surrounding protein coat or a capsid.

4. Viruses, continued They bind to specific membrane proteins on host.
They are much smaller than bacteria and can pass through filters.
Viruses- obligate intracellular parasites  must have a host cell!
Cannot make their own proteins
NOT living
They don’t meet all eight characteristics of life.
Why?
Can’t reproduce on their own
Don’t grow and develop

6. Viral Infection
Most viruses are highly specific to the cells they infect
Plant viruses only infect plants
Animal viruses only infect certain species of animals
Bacterial viruses (called bacteriophages) only infect bacteria

7. 5 Basic Steps of Virus Replication
Attachment – Virus attaches to host
Invasion (Penetration)– Injects DNA or RNA
Replication & Synthesis – hijacks host machinery to make new DNA/RNA & new capsids
Assembly – puts viral DNA/RNA into capsids, envelopes
Release – New Viruses Lyses (break out of) cell, releasing more viruses to infect new cells

8. Viral Life Cycles Lytic Cycle: virus injects nucleic acid
virus makes copies of itself
it causes host cell to lyse, or “burst,” releasing copies

9. The Lytic Cycle Bacteriophage protein coat Bacteriophage DNA
Bacterial chromosome
Bacteriophage attaches to bacterium’s cell wall
Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles thatcan attack other cells.
Bacteriophage injects DNA into bacterium
Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles
Bacteriophage takes over bacterium’s metabolism, causing synthesis of new bacteriophage proteins and nucleic acids
Bacteriophage
Bacteriophage DNA
Bacteriophage protein
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10. Viral Life Cycles Lysogenic Cycle: Virus injects nucleic acid
Viral nucleic acid turns into a prophage (plasmid) by embedding itself in the host cell’s DNA
Host cell with viral nucleic acid replicates and divides normally
In unfavorable conditions, the virus enters a lytic cycle

11. The Lysogenic Cycle Lytic Cycle Lysogenic Cycle Bacteriophage DNA
Bacterial
chromosome
Bacteriophage injects DNA into bacterium
Bacteriophage DNA (prophage) can exit the bacterial chromosome
Lytic Cycle
Lysogenic Cycle
Bacteriophage
DNA forms a circle
Bacteriophage DNA (prophage) may replicate with bacterium for many generations
Bacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can attack other cells
Prophage
Bacteriophage proteins and nucleic acids assemble into complete bacteriophage particles
Bacteriophage DNA inserts itself into bacterial chromosome
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13. Virus Dangers
Viruses cause yellow fever, rabies, polio, AIDS (HIV), influenza (the flu) , Herpes, Hepatitis & the even the common cold
You cannot “cure” a viral infection; you can only treat symptoms and prevent it with vaccines.
Vaccines – a preparation of a weakened or killed virus that is injected into the body to increase immunity (i.e. it causes your body to make more white blood cells).

14. Examples of Viruses…
Retrovirus: uses RNA; must copy genetic info backwards (ex: HIV AIDS virus)
Prions: protein infectious particles similar to viruses, only they do not have nucleic acids (i.e. No DNA or RNA)
might cause Mad Cow Disease