2 VirusesViruses: 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 proteinsNOT livingThey don’t meet all eight characteristics of life.Why?Can’t reproduce on their ownDon’t grow and develop
6 Viral InfectionMost viruses are highly specific to the cells they infectPlant viruses only infect plantsAnimal viruses only infect certain species of animalsBacterial viruses (called bacteriophages) only infect bacteria
7 5 Basic Steps of Virus Replication Attachment – Virus attaches to hostInvasion (Penetration)– Injects DNA or RNAReplication & Synthesis – hijacks host machinery to make new DNA/RNA & new capsidsAssembly – puts viral DNA/RNA into capsids, envelopesRelease – 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 itselfit causes host cell to lyse, or “burst,” releasing copies
9 The Lytic Cycle Bacteriophage protein coat Bacteriophage DNA Bacterial chromosomeBacteriophage attaches to bacterium’s cell wallBacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles thatcan attack other cells.Bacteriophage injects DNA into bacteriumBacteriophage proteins and nucleic acids assemble into complete bacteriophage particlesBacteriophage takes over bacterium’s metabolism, causing synthesis of new bacteriophage proteins and nucleic acidsBacteriophageBacteriophage DNABacteriophage proteinGo to Section:
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 DNAHost cell with viral nucleic acid replicates and divides normallyIn unfavorable conditions, the virus enters a lytic cycle
11 The Lysogenic Cycle Lytic Cycle Lysogenic Cycle Bacteriophage DNA BacterialchromosomeBacteriophage injects DNA into bacteriumBacteriophage DNA (prophage) can exit the bacterial chromosomeLytic CycleLysogenic CycleBacteriophageDNA forms a circleBacteriophage DNA (prophage) may replicate with bacterium for many generationsBacteriophage enzyme lyses the bacterium’s cell wall, releasing new bacteriophage particles that can attack other cellsProphageBacteriophage proteins and nucleic acids assemble into complete bacteriophage particlesBacteriophage DNA inserts itself into bacterial chromosomeGo to Section:
13 Virus DangersViruses cause yellow fever, rabies, polio, AIDS (HIV), influenza (the flu) , Herpes, Hepatitis & the even the common coldYou 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