Unit 5: Classification and Kingdoms Viruses

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

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

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

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

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

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

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 Go to Section:

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

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 Go to Section:

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).

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